abstract stringlengths 3 192k | title stringlengths 4 857 |
|---|---|
the recent discoveries of telluric exoplanets in the habitable zone of different stars have led to questioning the nature of their atmosphere, which is required to determine their habitability. atmospheric escape is one of the challenging problems to be solved: simply adapting what is currently observed in the solar system is doomed to fail due to the large variations in the conditions encountered around other stars. a better strategy is to review the different processes that shaped planetary atmosphere and to evaluate their importance depending upon the stellar conditions. this approach allowed to show that processes like ion-pickup were a more important way to lose atmosphere at mars in the past. we reviewed the different escape mechanisms and their magnitude in function of the different conditions. this led us to discover discrepancies in the current literature concerning problems such as the xenon paradox or the importance of a magnetic field in protecting an atmosphere. this shows that one should be very careful before claiming the presence of an atmosphere on planets in the habitable zone of their m-dwarfs: new criterion such as the alfven surface location with respect to the planet should be taken into account a-priori. overall, the habitability of a planet should not be claimed only on by its location in the habitable zone but also after careful analysis of the interaction between its atmosphere and its parent star [gronoff et al. 2020]. gronoff, g., arras, p., baraka, s., bell, j. m., cessateur, g., cohen, o., et al. ( 2020). atmospheric escape processes and planetary atmospheric evolution. journal of geophysical research: space physics, 125, e2019ja027639. https://doi.org/10.1029/2019ja027639 | atmospheric escape processes and planetary atmospheric evolution: from misconceptions to challenges |
there are nearly 200 known planets around m dwarfs, but only one system around an ultra-cool (>m7) dwarf: trappist-1. ultra-cool dwarfs are arguably the most promising hosts for atmospheric and biosignature detection in transiting planets because of the enhanced feature contrast in transit and eclipse spectroscopy. we are conducting a spitzer cycle 14 survey to continuously monitor 15 of the brightest ultra-cool dwarfs over 3 days. to maximize the probability of detecting transiting planets, we have selected only targets seen close to equator-on. spin-orbit alignment expectations dictate that the planetary systems around these ultra-cool dwarfs should also be oriented nearly edge-on. we have detected our first rocky transiting exoplanet: a potentially habitable sub-earth around a 0.076 solar-mass l1.5 dwarf. our three-day sequence shows only a single transit. we seek follow-up monitoring to determine the period of the planet and detect any additional planets in the system. if the planet is confirmed to be in the habitable zone, it will become a top jwst target for atmospheric characterization and biogenic gas detection. | confirming the first exoplanet around an l dwarf |
editors note:in these last two weeks of 2017, well be looking at a few selections that we havent yet discussed on aas nova from among the most-downloaded paperspublished in aas journals this year. the usual posting schedule will resume in january.detection of the atmosphere of the 1.6 m exoplanet gj 1132 bpublished march2017main takeaway:an atmosphere was detected around the roughly earth-size exoplanet gj 1132 b using a telescope at the european southern observatory in chile. a team of scientists led byjohn southworth (keele university) found features indicating the presence of an atmosphere in theobservationsof this 1.6-earth-mass planet as it transits an m-dwarf host star. this is the lowest-mass planet with a detected atmosphere thus far.why its interesting:m dwarfs are among the most common stars in our galaxy, and weve found manyearth-sizeexoplanets in or near the habitable zones around m-dwarf hosts. but m dwarfs are also more magnetically active than stars like our sun, suggesting that the planets in m-dwarfhabitable zones may not be able to support life due to stellar activity eroding their atmospheres. the detection of an atmosphere around gj 1132 b suggests that some planets orbiting m dwarfsare able to retain their atmospheres which meansthat these planetsmay be an interesting place to search for life after all.how the atmosphere was detected:the measured planetary radius for gj 1132 b as a function of the wavelength used to observe it. [southworth et al. 2017]when measuring the radius of gj 1132 b based on its transits, the authors noticed that the planet appeared to be largerwhen observed in some wavelengths than in others. this can beexplained if the planet has asurface radius of 1.4 earth radii, overlaid by an atmosphere that extends out another few tenths of an earth radius. the atmosphere, which may consist of water vapor or methane, is transparent to some wavelengths and absorbs others which is why the apparent size of the planet changes acrosswavelength bands.citationjohn southworth et al 2017 aj 153 191. doi:10.3847/1538-3881/aa6477 | selections from 2017: atmosphere around an earth-like planet |
astronomers are becoming more interested in finding an earth-like planet in the liquid water, or habitable, zone of the star it is orbiting. with today's methods for finding exoplanets (transit, radial velocity) being more efficient when observing smaller stars with planets closer to them, the best targets for modern instruments are m-dwarf stars. m-dwarf stars are also known to have intense flares that must be considered when determining if stars of this type can host a planet with an atmosphere, let alone sustain life. more information about the star's ultraviolet characteristics is needed to understand the effects of heating and photochemistry on the exoplanet's atmosphere. contemporaneous optical light curves also provide information on the flare chromospheric continuum emission and serve as proxy tracers of high energy behavior. the mini-muscles treasury survey focuses on observations of two m-dwarf stars from the target list of the ongoing mega-muscles treasury survey: gj 674 and gj 729. using u-band photometric observations from the los cumbres observatory, mini-muscles created light curves of the stars and compared them to simultaneous uv spectroscopic observations from hst. the lco observations of gj 674 missed a large flare seen by hst but was able to trace the post-flare light curve of the star. for gj 729, a simultaneous flare observation was successfully observed. in this poster, we present the ongoing analysis of the light curves and the correlation between the uv and optical activity. | the mini-muscles treasury survey: measurements of the ultraviolet stellar characteristics of low-mass exoplanetary systems |
in this study we explore and identify the minimum eccentricity thresholds for deglaciation of globally ice-covered extrasolar planets with different obliquities. using a one-dimensional energy balance model, we calculate the latitudinal ice thickness and extent on earth-like planets receiving different values of instellation from a g-dwarf star, for obliquities from 0° to 90°. for orbiting planets that are completely frozen at apoastron, we find that the minimum eccentricity required to fully thaw the ice sheets at periastron decreases with increasing instellation. increases in obliquity raise these minima, and this effect is most strongly apparent at eccentricity values below ~0.75. however, the influence of obliquity diminishes at higher values of eccentricity, and has little effect on planets with extreme eccentricities above ~0.85, due to the increasing effectiveness of larger eccentricity as the primary means of thawing the ice. additionally, we find that the region where seasonal ice is present in the eccentricity-instellation parameter space diminishes with increasing obliquity. this diminishment is due to rapidly-increasing ice sheet thickness on planets with initially thin ice at low obliquities, preventing the occurrence of seasonal ice. outside of this region, the planet is either completely frozen or fully thawed throughout the entire planet's year. our work has important implications for planets discovered at large orbital distances from their parent stars and whose eccentricities and obliquities are currently unconstrained by observations. such planets may exhibit temporal surface habitability despite technically orbiting outside of their host stars' traditional habitable zones. | eccentricity thresholds for planetary deglaciation at varying obliquity |
we discuss the implications on the future evolution of planetary systems orbiting red giant stars, if physical reactions involving the production of axions and the enhanced decay of plasmons into neutrinos, as a consequence of neutrinos having a magnetic dipole moment, are assumed to be happening simultaneously within the stellar core. simulations were created by employing a numerical code to calculate the physical properties of stellar models from the main sequence to the end of the red giant phase, for stars with a mass range between 0.9 and 1.9 m⊙ and we analyze the most noticeable differences to canonical stellar evolution. two exoplanet systems, known for harboring single red giant stars, were modelled by using the predicted stellar properties to calculate the temporal variation of the habitable zone and the orbital distance. | implications of non-standard physics on the future evolution of exoplanets orbiting red giant stars. |
we present the mirecle (mir exoplanet climate explorer) mission concept, a concept for a moderately sized (2-meter) cryogenic telescope with broad wavelength coverage (4 - 25 um) and ultra-stable detectors capable of conducting in-depth characterization of a statistically significant sample of planets around ultra-cool dwarfs, several of which will be terrestrial planets in their host stars's habitable zones. mirecle would provide unprecedented sensitivity in the mir over long observing baselines, providing the opportunity for phase-resolved and secondary eclipse spectroscopy of cool (300k) planets. spectroscopic characterization of terrestrial atmospheres will provide constraints for the distribution of planets with tenuous vs. substantial atmospheres, map tidally-dominated volcanically active planets, and explore the transition across the inner edge of the surface-water regime. for the few brightest targets, the detection of specific combinations of molecules would provide evidence of biological activity. for gas-rich planets, mirecle would produce phase-resolved maps of planets across a wide range of mass and temperature. this comprehensive survey would also determine which subset of potentially habitable worlds would be best for future in-depth atmospheric characterization using larger aperture telescopes. we will present the science case and sensitivity calculations for the mission, as well as the key technical aspects of the instrumentation that will provide the stability floor of 5 ppm or better over multi-day timescales needed to measure thermal phase curves of hz planets. | mirecle: exploring the nearest m-earths through ultra-stable mid-ir transit and phase-curve spectroscopy |
locating planets in circumstellar habitable zones is a priority for many exoplanet surveys. space-based and ground-based surveys alike require robust toolsets to aid in target selection and mission planning. we present the catalog of earth-like exoplanet survey targets (celesta), a database of habitable zones around 36,000 nearby stars. we calculated stellar parameters, including effective temperatures, masses, and radii, and we quantified the orbital distances and periods corresponding to the circumstellar habitable zones. we gauged the accuracy of our predictions by contrasting celesta's computed parameters to observational data. we ascertain a potential return on investment by computing the number of habitable zones probed for a given survey duration. a versatile framework for extending the functionality of celesta into the future enables ongoing comparisons to new observations, and recalculations when updates to habitable zone models, stellar temperatures, or parallax data become available. we expect to upgrade and expand celesta using data from the gaia mission as the data becomes available. | celesta: a catalog of earth-like exoplanet survey targets |
indirect detections of exoplanets rely heavily on the properties of their host stars. however, stellar features can sometimes masquerade as planetary signals. this issue is especially prominent for m dwarfs. so how do we know for sure if weve found a planet around an m dwarf?starring m dwarfsdespite the observational challenges they pose, m dwarfs present exciting prospectsfor exoplanet science. with their low masses, they are especially susceptible to thegravitational influence of any orbiting planets. when an m dwarf is tugged on by an orbiting planet, this affects the stars radial velocity, creating a strong doppler signal that we can detect in its spectrum.the extent of the habitable zone (highlighted in green) for a stars, g stars, and m stars. as indicated in the figure, the sun is a g star. [nasa]in addition, the habitable zone of an m dwarf is located very close to the star. this means that planets in the habitable zone of an m dwarf would produce a stronger doppler signal than planets in the habitable zone of a higher mass, hotter a star.but theres a catch! while planets can produce detectable doppler signals, so can m dwarfs themselves. starspots, or cool regions on the surface of stars, can be prominent in the stellar spectra, and as an m dwarf rotates, starspots can mimic the effects of a planet.so whats to be done? m dwarfs are complicated beasts, so astronomers have been attempting to characterize them in more detail. in a new study, a group of researchers led by paul robertson (the university of california, irvine) used optical and near-infrared observations of m dwarfs to understand the effects of starspots and other surface features.an artists impression of an m dwarf flaring like ad leo was observed to do so earlier this year. however, ap leo is not confirmed to host any planets. [national astronomical observatory of japan]spinning around and aroundfor their study, robertson and collaborators selected four rapidly-rotating m dwarfs. here, rapid means the rotational speed at the surface of the star is about 3 to 11 kilometers per second. three of the stars in the sample were chosen because they were similar to the fourth, ad leo. ad leo is only 16 light-years from the earth and has been studied extensively in the context of its magnetic activity, flares, and starspots.robertson and collaborators pulled observations from a number of instruments, including the transiting exoplanet survey satellite and the harps spectrograph. they also used data taken by the habitable-zone planet finder on the hobby-eberly telescope, which was built especially for finding low-mass planets around m dwarfs.separating out planetary signalsthe phased radial-velocity/doppler signals from the m dwarf gj 3959 as observed by the habitable-zone planet finder (hpf) and the high resolution echelle spectrometer (hires). the phased signal represents a stacking of the observations based on the rotational period of gj 3959, which is about half a day. [robertson et al. 2020]all four m dwarfs showed prominent doppler signals that persisted for longer than expected hundreds of stellar rotations! and also syncedup with the stars rotational periods. since the stars in this sample arent known exoplanet hosts, these stable doppler signals are likely caused by surprisingly persistent surface features like starspots.these findings contain a blessing and a curse for exoplanet searchers. any planets whose orbital period is similar to the rotational period of the star will be very hard to distinguish. the longevity of the signals means itll likely be a long wait for the surface features to die out so we can verify if a planet was contributing to the signal. however, since the doppler signals are stable and fairly predictable, they could be modeled and removed.this study highlights the challenges of searching for planets around m dwarfs. but the finds would definitely be worth the trouble!citation:persistent starspot signals on m dwarfs: multiwavelength doppler observations with the habitable-zone planet finder and keck/hires, paul robertson et al 2020 apj 897 125. doi:10.3847/1538-4357/ab989f | to separate starspots from planets |
currently, the strongest remotely detectable biosignature in the earth's atmosphere is molecular oxygen (o2) produced during photosynthesis. however, recent studies of geochemical signatures on earth-like exoplanets suggest that for most of them, atmospheric o2 would not be detectable by a remote observer, except during the last ~500 myr of evolution. during a long period in the earth's history (2.0-0.7 gyr ago), o2 was likely present in the atmosphere but in low concentrations, estimated at ~0.1-1% of the current level. although spectral manifestations of o2 are weak at such low concentrations; however, ozone (o3) molecules, which are in a photochemical equilibrium with such low o2 concentrations, cause noticeable spectral features in the hartley-huggins uv band (~0.25 µm), with a weaker manifestation in the medium ir-region at about 9.7 µm. thus, taking the earth's history as an informative example (proxy), it can be concluded that a category of exoplanets may exist for which the ordinary atmospheric biosignature can only be identified in the uv range. accordingly, the article emphasizes the importance of planning for uv observation capabilities when designing future space telescopes for direct observations of exoplanets and their atmospheres, such as the world space observatory-uv (wso-uv), habitable exoplanet observatory (habex), or large uv/optical/infrared surveyor (luvoir), for the detection of ozone o3 in the atmospheres of planets with intermediate oxidation states. the article also discusses mitigation strategies for the so-called false positives, i.e., detection of o3 generated in abiotic processes. it also emphasizes the importance and broad implications of studying the earth's history as a window to understanding potential biosignatures for exoplanets and the importance of uv observations for identifying habitable exoplanets with next-generation space telescopes. | exoplanet habitability: potential o2/o3 biosignatures in the ultraviolet |
terrestrial exoplanets orbiting within or near their host stars' habitable zone are potentially apt for life. it has been proposed that time-series measurements of reflected starlight from such planets will reveal their rotational period, main surface features and some atmospheric information. from imagery obtained with the akatsuki spacecraft, here we show that venus' brightness at 283, 365, and 2020 nm is modulated by one or both of two periods of 3.7 and 4.6 days, and typical amplitudes <10% but occasional events of 20-40%. the modulations are unrelated to the solid-body rotation; they are caused by planetary-scale waves superimposed on the super-rotating winds. here we propose that two modulation periods whose ratio of large-to-small values is not an integer number imply the existence of an atmosphere if detected at an exoplanet, but it remains ambiguous whether the atmosphere is optically thin or thick, as for earth or venus respectively. multi-wavelength and long temporal baseline observations may be required to decide between these scenarios. ultimately, venus represents a false positive for interpretations of brightness modulations of terrestrial exoplanets in terms of surface features. this study is published recently (https://doi.org/10.1038/s41467-020-19385-6). | taking the photometric pulse of venus, our nearest terrestrial planet: probing atmospheric super-rotation rather than surface features |
the habitable exoplanet observatory mission (habex) is one of four missions under study for the 2020 astrophysics decadal survey. its goal is to directly image and spectroscopically characterize planets in the habitable zone around nearby sun-like stars. additionally, habex will perform a broad range of general astrophysics science enabled by 115 to 1700 nm spectral range and 3x3 arc-minute fov instruments. critical to achieving its science goals is a large, ultra-stable uv/optical/near-ir (uvoir) telescope. the baseline habex telescope is a 4-meter off-axis unobscured three-mirroranastigmatic, diffraction limited at 400 nm with wavefront stability on the order of a few 10s of picometers. this paper summarizes the opto-mechanical design of the habex baseline optical telescope assembly, including a discussion of how science requirements drive the telescope's specifications, and presents analysis that the baseline telescope structure meets its specified tolerances. | habitable-zone observatory (habex) baseline 4-m telescope design and predicted performance |
the irradiation of protoplanetary disks by high-energy radiation from magnetic and accretion activity at low-mass, pre-ms stars likely plays an essential role in regulating exoplanet formation around such stars. to provide the x-ray data necessary to address the problem of the dissipation of protoplanetary disks around the lowest-mass stars, we propose a survey of a sample of previously established and newly-discovered mid- to late-type m type members of the nearby tw hya association (age 8 myr), most of which were the subjects of our recent alma survey to detect dusty disks. the combined chandra and alma survey of the twa will provide a unique resource with which to investigate x-ray-induced photoevaporation of disks orbiting very low-mass stars and massive brown dwarfs. | x-rays from young low-mass stars: inhospitable habitable zones? |
oxygenic photosynthetic organisms (opos) are primary producers on earth and generate surface and atmospheric biosignatures, making them ideal targets to search for life from remote on earth-like exoplanets orbiting stars different from the sun, such as m-dwarfs. these stars emit very low light in the visible and most light in the far-red, an issue for opos, which mostly utilize visible light to photosynthesize and grow. after successfully testing procaryotic opos (cyanobacteria) under a simulated m-dwarf star spectrum (m7, 365–850 nm) generated through a custom-made lamp, we tested several eukaryotic opos: microalgae (dixoniella giordanoi, microchloropsis gaditana, chromera velia, chlorella vulgaris), a non-vascular plant (physcomitrium patens), and a vascular plant (arabidopsis thaliana). we assessed their growth and photosynthetic efficiency under three light conditions: m7, solar (sol) simulated spectra, and far-red light (fr). microalgae grew similarly in sol and m7, while the moss p. patens showed slower growth in m7 with respect to sol. a. thaliana grew similarly in sol and m7, showing traits typical of shade-avoidance syndrome. overall, the synergistic effect of visible and far-red light, also known as the emerson enhancing effect, could explain the growth in m7 for all organisms. these results lead to reconsidering the possibility and capability of the growth of opos and are promising for finding biosignatures on exoplanets orbiting the habitable zone of distant stars. | growth and photosynthetic efficiency of microalgae and plants with different levels of complexity exposed to a simulated m-dwarf starlight |
the habex concept telescope is optimized for direct imaging and spectroscopy of potentially habitable exoplanets, and also enables a wide range of general astrophysics science. the design strategy chose mature technologies and leveraged in-development technologies to minimize risk and possibly reach technology readiness level 5 by 2026 for architecture a. we update the technology maturity roadmap with technology advances in the past year and expand it to include an architecture option which is a 3.2 m diameter on-axis segmented aperture with a starshade only. the starshade suppresses starlight before it enters the telescope, allowing the telescope optical performance and stability to be significantly looser than for a coronagraph, thus enabling a segmented primary mirror design that can meet stability requirements with minimal advancement from the state of the art. in this poster we assess the exoplanet-driven technologies, including elements of coronagraphs, starshades, mirrors, jitter mitigation, segment stability, wavefront control, and detectors. © california institute of technology 2018. all rights reserved. government sponsorship acknowledged. | updated technology roadmap for the habitable-zone exoplanet imaging observatory (habex) concept |
the exoplanet community is eagerly searching data from the transiting exoplanet survey satellite (tess) for m dwarfs hosting small planets in their habitable zones. for my thesis work, i plan to look beyond the habitable zone to find the coldest planets orbiting m dwarf stars to determine their occurrence rate. to identify these planets in the tess (and potentially k2) light curves, i am designing a pipeline recognizing both single- and multiply-transiting long-period planets. as nearly half of these long-period planets will be detected as single-transit events, i look to the tess extended mission to recover second transits for many of these targets which will then offer the potential to find their true periods through ground based follow-up. i will quantify the completeness and reliability of my pipeline by implementing injection/recovery tests. i will then calculate the occurrence rate of cold m dwarf planets as a function of orbital period and planet radius. by studying this hitherto underexplored area of parameter space, this work can be applied towards understanding the demographics of cold m dwarf planets as well as providing candidates for future mass and atmospheric characterization. based on a simulated planet catalog (barclay et al., 2018), i anticipate 27 single-transiting and 32 multiply-transiting m dwarf planets with p > 20 days from the tess primary mission. i hope to have early results at the time of this meeting. | a survey of the coldest planets around low-mass stars |
we began with the sample of planet host stars in the california-kepler survey (cks) sample. the cks sample selection, spectroscopic observations, and spectroscopic analysis are described in detail in petigura et al. (2017aj....154..107p). in brief, the sample was initially constructed by selecting all kepler objects of interest (kois) brighter than kp=14.2 mag. a koi is a kepler target star that showed periodic photometric dimmings indicative of planet transits. however, not all kois have received the necessary follow-up attention needed to confirm the planets. over the course of the cks project, we included additional targets to cover different planet populations, including multi-candidate hosts, ultra-short-period candidates, and habitable-zone candidates. we cross-matched the cks sample with the gaia dr2 catalog (cat. i/345) by querying all gaia sources within 1 arcsec of the kic coordinates. in rare cases, gaia detected more than one source within 1 arcsec, and we selected the source with the smallest difference between g and kp magnitudes. we cross-matched 1257 targets in this way. (3 data files). | vizier online data catalog: california-kepler survey. vii. planet radius gap (fulton+, 2018) |
future direct imaging missions such as habex and luvoir aim to catalog and characterize earth-mass analogs around nearby stars. the exoplanet yield of these missions will be dependent on the frequency of earth-like planets, and potentially the a priori knowledge of which stars specifically host suitable planetary systems. ground or space based radial velocity surveys can potentially perform the pre-selection of targets and assist in the optimization of observation times, as opposed to an uninformed direct imaging survey. in this paper, we present our framework for simulating future radial velocity surveys of nearby stars in support of direct imaging missions. we generate lists of exposure times, observation time-series, and radial velocity time-series given a direct imaging target list. we generate simulated surveys for a proposed set of telescopes and precise radial velocity spectrographs spanning a set of plausible global-network architectures that may be considered for next generation extremely precise radial velocity surveys. we also develop figures of merit for observation frequency and planet detection sensitivity, and compare these across architectures. from these, we draw conclusions, given our stated assumptions and caveats, to optimize the yield of future radial velocity surveys in support of direct imaging missions. we find that all of our considered surveys obtain sufficient numbers of precise observations to meet the minimum theoretical white noise detection sensitivity for earth-mass habitable zone planets, with margin to explore systematic effects due to stellar activity and correlated noise. | simulations for planning next-generation exoplanet radial velocity surveys |
newly-discovered exoplanets from tess provide the opportunity to assessthe relative importance of atmospheric escape mechanisms proposed toexplain the bimodal distribution of super-earths and mini-neptunesdiscovered with kepler. we will measure x-ray spectra and fluxes of thehost stars of a sample of tess planets as inputs to models comparingxuv-driven photoevaporation with core-powered mass loss. determiningthe relative importance of these mechanisms is important for studies ofexoplanet habitability, particularly around m-dwarfs where the cumulativexuv irradiation in the habitable zone is much higher. | testing the origin of the exoplanet radius valley with new systems from tess |
future exoplanet direct imaging missions aim to maximize the number of habitable earth-like exoplanets discoverable in their limited mission time. state of the art exoplanet orbit fitting techniques only incorporate astrometry and require at least three exoplanets detections to determine if the exoplanet is in the habitable zone. decreasing the number of detections required to orbit fit increases telescope operational efficiency. in this work, we use our analytical and numerical techniques in a monte carlo to orbit fit exoplanets from simulated direct-imaging measurements using astrometry and photometry. we achieve an average orbit fit uncertainty less than 0.15 au. | direct imaging orbit fits in two detections |
the discovery of the trappist-1 system, which consists of an ultra cool m-dwarf star orbited by 7 planets, 3 of which are located in the habitable zone, has demonstrated that these types of plane-tary systems around dwarf stars are very common. such systems are well suited for the study of exoplanets. in particular the search for bio-signatures in the atmosphere of planets in the habita-ble zone around m-stars will be a high-priority science goal of future space missions. the mid-infrared (mid-ir) band between 6 and 15 microns is probably the best available band for this science, because the band contains spectral lines of methane, ozone, and nitrous oxide. the coexistence of those in a planet's atmosphere would be a very strong indicator for life on the planet. mid-ir transit spectrometers on future space missions such as origins space telescope (ost) will be the instrument of choice to detect these bio-signatures in exoplanets around m-dwarfs. however, current mid-ir detectors are based on impurity band conduction (ibc) devices such as si:as detectors, which have significant problems with stability. as a result, those detectors are not expected to provide the required stability of 5 ppm needed for a reliable detection of the aforementioned spectral lines. while efforts are under way to improve ibc detectors, it is un-clear how far the performance can be improved. here we propose the development of an ultra-stable mid-ir array spectrometer demonstration for exoplanet transits (miraset), which includes a calibration system that, as we show, is needed to achieve the required sensitivity for the detection of atmospheric bio-signatures in habitable-zone planets around m-dwarfs. the spectrometer will be demonstrated with arrays of transition edge sensor detectors (tes). these devices are known to be intrinsically very stable and the required detector parameters (sensitivity, dynamic range) for space based mid-ir transit spectroscopy can be easily met with existing devices. no new detector developments are re-quired. this project will include the development of a high-accuracy calibration system with a stable reference source which itself will be monitored by an out of band (0.5 μm) detector at a wavelength at which the precision of the measurements exceeds that of an in-band calibration. this scheme will allow for real time monitoring of the detector gain, which we anticipate will result in a background limited performance with the required stability of better than 5 ppm for the detection of bio-signatures in a designated spectrometer flying e.g. on the ost space tele-scope, and as such will help to answer one of nasa's main questions: "are we alone?" | development of an ultra-stable mid-infrared detector array for space-based exoplanet transit spectroscopy |
the discovery of a planetary candidate orbiting the habitable zone of our closest neighbor, proxima centauri (gl 551), shook the planetary community as few other discoveries have done in recent years. it not only showed that the nearest star to the sun could host a planetary system but also that, given the right conditions, it could host a habitable rocky planet. 2020 brought back the attention to proxima with the announcement of a second planetary candidate in the system, the confirmation of proxima b, using espresso, and the suggestion of a third, very low mass. now, after an intense observational campaign, we can confidently say there's a third companion with just one quarter of the mass of the earth. proxima d is one of the lightest exoplanets known to date. it orbits its parent star at 0.029 au, with a period of 5.12 days. it's induced rv semi-amplitude, of just 39 cm/s on a v-mag 11 star, highlights the capabilities of the espresso spectrograph, installed at the vlt telescope array of the paranal observatory. the signal has an amplitude that is just 1/5 of the amplitude of the activity-induced radial velocity signal, which shows that, under the right conditions, current techniques can detect planetary signals much smaller than activity signals. the discovery of proxima d opens the door to the characterization of the population of very low mass planets of the solar neighborhood. it shows that radial velocity studies are now capable of detecting exoplanets with masses similar to the earth and much smaller. | a short-period sub-earth orbiting proxima centauri |
in every batch of detections from the kepler spacecraft, some transit signals get relegated to false positive status by an automated vetting pipeline. how do we ensure that real exoplanet detections dont accidentally get discarded by the pipeline?the kepler false positive working group is on the case and they just rescued quite a find from being relegated to a false-positive fate.to be a planet candidatean illustration of some of the planetary systems discovered by the kepler spacecraft. the stars at the centers of these systems are not pictured. [nasa ames/uc santa cruz]since keplers launch in 2009, this hard-working satellite has found signals from thousands of candidate transiting exoplanets. but all transit signals arent just immediately declared planet candidates!the first hint in kepler data of a potential transiting planet is whats known as a threshold crossing event (tce). that tce could either be a true signal from a planet transiting across the face of its host star, or it could be a false positive or false alarm a signal mimicking a transiting planet thats instead caused by a background eclipsing binary system, noise in the data, instrumental artifacts, etc.early on in the kepler mission, every tce was reviewed by a team of scientists and classified as a true planet candidate or a false positive. but as the mission ramped up and data volume grew, scientists turned to an automated pipeline aptly named the robovetter to categorize the tces.the transit signals of kepler-1649 b (top; previously known) and c (bottom; newly discovered), in the stars light curve. [adapted from vanderburg et al. 2020]human vs. machinethe automated approach has many advantages: we can process larger volumes of data, and the statistical uniformity allows us to make inferences about the sample completeness. but its inevitable that the robovetter will sometimes be wrong, misclassifying a true planet as a false positive.to address this, the kepler false positive working group was established to visually inspect all signals the robovetter classified as false positives and confirm the categorization. this process allows us to improve the robovetters algorithms and it also opens the door to new discoveries hidden in old data.such is the case with kepler-1649c, a planet candidate that was incorrectly categorized by the robovetter as a false positive. in a new study led by andrew vanderburg (nasa sagan fellow at the university of texas at austin), a team of scientists presents their rescue of this sneaky planet.earth-like discoverythe locations of kepler-1649 planets b and c relative to the stars optimistic (light green) and conservative (dark green) habitable zone. [vanderburg et al. 2020]kepler-1649c is a planet the same size as earth that orbits around its m-dwarf host star once every ~20 days, placing it firmly in its host stars habitable zone. its star also hosts a previously known inner planet that appears to be equivalent to venus in its size and the amount of flux it receives.how did the robovetter miss this important habitable-zone, earth-like planet? vanderburg and collaborators suspect that the pipeline was fooled into mistaking kepler-1649s location due to the stars high proper motion. this introduced noise into the inferred light curve, making the robovetter think the transit signal wasnt real.vanderburg and collaborators point out that there are likely hundreds of undiscovered planets left in the extended kepler mission data. while automated pipelines do a great job of doing the heavy lifting, the discovery of kepler-1649c goes to show that theres value in having a human eye checking results.citationa habitable-zone earth-sized planet rescued from false positive status, andrew vanderburg et al 2020 apjl 893 l27. doi:10.3847/2041-8213/ab84e5 | rescuing an overlooked planet |
g9-40 was observed by the kepler spacecraft as part of campaign 16 of the k2 mission. it was proposed as a k2 campaign 16 target by the following programs: go16005lc (pi: crossfield), go16009lc (pi: charbonneau), go16052lc (pi: stello), and go16083lc (pi: coughlin). the star was monitored in long cadence mode (30 minute cadence) for 80 days from 2017-december-7 to 2018-february-25. we obtained four visits of g9-40 with the habitable-zone planet finder (hpf) spectrograph with the goal to measure its radial velocity (rv) variation as a function of time. hpf is a high-resolution (r~55000) nir spectrograph recently commissioned on the 10m hobby-eberly telescope (het) in texas covering the information-rich z, y, and j bands from 810 to 1280nm. (2 data files). | vizier online data catalog: flux & rvs of the dwarf g9-40 with k2 & hpf (stefansson+, 2020) |
exoplanet surveys have revealed a number of earth-sized planets orbiting in or near the so-called habitable zones. finding life on these planets is one of the key goals of the ongoing and future exoplanet observations. in this article, we discuss possible schemes to assess the life hypothesis, highlighting the importance of comprehensive understanding of the surface environment. we also introduce approaches to constrain it observationally, with a focus on the potential of high-resolution spectroscopy in the mid-infrared domain for identifying several atmospheric molecules. | future atmospheric observations of temperate terrestrial planets in search of inhabited worlds |
our current picture of exoplanetary systems is unfinished, and the search for and characterization of habitable planets in these partially explored systems is still ongoing. to address this, we combined the specific yet incomplete information about any given multi-planet system with population-level statistical knowledge and developed dynamite (dynamical multi-planet injection tester) to predict the presence, locations, and sizes of previously unknown planets in these systems. determining the physical parameters and orbital dynamics of potentially habitable exoplanets will provide a wealth of information that can be used for targeted follow-up observations to assess the likelihood of habitability. our analysis examines multi-planet systems individually and in ensembles, looking for additional planets and categorizing their potential for habitability. tests performed on known systems show successful predictions of the approximate period and sizes of these planets, which are sensitive out to rocky planets in the habitable zones of g stars and smaller. we will share the latest dynamite results for currently hidden worlds in the habitable zone around nearby stars and their observational signatures. in the future, we expect to be able to predict which new exoplanet systems are most likely to contain an earth-like planet as these systems are discovered. | finding missing earths: an integrated analysis of multi-planet systems and assessing likelihood of potentially habitable worlds |
we detail here how the wfirst coronagraph instrument (cgi) will benefit potential future flagship direct imaging missions aimed at the spectroscopic characterization of exoplanets, including small rocky planets in the habitable zone. cgi will demonstrate for the first time in spacemany of the key technologies required: pointing stabilization at the milli-arcsecond level, autonomous sub-nanometer wavefront sensing and control, broad-band coronagraphic imaging and spectroscopy, point source detection using ultra-low noise photon counting ccds, and accurate speckles subtraction. all of these technologies will be validated in a yet unexplored high contrast / low source flux regime, at levels commensurate with the needs of future flagship direct imaging missions aimed at the characterization of exoplanetary systems, including imaging of exozodiacal dust and spectroscopy of exoplanets in the habitable zone. | the wfirst coronagraph instrument: a major stepping stone in the preparation of future exoplanet direct imaging missions |
a major bottleneck for the exploitation of data from the kepler mission for stellar astrophysics and exoplanet research has been the lack of precise radii and evolutionary states for most of the observed stars. we present revised radii of 180,000 kepler stars derived by combining parallaxes from the gaia data release 2 with the dr25 kepler stellar properties catalog. the median radius precision is ≍8%, a typical improvement by a factor of 4-5 over previous estimates for typical kepler stars. we find that ≍67% of all kepler targets are main-sequence stars, ≍21% are subgiants, and ≍12% are red giants, demonstrating that subgiant contamination is less severe than some previous estimates and that kepler targets are mostly main-sequence stars. using the revised stellar radii, we recalculate the radii for > 4000 exoplanets. we confirm the presence of a gap in the radius distribution of small, close-in planets, and its location may be at a slightly larger radius when compared to previous results. furthermore, we find several confirmed exoplanets occupying a previously described “hot super-earth desert” at high irradiance, show the relation between a gas-giant planet’s radius and its incident flux, and establish a bona fide sample of 38 planets smaller than two earth radii in circumstellar habitable zones. the results presented here demonstrate the potential for transformative characterization of stellar and exoplanet populations using gaia data. | precise properties of kepler stars and planets in the gaia era |
stellar variability on short timescales has been largely unexplored until recently, but is relevant in the era of transient surveys like the zwicky transient facility (zft), the large synoptic survey telescope (lsst), and in searches for optical counterparts to gravitational wave events detected by the laser interferometer gravitational-wave observatory (ligo). the decam minute cadence survey is the first high cadence survey of its kind, observing a total of 9 square degrees of the sky at a cadence of ~90 sec over 8 consecutive half-nights per 3 square degree field. we present minute cadence photometry of ~100,000 point sources. we use these data to search for eclipse-like events consistent with a planet in the habitable zone of a white dwarf and other sources of variability. we do not find any significant evidence for minute-long transits around our targets, hence we rule out planetary transits around the ~1000 white dwarfs that should be present in this field. our results allow us to place stringent constraints on the frequency of planets orbiting white dwarfs in the habitable zone. additionally, we identify ~150 variable sources, the majority of which are previously unknown systems. these include 77 detached or contact stellar binaries, two eclipsing white dwarf + m dwarf binaries, 27 delta scuti, 13 rr lyrae, six zz ceti, and one subdwarf a or low-mass white dwarf pulsator candidate. upcoming surveys like superwasp, the next-generation transit survey, the transiting exoplanet survey satellite, and ztf will obtain high-cadence observations for millions of stars, allowing us to probe the variability of the night sky on timescales that remain largely unstudied. | the decam minute cadence survey: a search for habitable planets around white dwarfs |
many terrestrial planets in the habitable zones around m-dwarf stars may be water-poor as a result of their host stars' prolonged pre-main-sequence phase. recent work revealed multiple moist climate equilibrium states on such arid and synchronously rotating planets with limited surface water reservoirs that can either be converged around the substellar region as an "oasis" or stably trapped on the nightside surface as ice. the former state resembles titan's polar methane lakes in the summer hemisphere surrounded by dry deserts; the latter one resembles present mars with polar co2 icecaps in the winter hemisphere. fully explaining the emergence of multiple moist climate systems requires a detailed understanding of the water vapor distribution in the atmosphere. here we diagnose the humidity distribution in idealized general circulation model (gcm) simulations of arid rocky exoplanets and use the "tracer of last saturation" technique to study how atmospheric dynamics and thermodynamics shape the humidity distribution. we show that the multiple moist climate states arise from the cold-trapping competition between the substellar upper atmosphere and cold surface regions. we further find that fast synchronously rotating planets tend to trap surface water on the nightside because of their weak atmospheric and strong surface cold traps compared to the slow rotating ones. these results elucidate the nature of the water cycle on arid rocky exoplanets and will aid the interpretation of atmospheric observations in the future. | multiple moist climate equilibrium states on arid rocky m-dwarf planets: a last-saturation tracer analysis |
i performed the evryflare survey using observations from the evryscope array of small telescopes and the transiting exoplanet survey satellite (tess) to answer two questions: (1) how frequently are superflares emitted from the nearby cool stars, both in the present and in the first 200 myr after formation? (2) what impact does superflare uv emission have on planetary atmospheres and surface habitability of planets orbiting cool stars? stellar flares are stochastic events that occur when a star's magnetic field re-connects, releasing radiation across the electromagnetic spectrum. rocky planets in the habitable zones (hz) of m-dwarfs are often subjected to superflares, events of at least 1033 erg and 10-1000x the energy of the largest solar flares. frequent superflares can erode the ozone layer of an earth-like atmosphere and allow lethal amounts of uv flux to reach the surface. conversely, too few flares may result in insufficient uv radiation to power pre-biotic chemistry due to the inherent faintness of m-dwarfs in the uv. cool stars often exhibit superflares. cool stars are the most common type of star, and are known to frequently host rocky planets. as a result, they may host most of the universe's earth-size hz planets. the evryflare survey detected hundreds of superflares from hundreds of nearby cool stars, allowing me to measure the dependence of superflare rates on stellar mass, age, rotation, and starspot coverage. the evryflare survey detected the first superflare from proxima cen, the nearest host star to a rocky planet in the habitable zone. i will discuss the effects of superflares on ozone loss to planetary atmospheres, including that of proxima b. i present the largest-ever survey of simultaneous observations of dozens of m-dwarf superflares with evryscope and tess. statistical scaling laws between energy and temperature demonstrate for the first time in a large sample of m-dwarf superflares that higher-energy events are hotter events. i quantify the amount of time superflares emit at very high temperatures, and determine the uv-c flux reaching the hz of young stars reaches values up to 103 w m-2, 1000x the time-averaged xuv flux from proxima cen. | investigating exoplanet habitability and the stellar magnetism of cool stars across half the southern sky via superflares, starspots, and stellar rotation. |
a critical component of a habitable planet is its ability to stabilize its climate over long timescales. in a new study, scientists explore whether a world covered in water can keep its climate as stable as an earth-like, continental world.the carbon goes round and rounddiagram of the physical and chemical processes (top panel) and feedback loops (bottom panel) associated with the carbonatesilicate cycle. click to enlarge. [gretashum]over the span of millions of years, a planets host star might gradually dim or brighten, or the planets volcanic outgassing patterns might slowly shift. if evolution like this also caused dramatic changes in the overall climate of a planet, this would spell bad news for habitability: the planet might not be able to retain liquid water over timescales long enough for life to form and evolve.so how do you keep a climate stable against these slow shifts? one crucial factor is having a carbonatesilicate cycle. this cycle dictates how carbon is moved around a planet, sometimes burying it deep below the planets surface, sometimes releasing it out into the atmosphere.on earth, a simplified description of the carbonatesilicate cycle is:atmospheric carbon dioxide dissolves in rainwater, forming carbonic acid, which falls to the ground.over long timescales, weathering from this weak acid dissolves silicate rocks, and the dissolved products are carried to the oceans, where they accumulate.subduction of the seafloor carries the products to great depths, where they reform into silicates and gaseous carbon dioxide.the carbon dioxide is restored to the atmosphere by volcanism.when negative feedback is a good thingin an ideal scenario, the carbonate-silicate cycle acts as a planets thermostat, with negative feedback loops keeping the temperature of the planet in balance. if oceans freeze over, silicate weathering slows, causing atmospheric carbon dioxide to accumulate and warm the planet via the greenhouse effect. if the planet heats, rainfall increases and silicate weathering speeds up, removing carbon from the atmosphere and cooling the planet.illustration of a water world around a cool, dim star. the ability of a planet to stabilize its temperature as its host star evolves is important to habitability. [m. kornmesser/hubble/esa]this cycle only stabilizes the climate against very slow external changes, like a stars gradual dimming so this isnt the solution to our current global warming crisis caused by fossil fuel emissions. nonetheless, its an important component when considering the general habitability of other worlds.in a new study, scientists benjamin hayworth and bradford foley (pennsylvania state university) consider how this cycle might be affected by the geography of a planet. will worlds covered in water do a better or worse job of keeping their climates stable?stability from the seaclimate buffering capacity of planets with varied ocean coverage, for changes in stellar luminosity. colored curves correspond to different fractional ocean coverage. lower values of dt/dl, the change in surface temperature with luminosity, mean the planet is better at stabilizing its climate. [adapted from hayworth foley 2020]hayworth and foley point out that both continental land and seafloors experience silicate weathering and participate in the carbonatesilicate cycle of a planet. the weathering rates for continental land and the seafloor, however, depend differently on the planets surface temperature and the partial pressure of carbon dioxide.by accounting for these different dependencies in climate and weathering models, the authors show that water worlds which are dominated by seafloor weathering are actually better than their continental counterparts at stabilizing planet-wide temperatures against gradual changes in host star luminosity.this means that temperate climates can exist over a wider range of stellar luminosities for water worlds than for continental planets, and they can stay stable for longer indicating that these worlds may be worthwhile targets in the search for life.citationwaterworlds may have better climate buffering capacities than their continental counterparts, benjamin p. c. hayworth and bradford j. foley 2020 apjl 902 l10. doi:10.3847/2041-8213/abb882 | climate stabilization on distant worlds |
the potential solar tides in an ancient venusian ocean is simulated using a dedicated numerical tidal model. a series of simulations with ocean depths varying between 500-4500m and rotational periods ranging from -243 to 64 earth days were used to calculate tidal dissipation rates and the resulting tidal torque. the results show that tidal dissipation rates on venus could have varied over 3 orders of magnitude depending on rotational period and depth, with the most energetic simulations dissipating nearly as much tidal energy as the solar tide does in earth's oceans today. this occurs at all depth configurations when the rotation period is close to one earth day. the associated tidal torque is large and of the same order of magnitude as today's total tidal torque on earth and an order of magnitude below the atmospheric torque reported for present-day venus. consequently, an ocean tide on ancient venus, albeit probably short-lived in geological terms, could have had significant effects on the rotational history of the planet if its rotation rate was faster than today. these calculations have important implications for the rotational periods of exoplanetary worlds and the location of the inner edge of the liquid water habitable zone. | tides on ancient venus and applications to similar exoplanets |
plato (planetary transits and oscillations of stars) is the m3 mission in the esa's cosmic vision 2015-2025 programme. it aims at finding a large number of exoplanets, at characterizing their bulk density with emphasis on the properties of terrestrial planets in the habitable zone around solar-like stars, and at studying exoplanetary systems evolution. in this talk we will address the way the payload, the preparatory science, and the ground segment are implemented and how the community can be involved in the plato preparatory and follow-up activities. | plato: the instrument and the science preparation |
m dwarfs are the most feasible targets for finding planets orbiting in the habitable zone (hz). however, their intense magnetic activity makes it very challenging for these planets to sustain an atmosphere. using mhd, we model the coronae of planet hosting stars and the interaction of the stellar wind with the planets' atmosphere in order to asses their "habitability". these space weather conditions could result in strong atmospheric stripping and evaporation and should be taken into account for any realistic assessment of habitability.in this talk i will discuss these conditions for the exciting newly discovered temperate terrestrial planets around proxima centauri and trappist-1, and will highlight a new and quite extreme magnetospheric configuration that most of the trappist-1 planets are likely to experience. | space weather characterization of exoplanets |
the interest in the possibility of life on venus is driven not just by curiosity about life originating in another earth-like environment, but because of the possibility that life may be playing a critical role in the planet"s present, and possibly its past, atmospheric state. the brilliance of venus in the night sky (as viewed from earth) is due to its highly reflective cloud cover, about 28 km thick at the equator. its spectral albedo is about 90% at wavelengths > 500 nm, but it drops gradually to about 40% around 370 nm before rising slightly at shorter wavelengths. this albedo drop is due to the presence of several absorbers in the atmosphere and the cloud cover. a very large fraction of the energy absorbed by venus is at ultraviolet wavelengths with sulfur dioxide above the clouds contributing to the absorption below 330 nm; however, the identities of the other absorbers remain unknown. the inability to identify the absorbers that are responsible for determining the radiative energy balance of venus over the last century is a major impediment to understanding how the planet "works", a major component of nasa"s efforts in planetary exploration. limaye et al. (astrobiology 18, 1181-1198, 2018) presented a hypothesis suggesting that cloud-based microbial life could be contributors to the spectral signatures of venus" clouds, building upon previous suggestions of the possibility of life in the clouds of venus.four interconnected themes for the exploration of venus as an astrobiology target are: - (i) investigations focused on the likelihood that liquid water existed on the surface in the past leading to the potential for the origin and evolution of life, (ii) investigations into the potential for habitable zones within venus" clouds and venus-like atmospheres, (iii) theoretical investigations into how active aerobiology may impact the radiative energy balance of venus" clouds and venus-like atmospheres, and (iv) application of these investigative themes towards better understanding the atmospheric dynamics and habitability of exoplanets. these themes can serve as a basis for proposed venus astrobiology objectives and suggestions for measurements for future missions, as per the goals and objectives developed by the venus exploration analysis group (vexag), which is sponsored by nasa to plan for the future exploration of venus. a venus collection to be published in astrobiology journal in 2021 will include papers from the "habitability of the venus cloud layer", moscow (october 2019) workshop. | venus, an astrobiology target |
the sun is currently moving through a rich and complex suite of partially ionized, warm, interstellar clouds. an interface, dictated by pressure balance, magnetic fields, and charge exchange, signifies the interaction between a outward moving stellar wind and the inward force of the surrounding local interstellar medium (lism). as stars and the ism each comprise roughly half of the luminous matter in galaxies, these interactions are ubiquitous. the interaction is also dynamic. while stellar wind strengths change slowly, at least during the main sequence, the density of interstellar clouds ranges by more than six orders of magnitude. the implication is that the solar heliosphere and stellar astrospheres are permanent features of planetary systems and are constantly changing. the stellar magnetic field and particle interactions can lead to a filtering of low and high-energy (i.e., cosmic rays) particles. the particles that make it through the filter can be deposited in the atmospheres and on the surfaces of planets in the system. therefore, the galactic interstellar environment, and the resulting interface with the star, can potentially have an influence on the habitability of a planet. this is a refined and planetary system specific version of the galactic habitable zone paradigm. i will discuss work to measure and model the morphology and physical properties of the lism. these clouds reside in our immediate cosmic neighborhood, within 20 pc. this same volume contains several detected astrospheres and the nearest and most favorable exoplanetary systems with planets in habitable zone orbits. i will also discuss the the influence the lism has on observations to characterize the atmospheres of exoplanets. finally, i will present results of a project to observe the interstellar environment that the sun traversed in its most recent past (e.g., within the last 5 myr) and evaluate the corresponding impact on our historical heliosphere. while perhaps not a primary driver of habitability in general, it is worth exploring the impact of these planetary system-ism interactions, and identifying extreme systems where it may have a significant influence on habitability. | our inhabited heliosphere: the implications of stellar motion through galactic interstellar clouds on planetary atmospheres |
exoplanet orbital eccentricities encode key information about planetary system formation and evolution. for m dwarf planets in particular, the proximity of the habitable zone to the host star makes eccentricity additionally important for understanding habitability. however, constraints on orbital eccentricity typically require long and resource-intensive radial velocity campaigns. we summarize an investigation to extract eccentricity measurements of 143 planets orbiting early- to mid-m dwarfs across 81 systems, employing only their light curves from kepler and host star density constraints from spectroscopy and gaia. the "photoeccentric effect," described in exoplanet literature, enables the measurement of orbital eccentricity from a careful combination of transit duration and kepler's 3rd law: both are linked to the stellar density. a disagreement between the stellar density inferred from the measured transit duration, and the density from kepler's 3rd law, points to a non-zero orbital eccentricity. employing our machinery in a standardized way on each of the 143 exoplanets, we derive an eccentricity posterior distribution for each. through a hierarchical bayesian analysis, we apply these individual posteriors to derive a population-level eccentricity distribution for planets around early- to mid-m dwarfs. comparisons between the eccentricity distribution for singly- and multiply-transiting systems, as well as the distribution between m dwarfs and fgk dwarfs, help illuminate the dynamical states of m dwarf planets at a population level. | the orbital eccentricities of the kepler m dwarf planets: a population-level view of planet dynamics around small stars |
there have been discovered many exoplanets and the number of terrestrial exoplanet detection increases rapidly in recent ten years. many terrestrial exoplanets or super-earth are found around low mass stars such as m dwarfs. a red dwarf (m type star) has comparatively narrow habitable zone, which is very close to the host star, and exoplanets are considered to be exposed to extreme levels of x-ray and ultraviolet (uv) radiation. classic equilibrium tide theories predicts that k or m-type stars induce strong tidal effects on potentially habitable exoplanets, and tidal locking is possible for most planets in the habitable zones of k and m dwarf stars [e.g., barnes, 2017]. when a planet has dipole magnetic field and rapid rotation, superposition of the stellar wind induced and corotation electric fields results in the tear-drop-shaped region of the closed drift, where planetary ionized atmosphere can fill the magnetic flux tubes along the field lines. the region is characterized with cold dense planetary plasma and called as the plasmasphere. in this study, a simple estimation method of the size of terrestrial exoplanetary plasmasphere is shown based on the knowledge of the solar system planets. we considered the role of rapid rotation of the atmosphere (superrotation) in the formation of the plasmasphere of tidally-locked exoplanets. many gcms of exoplanets show that the circulations of typical tidally locked terrestrial exoplanets can become superrotation [e.g., showman+, 2013]. however, the horizontal circulation in the thermosphere is far from understood [e.g., machado+, 2017]. the results indicate that earth-like magnetized exoplanet can have a plasmasphere with a size of 4-6 times of the planetary radius. the size of the plasmasphere depends on the superrotation speed of the thermosphere, ionospheric conductance, stellar wind dynamic pressure, and imf cone angle. if the exoplanet has a co2-rich atmosphere, the results suggest the fuvabsorption of plasmaspheric c+ ions might be observable by space telescopes. since the plasmasphere formation requires the existence of both the thick atmosphere and global intrinsic magnetic field, the observation of plasmasphere can provide possible evidence and clues of the exoplanetary atmosphere and intrinsic magnetic field. | plasmasphere formation around terrestrial exoplanets: possible evidence of the exoplanetary intrinsic magnetic field and atmosphere |
silicate weathering is a key component of the carbonate-silicate cycle (carbon cycle) that draws down co2 from the atmosphere for eventual burial and long-term storage in the planetary interior. this process is thought to provide an essential negative feedback to the carbon cycle to maintain temperate climates on earth and earth-like planets. we model chemical kinetics and thermodynamics to determine weathering rates for three types of rocks based on earth's continental and oceanic crust, and its upper mantle. we find that thermodynamic weathering rates of a continental crust-like lithology are about one to two orders of magnitude lower than those of a lithology characteristic of the oceanic crust. our results show that the weathering of mineral assemblages in a given rock, rather than individual minerals, is crucial to determine weathering rates at planetary surfaces. we show that when the co2 partial pressure decreases or surface temperature increases, thermodynamics rather than kinetics exerts a strong control on weathering. the results imply that the temperature-sensitivity of thermodynamically-limited silicate weathering may instigate a positive feedback to the carbon cycle, in which the weathering rate decreases as the surface temperature increases. our radiative transfer model of the atmosphere shows that silicate weathering has profound implications on the extent of the habitable zone. | the impact of silicate weathering on exoplanet atmospheres and the habitable zone |
the james webb space telescope (jwst) will offer the first opportunity to characterize terrestrial exoplanets transiting nearby m dwarf stars with sufficient precision to identify high mean molecular weight atmospheres. trappist-1's seven known transiting earth-sized exoplanets are particularly amenable to atmospheric characterization due to the star's small radius and cool effective temperature, both of which increase the detectability of atmospheric spectral features. to help the community prepare for jwst observations, we use self-consistent climate and photochemical models of plausible post-ocean-loss and habitable environments for the trappist-1 exoplanets to directly compare transmission and emission spectroscopy with various jwst instruments and observing modes. we find that transmission spectroscopy with nirspec prism could lead to a modest detection of atmospheric spectral features (predominantly due to co2) in fewer than 10 transits for all seven trappist-1 planets, if they lack high altitude aerosols. if the trappist-1 planets possess self-consistent venus-like h2so4 aerosols, atmospheres may require 3-4 times the number of transits to detect, thus mirroring the current high-altitude aerosol paradigm seen for hot jupiters and super-earths. water may be prohibitively difficult to detect in both venus-like and habitable atmospheres due to its presence lower in the atmosphere where transmission spectra are insensitive. although the presence of biogenic o2 will likely be extremely challenging to detect for the trappist-1 habitable zone planets, abiotically produced o2 may be detectable for all seven trappist-1 planets via large o2-o2 collisionally-induced absorption features at 1.06 and 1.27 µm, which would implicate a post-runaway environment. our results constitute a suite of hypotheses on the nature and detectability of highly-evolved terrestrial exoplanet atmospheres that may be readily tested with jwst. | simulating the detectability and characterization of the trappist-1 exoplanet atmospheres with jwst |
since the early 1990s, it has been known that early mars river-forming climates require slightly more greenhouse forcing than can be explained by the greenhouse effect of co2 and h2o vapor. this mis-match is a challenge to our understanding of the circumstellar habitable zone. i will present new evidence that the mis-match between mars geologic data and models is more severe than has previously been recognized, report new 3d simulations of a long-term warming mechanism that can match the mars data, and review lessons from early mars that can help us interpret data from habitable-zone exoplanets. to get a river-forming climate, models so far have assumed near-optimal pco2 (1-2 bar) plus an extra greenhouse gas (e.g., h2). however, our new analysis of the distribution of ~3 ga fluvial landforms indicates that they formed at low average atmospheric pressure and therefore at low average pco2. this result raises the likelihood of false negatives in the search for habitable exoplanets, and weakens (although it does not remove) the correlation between pco2 and surface habitability. we do not know what supplied the necessary extra non-co2 warming. an attractive candidate is warming by high-altitude water ice clouds. we present new marswrf simulations of this warming mechanism. finally, i will review work by our collaboration and by others on trends, rhythms and aberrations in early mars climate, with a focus on the implications for interpreting data from habitable zone exoplanets. for example, there is strong evidence that early mars river-forming climates switched on and off on timescales (103-107 yr) that could fool telescopic observers into thinking that a habitable planet was uninhabitable, or vice versa. | lessons from early mars for models of surface habitability on exoplanets |
rocky extrasolar planets orbiting m dwarfs are prime targets in the search for habitable surface conditions and biosignatures with near-future telescopes like the james webb space telescope (jwst) and the extremely large telescope (elt). even for the closest known targets the capabilities to characterize earth-like or co2-dominated atmospheres with jwst or elt might still be limited to a few molecules such as co2 or ch4. hence it would be difficult to draw conclusions on the surface conditions and potential habitability of these planets. in clear h2-he atmospheres the molecular features in transmission spectra could be much larger and hence potential biosignatures might be detectable.in this study, we investigate the detectability of the potential biosignatures nh3, ph3, ch3cl, and n2o, assuming different h2-he atmospheres for the habitable zone super-earth lhs 1140 b. recent observations of the atmosphere of lhs 1140 b suggest that the planet might hold a clear h2-dominated atmosphere and might show an absorption feature around 1.4 µm due to h2o or ch4 absorption. here we use the coupled convective-climate-photochemistry model 1d-terra to simulate h2 atmospheres of lhs 1140 b with different amounts of ch4 and assuming that the planet has an ocean and a biosphere.the destruction of the potential biosignatures nh3, ph3, ch3cl, and n2o shows a weak dependence on the concentrations of ch4. for weak abundances of ch4 only 5 to 10 transits are required to detect these molecules with jwst or elt. however, for ch4 surface mixing ratios of a few percent only nh3 and n2o might be detectable with less than 10 transits. a scenario with large abundances of ch4 is consistent with the spectral feature at 1.4 µm and such an atmosphere might allow habitable surface temperatures. if this spectral feature at 1.4 µm originates from h2o absorption, the planet is likely not habitable at the surface. | detectability of biosignatures on lhs 1140 b |
the next phase of exoplanet science will focus on characterizing exoplanet atmospheres, including those of low-mass, terrestrial planets. a comprehensive understanding of possible biosignatures that may be detected with the next generation of ground and space telescopes is warranted. while some biosignature gases, such as oxygen and phosphine, have recently been reviewed in depth (meadows et al. 2018 and sousa-silva et al. 2020), these will likely be extremely difficult to detect with jwst. in contrast, while it has not been thoroughly reviewed, methane at earth-like biogenic fluxes is one of the only biosignatures that may be readily detectable with jwst (krissansen-totton et al. 2018a). in fact, an early earth-like, methane-rich atmosphere would be easier to detect with jwst than modern earth's oxygen-rich atmosphere (ibid). here we present our preliminary work on a comprehensive review of methane biosignatures and false positives. biogenic methane production, or methanogenesis, is a simpler metabolism than oxygenic photosynthesis, that is carried out by anaerobic microbes (i.e., those not requiring oxygen for growth). methanogens use either co2 and h2 or acetate as substrates (schwieterman et al. 2018). this process could be widespread due to the likely ubiquity of the co2+h2 redox couple in terrestrial planet atmospheres, and the antiquity of methanogenesis on earth (wolfe and fournier 2018). we briefly review the current understanding of the origin and evolution of methanogens, the organisms responsible for methanogenesis, and how this process relates to origins-of-life theories. when ch4 is invoked as a possible biosignature it is often included with a strongly oxidizing companion gas (e.g., co2 or o2/o3). this is because it is difficult to explain abundant ch4 if a terrestrial planet atmosphere's redox state is more oxidizing so that the thermodynamically stable form of carbon would not be ch4 (schwieterman et al. 2018). however, even in atmospheres devoid of oxygen, ch4 has a short photochemical lifetime on habitable zone rocky planets, and the large fluxes required to sustain high ch4 abundances are likely much greater than could be supplied by abiotic processes (e.g., magmatic outgassing, serpentinization) (krissansen-totton et al. 2018b, wogan et al. 2020). in addition, many abiotic, geological processes that produce ch4 are expected to also produce abundant co, which life readily consumes, so the presence of ch4 and co2 but absence of co strengthens the case for biogenicity (krissansen-totton et al. 2018b). although ch4+co2 (minus co) might coexist in thermodynamic equilibrium on planets without large surface oceans (woitke et al. 2020), in practice, such atmospheres would be photochemically unstable and, in particular, the ch4 would have a short lifetime (less than ~1 myrs). in addition to briefly discussing methane on mars and titan, we review the presence of methane in jovian and sub-neptune planet atmospheres. in many giant planets, methane is the most abundant carbon-bearing gas and can be replenished indefinitely because, although methane is photodissociated in the upper atmosphere, hydrogen is never depleted and carbon and hydrogen can recombine deeper in the atmosphere where temperatures and pressures are high enough for methane production to be thermodynamically favorable and kinetically viable (moses et al. 2013). on the other hand, terrestrial planets with high mean molecular weight atmospheres do not have deep enough atmospheres to replenish methane without an additional source (abiotic or biotic). in terrestrial atmospheres without a replenishment source, methane is photodissociated and hydrogen is lost to space on short timescales (~10s of thousands of years for ~1 bar atmospheres). for planets in the sub-neptune regime, we seek to determine how much atmosphere is necessary for a planet to sustain methane via thermodynamic recombination against photodissociation. in summary, for terrestrial planets to have methane-rich atmospheres, the methane must be constantly replenished. we explore to what extent abiotic ch4 replenishment is possible based on prior work on abiotic methane sources including water-rock reactions, volcanic outgassing, and impacts (e.g., etiope & lollar 2013, wogan et al. 2020, kress & mckay 2003). we review methane false positives on terrestrial planets and determine if they are likely to produce methane fluxes as large as those caused by known biogenic sources. through this comprehensive review, we will develop a framework for identifying methane biosignatures and discuss detectability prospects with jwst. | methane: the ideal biosignature for the jwst era? |
on terrestrial planets, the abundances of key trace gases such as methane ch4 are controlled by photochemistry and source fluxes such as the rate of volcanic outgassing, water-rock reactions, or biological production. the interpretation of ch4 as a biosignature is thus ultimately dependent on the production flux inferred from its abundance and the likelihood that this flux could be produced by geological sources alone. prior work has shown that the buildup of ch4 in the atmosphere at a given flux is highly favored for planets with oxygen-rich atmospheres orbiting k and m dwarfs, relative to sun-like stars. however, relatively limited attention has been given to anoxic, archean-like atmospheres and their flux-abundance relationships. we use a photochemical model to predict the atmospheric ch4 mixing ratio as a function of its production rate for anoxic planets in the habitable zones of fgkm stars. we then compare the fluxes to those produced by primitive bacterial biospheres and geologic processes to evaluate what levels of ch4 would suggest biological activity. we find that the flux-abundance relationships, photochemical destruction pathways, and ultimate detectability of ch4 in anoxic exoplanets are highly dependent on the host star spectrum (figure 1). for example, at low abundances ch4 destruction by the oh radical dominates for anoxic planets orbiting all stellar types but is much less efficient for anoxic planets orbiting m dwarfs, which may challenge biosignature interpretations based on co2-ch4 disequilibrium. in contrast, at high methane fluxes/abundances, direct photolysis of ch4 is the dominant loss channel for mgk stars and at sufficiently high fluxes their flux-abundance relationship converges. the surface flux at which this convergence occurs is dependent on the distribution of nuv and fuv photons. we find that a biosphere similarly productive to that of archean-earth would likely produce similar methane mixing ratios on habitable anoxic planets with mgk stellar hosts (in contrast to oxygen-rich cases where the mixing ratios can be enhanced by a factor of ~1,000 when comparing an m dwarf host to a g dwarf host). for f dwarf stars, the dominant loss channel for methane is always oh, which would limit methane accumulation but also strongly reduces the potential for biosignature false positives. this research was supported by exobiology grant 18-exo18-0005. | evaluating methane as a biosignature on habitable anoxic planets orbiting fgkm stars |
exoplanetary systems are seen outside the solar system which may be similar or may vary drastically with the solar system. a comparative study between seventeen selected exoplanetary systems with solar system is carried out taking some physical parameters as probes with the hope that a clue to the habitable system outside the solar system can be found. selected exoplanets are revolving around a sun like star and have more than one confirmed planetary companions and all these are detected using transit method. the details about the exoplanets and the host stars are collected from the exoplanetary encyclopedia available online. one of the inferences got from the study is that most of the studied exoplanets are nearer to its host stars unlike the solar system. ninety eight percent of the selected confirmed exoplanets are situated out of the habitable zone boundary of the host star and are at a high inclination with respect to their host stars. the characteristics of the selected exoplanetary systems show similarities as well as differences from the solar system planets. a modified and more general bode-titius empirical relation is also suggested, for predicting the position of hitherto undetected exoplanets. obviously there is a broad array of enhancements which can be pursued following on this line of study. | a comparative study between solar system and exoplanetary systems and generalisation of bode-titius empirical formula |
catalogue of 767 tidally locked rocky exoplanets (rp<=1.23r{earth} with a subset of 14 planets inhabiting the circumstellar habitable zone (chz) of their host stars. for each exoplanet in our sample, parameters for planetary mass, planetary radius, semi-major axis, eccentricity, stellar mass, and stellar age are given. we execute 10000 monte carlo simulations that include uncertainties on all parameters to determine the median and 68% confidence intervals on tidal stress and tidal heating rates. (1 data file). | vizier online data catalog: catalogue of tidally locked rocky exoplanets (mcintyre, 2022) |
in this communication we present the first experimental results obtained on the crossed-cubes nuller (ccn), that is a new type of achromatic phase shifter (aps) based on a pair of crossed beamsplitter cubes. we review the general principle of the ccn, now restricted to two interferometric outputs for achieving better performance, and describe the experimental apparatus developed in our laboratory. it is cheap, compact, and easy to align. the results demonstrate a high extinction rate in monochromatic light and confirm that the device is insensitive to its polarization state. finally, the first lessons from the experiment are summarized and discussed in view of future space missions searching for extrasolar planets located in the habitable zone, either based on a coronagraphic telescope or a sparse-aperture nulling interferometer. | experimental demonstration of a crossed cubes nuller for coronagraphy and interferometry |
this activity has been developed as a resource for the "eu space awareness" educational programme. as part of the suite "our fragile planet" together with the "climate box" it addresses aspects of weather phenomena, the earth's climate and climate change as well as earth observation efforts like in the european "copernicus" programme. this activity uses a simple analogue for the power of radiation received at a given distance from a star. a photovoltaic cell is connected to an electric motor. depending on the power received on the cell, the motor begins to move. it changes also its speed with respect to the distance between the cell and the lamp. this can be interpreted as a model for a planetary system and its habitable zone. the "engine of life" moves as soon as the receiving power is big enough to sustain its operation. the distance, where the motor stops, can be interpreted as the outer edge of the habitable zone. as a second activity, the students will reconstruct the orbits of a real exoplanetary system by drawing a scaled model. in addition, they will calculate and superimpose a realistic circumstellar habitable zone and discuss its elements. | the engine of life |
one of the most compelling questions for exoplanets located within the potential habitable zone of their host star is whether they are able to retain an atmosphere. the primary approach that has been taken to answer this question is to determine how much of the atmosphere, with a particular focus on water, can be lost during energy limited escape. this approach has been a critical first step in determining the maximum loss rates of exoplanet atmospheres. we have conducted a pilot study that builds on this work by mapping out pathways for atmospheric evolution of trappist-1e. we have done this by adding the influence of non-hydrodynamic thermal escape, and potential contributions by volcanic outgassing and impacts, and photochemistry. through this study we have determined upper and lower limits for the abundance of important volatiles as a function of time. | atmospheric loss and photochemistry of exoplanets around m dwarfs |
the census of planets orbiting the center of mass of binary stars is rapidly growing. the question of stability for circumbinary planets has been the focus of much recent research. we review this work and present results of new simulations, from which we find criteria for the long term stability of exoplanets orbiting binaries. we are especially concerned with the potential stability of planets in habitable zones surrounding binaries. for this purpose, we merge a long-term orbital stability study with an analysis of the rotational evolution of the stellar components. the stellar evolution and rotational study enables estimates of stellar activity, and the effects on the magnetospheres and atmospheres of planets over the course of history for a potentially habitable circumbinary planet. we find that the long-term orbital stability of circumbinary habitable zone depends sensitively on the initial orbit of the binary and on the masses of the stars. we find that stellar twins (binary mass ratio ~ 1) and binaries with circular orbits provide the most stable solutions. however, if the binary orbit evolves too rapidly, planets may be lost due to changes in resonance locations. a subset of binaries are identified possessing both stable orbital solutions for multiple planets in the habitable zone and reduced stellar aggression due to tidal torqueing of the stellar rotation. | an investigation of circumbinary planet orbital stability and habitability to identify potential planetary systems with several habitable planets |
oxygen is widely considered to be a reliable biosignature in the search for life elsewhere [1]. however, several mechanisms create abiotic oxygen, or oxygen from non-biogenic sources. these mechanisms must be thoroughly characterized to rule out false positive identifications of life in the future. previous experiments from 1-d planetary evolution models have predicted significant abiotic oxygen accumulation resulting from a variety of initial volatile inventories, but the production of abiotic oxygen from atmospheric water loss after the magma-ocean phase did not consider the effect of planetary obliquity [2]. 3-d general circulation models (gcms) have shown that stratospheric humidity increases on planets with higher degrees of obliquity through enhanced seasonality, which should result in significant levels of atmospheric water loss [3]. however, the minimum obliquity necessary to trigger significant stratospheric water vapor accumulation and subsequent water loss has not been studied. we address this unknown obliquity parameter by using the exoplasim gcm to quantify stratospheric humidity for a range of obliquity, eccentricity, insolation, and co2 levels. we then develop a parameterization for the 3-d stratospheric humidity concentration that can be used in long-term 1-d planetary evolution models to study the effect of higher degrees of obliquity on atmospheric water loss. the exoplasim gcm results support those seen from previous studies, with the stratospheric humidity increasing with increasing obliquity values [3]. our results also reveal a marked regime change in the global humidity distribution after the planetary obliquity value passes the critical value of 54o. finally, we will discuss the consequences of these results in the context of planetary habitability and exoplanet life detection. [1] meadows, v. s. (2017), astrobiology, 1022–1052. [2] krissansen‑totton, j., fortney, j. j., nimmo, f., & wogan, n. (2021), agu advances, 2(2). [3] kang, w. (2019), the astrophysical journal, 877(1), l6. | the effect of obliquity on abiotic oxygen production in habitable zone planet atmospheres |
the recent detection of earth-sized planets in the habitable zone of proxima centauri, trappist-1, and many other nearby m-type stars (which consist some 75% of the stars) has led to speculations, whether liquid water and life actually exist on these planets. defining the bio-habitable zone, where liquid water and complex organic molecules can survive on at least part of the planetary surface, we suggest that planets orbiting m-type stars may have life-supporting conditions for a wide range of atmospheric properties (wandel2018). we extend this analysis to synchronously orbiting planets of k- and g-type stars and discuss the implications for the evolution and sustaining of life on planets of m- to g-type stars, in analogy to earth. | bio-habitability and life on planets of m-g-type stars |
barnard's star (gj 699) is a dim old red dwarf m4 v star. at 6 ly it is the 2nd nearest star system. until recently barnard star's claim to stardom is having the largest proper motion (mu = 10.4"/yr). adding to its fame, ribas et al. (2018 nature 563, 365) recently found that barnard's star hosts a super-earth exoplanet (mp sini = 3.2 me). barnard b has an orbital period of 233-d and semi-major axis of a = 0.40 au- i.e. at the same distance of mercury from the sun. however barnard's star is very dim (lbol = 0.0033lo) thus barnard b with an irradiance (relative to the earth) of s/se ~0.020 and thus it is cold (-170 c). so that there is little chance of liquid water and life on its frigid surface. barnard's star is a founding member of the villanova living with a red dwarf program (engle & guinan 2011, aspcs 451) from photometry started in 2003 we determined a rotation period of prot = 142+/-8 days. utilizing our period-age relation for red dwarfs (engle & guinan 2018 rnaas 2, 34) indicates an age of 8.5+/-0.9 ga. this gyro-chronological age agrees well with other age indicators that include large uvw space motions and low chromospheric ca ii hk and coronal x-ray emissions. from the available x-ray and uv data, we compute the x-ray and uv irradiances of the planet. all hope for life on barnard b may not be lost. as a super-earth, if not too massive, barnard b could have a large hot (iron?) core and possibly enhanced geothermal activity. geothermal heating could support "life zones" under its surface. we note that the surface temperature on jupiter's icy moon europa is similar to barnard b but because of tidal heating europa probably has liquid oceans under its icy surface. however, if the actual mass of the barnard b is much higher than ~5 me, its higher gravity could result in a thick h-he atmosphere and thus be a dwarf gas giant / mini-neptune. the angular separation of the barnard b from its star is ~220 mas. although very faint, it may be possible for the planet to be imaged by future very large telescopes. such observations will shed light on the nature of the planet's atmosphere/surface and potential habitability. this research is supported by grants from nasa that we gratefully acknowledge. | x-ray, uv, optical irradiances and age of barnard\'s star\'s new super-earth planet:—"can life find a way" on such cold planet? |
we describe a physically- and statistically-based method to infer the near-sun magnetic field of coronal mass ejections (cmes) and then extrapolate it to the inner heliosphere and beyond. besides a ballpark agreement with in-situ observations of interplanetary cmes (icmes) at l1, we use our estimates to show that earth does not seem to be at risk of an extinction-level atmospheric erosion or stripping by the magnetic pressure of extreme solar eruptions, even way above a carrington-type event. this does not seem to be the case with exoplanets, however, at least those orbiting in the classically defined habitability zones of magnetically active dwarf stars at orbital radii of a small fraction of 1 au. we show that the combination of stellar icmes and the tidally locking zone of mother stars, that quite likely does not allow these exoplanets to attain earth-like magnetic fields to shield themselves, probably render the existence of a proper atmosphere in them untenable. we propose, therefore, a critical revision of habitability criteria in these cases that would limit the number of target exoplanets considered as potential biosphere hosts. | a new spin to exoplanet habitability criteria |
the discovery of an earth-size (~1.3 me) planet, proxima b, orbiting in the habitable zone (hz) of the nearest star (d = 4.25 ly), has provided great impetus for the study of the potential habitability of other nearby hz planets. ribas et al. (2016, a&a in press) have shown, that in spite of the relatively high levels of magnetic-dynamo generated x-ray & uv radiation from its m5.5 v host star that the planet endures, there are pathways for the proxima b to possess an atmosphere, water and climate conditions to be potentially habitable. at a distance of 13 ly, the old (11.5 gyr) pop ii m1.5 star, kapteyn star, has been found to hosts two large earth mass planets, one of which - kapteyn b (m= 4.8 me; a = 0.17 au) is located near the mid-hz of host star (see englada-escude’ et al. 2014). unlike proxima b, the kapteyn b planet receives significantly less high energy radiation from its host star due the star’s lower magnetic activity and the planet’s greater distance from its host star (see guinan et al. 2016). recently three large earth size planets have been found orbiting the nearby (14 ly) solar-age m3 v star - wolf 1061 (wright et al. 2016). one of these planets, wolf 1061 c (m = 4.6 me; a = 0.084 au) is located in the star’s hz. as in the case of kapteyn b, wolf 1061 appears to receive less high energy radiation than proxima b. here we provide preliminary assessments of the effects of the host star’s high energy x-ray and uv photo-ionization radiation on the atmospheres and water inventories of the hosted planets. we compare the suitability of these three nearest planets for potential habitability and suitability for life.this research is supported by grants from nsf (rui) and nasa. | beyond proxima b: investigating the next nearest potentially habitable exoplanets: kapteyn b (13 ly) and wolf 1061 c (14 ly) - assessing their suitabilty for life |
the nasa kepler mission has discovered thousands of new planetary candidates, many of which have been confirmed through follow-up observations. a primary goal of the mission is to determine the occurrence rate of terrestrial-size planets within the habitable zone (hz) of their host stars. a major product of the habitable zone working group (hzwg) is a list of hz exoplanet candidates from the kepler data release 24 q1- q17 data vetting process [1]. we used a variety of criteria regarding hz boundaries and planetary sizes to produce complete lists of hz candidates, including a catalog of 104 candidates within the optimistic hz. we cross-matched our hz candidates with the data release 25 stellar properties and confirmed planet properties to provide robust stellar parameters and candidate dispositions. we also performed dynamical analysis simulations for multi-planet systems that contain candidates with radii less than two earth radii as a step toward validation of those systems. from this list we found 39 planet candidates greater than 3 earth radii residing in the optimistic habitable zone of their host star. while giant planets are not favored in the search for eta earth, they do indicate a potential for large, potentially rocky moons residing in the habitable zone. these giant planets can also provide a potential for a wider range of "habitable" incident flux due to additional energy sources from tidal energy, etc. thus we analyzed each giant planet, estimating their mass and then calculating the estimated radial velocity semi amplitudes of each planet for use in follow up observations. we then calculated the planets hill radius and determined the maximum angular separation of potential moons. this presentation will describe the highlights of the hz catalog giant planets and the plans for further validation of hz candidates and follow-up studies. fig. 1 - plots both the unconfirmed and confirmed giant (>3⊕r) kepler candidates expected radial velocity signatures vs the kepler magnitude of their host star. references: [1] kane, s.r., hill, m.l., kasting, j.f., et al. 2016, apj, 830, 1 | potential habitable zone exomoon candidates and radial velocity estimates for giant kepler hz candidates. |
nasa is currently studying concepts for flagship missions (habex and luvoir) with the goal of directly imaging a sample of earth-like planets in the habitable zones of nearby sun-like stars. a critical ingredient to these mission concept studies is the prevalence of such earth-like planets; if earth-like planets are intrinsically rare, larger and more expensive telescopes are required to conduct the survey. presently, the best constraints on the occurrence of earth-sized habitable-zone planets around sun-like stars come from the kepler mission, but the kepler planet-candidate catalog is incomplete and unreliable in this regime. as a result, published estimates of the occurrence rate of earth analogs, determined by extrapolating occurrence rates from larger and hotter planets, vary by over an order of magnitude. this level of uncertainty is an obstacle in the design of future flagship missions. we propose to apply state-of-the-art machine learning techniques to the search for exoplanets to address this problem. our objective is to make the first measurement of the occurrence rates for earth analogs that does not rely on extrapolations from larger and hotter planets. key to this objective will be our work to produce a new, more reliable catalog of kepler planet candidates with higher sensitivity to earth analogs. we will construct this catalog by performing a highly sensitive search for planet candidates, and using modern machine learning techniques to reliably select the true planet signals from the false alarms. we have recently demonstrated this strategy can indeed uncover previously missed exoplanets. increasing sensitivity to earth analogs in the kepler dataset, without sacrificing the reliability of the planet candidate catalog will enable more accurate occurrence rates. the result of our program will be the most sensitive catalog of kepler planet candidates produced to date. we will leverage this new data product to produce an accurate measurement of the occurrence of earth analogs. our progam will support the salary of a graduate student and summer salary for the pi. our proposed work is relevant to the astrophysics data analysis program because it is focused on analysis of archival data from the kepler mission. | completing kepler's census: using deep neural networks to measure the frequency of earth analogs |
the amount of light an exoplanet reflects and emits towards an observer waxes and wanes as the planet orbits through its phases. the amplitude and profile of reflection phase curves constrain the albedo of planetary surfaces and atmospheres, while the thermal amplitude and profile reveal temperature distributions and heat transport efficiencies, all providing valuable insight into the nature of exoplanet surfaces and atmospheres. in this dissertation i highlight the usefulness of utilizing full orbital phase curves in addition to occultation measurements, which provides a higher sensitivity to planetary photons at the expense of a more challenging data reduction. in the first few chapters of this dissertation, i introduce a novel non-parametric algorithm to produce clean, robust exoplanet phase curves, and apply it to separate ensembles of 115 neptunian and 50 terran exoplanets observed by the kepler satellite to measure an upper limit on the average albedo of kepler's neptunian planets, and make the first constraint on the average albedo of terran worlds. in the fourth chapter, i present the full orbital phase curve and occultation of the ultra-hot jupiter wasp-100b observed by the transiting exoplanet survey satellite (tess), and with the use of bayesian methods, present the first measurement of a phase shift of the thermal maximum among the phase curves observed by tess, the degree of which challenges the predicted efficiency of heat transport in the atmospheres of ultra-hot jupiters. in the final chapter, i present an example of how the nasa rocke-3d general circulation model can be used to explore the physical mechanisms that influence the habitability of terrestrial exoplanets, and then show how i generated phase curves from the 3-dimensional models to study the signals produced by simulated trappist-1 habitable-zone worlds. the work in this dissertation contributes valuable new information to the astronomical literature and provides avenues for further research on the nature of short-period exoplanets. | it's not just a phase: measuring the properties of short-period exoplanets from full orbital phase curves |
the recent discoveries of telluric exoplanets in the habitable zone of different stars have led to questioning the nature of their atmosphere, which is required to determine their habitability. atmospheric escape is one of the challenging problems to be solved: simply adapting what is currently observed in the solar system is doomed to fail due to the large variations in the conditions encountered around other stars. a better strategy is to review the different processes that shaped planetary atmospheres and to evaluate their importance depending upon the stellar conditions. this approach allowed us to show that processes like ion-pickup were a more important way to lose atmosphere at mars in the past. we reviewed the different escape mechanisms and their magnitude in function of the different conditions. this led us to discover discrepancies in the current literature concerning problems such as the xenon paradox or the importance of a magnetic field in protecting an atmosphere.this shows that one should be very careful before claiming the presence of an atmosphere on planets in the habitable zone of their m-dwarfs: new criteria such as the alfven surface location with respect to the planet should be taken into account a-priori.overall, the habitability of a planet should not be claimed only on by its location in the habitable zone but also after careful analysis of the interaction between its atmosphere and its parent star [gronoff et al. 2020]. gronoff, g., arras, p., baraka, s., bell, j. m., cessateur, g., cohen, o., et al. ( 2020). atmospheric escape processes and planetary atmospheric evolution. journal of geophysical research: space physics, 125, e2019ja027639. https://doi.org/10.1029/2019ja027639 | atmospheric escape processes and planetary atmospheric evolution: from misconceptions to challenges |
recent kepler and tess observations discovered many rocky exoplanets in habitable zones around active main-sequence stars. the upper atmospheres of exoplanets are subject to two important energy sources derived from their host stars. first, the stellar photon flux in the x-ray and xuv bands ionizes and heats the upper atmosphere, driving atmospheric heating, affecting the conductance, and enhancing atmospheric escape. second, the stellar wind's interaction with the exoplanet's intrinsic magnetic field transfers energy to the atmosphere through field aligned currents and poynting flux.that energy is dissipated in the high latitude cusp and auroral regions through joule heating which can inflate the atmosphere and also enhance the atmospheric escape rate. this presentation will discuss recent advances in modeling these energy inputs and their consequences for exoplanetary habitability. in particular, we present the development of a new model, the (exo) planetary ionosphere-thermosphere tool for research (planet-ittr). the model and its validation are presented as well as application of the model for two physical scenarios. first, we examine the determination of ionospheric conductance for planetary systems and present verification of the conductance calculation with widely used empirical models for modern earth. we will also model the case of elevated stellar xuv input appropriate for close-in exoplanets as well as the early venus and mars and discuss the consequences for the stellar wind magnetosphere coupling. second, we study the onset of hydrodynamic escape under conditions of enhanced stellar xuv flux. we will derive the loss time of hydrogen dominated primary atmospheres of terrestrial (exo)planets and sensitivity of the atmospheric loss time scale to various stellar inputs. | modeling atmospheric escape from magnetized rocky exoplanets with (exo) planetary ionosphere-thermosphere tool for research (exoplanet-ittr) |
habitable planets may be found on sun-like stars. in such systems, an earth analog can be sought that shares earth's albedo spectrum. we propose using an anomalous characteristic of the albedo of earth to identify earth 2.0 candidates. we present a specialized telescope adept at making the requisite observation. in the portion of solar spectrum where black body radiation has its peak intensity in green then decreasing into the shorter wavelength region toward the uv, the correlated spectral emission of earth's albedo shows a unique gain in emitted energy above 400 nm when compared to all other planets in our solar system. earth is luminescent in the near-uv band. the earth may be "a pale blue dot," but surprisingly it is bright in the near-uv. by way of contrast, neptune, a blue planet, loses flux in the near-uv in proportion to the sun's spectrum. the gain in reflected energy by earth in the region of interest is attributable to raleigh scattering in the atmosphere and a lowering of absorption in oceans. in the former, we see a more productive albedo scattering from earth's atmospheric gas abundances such as nitrogen. in the latter, the absorption curve from the sun spectrum in water dies out into the blue. we used earth observing satellite data to document the earth albedo spectrum. although photons are scarce when observing exoplanets with g-class parent stars in the region of interest from 350-550 nm, there are more photons than in longer wavelengths. gathering photons within perhaps four binned bands of 50 nm width we could test for a differential gain relative to a parent star in the near-uv. if a directly observed exoplanet shows a differential gain relative to its g-class parent star in the near-uv, the differential between parent star and child planet is a signal indicating an earth analog candidate. the novel telescope proposed to make this observation uses a gabor zone plate (gzp) as its primary objective. we show how this instrument enjoys a feature that isolates a star and its exoplanetary system from all proximate stars by use of its secondary spectrometer. the optical physics are a direct descendant from newton's dual prism experiment. the slit between the primary and secondary along with a secondary dispersive optic such as a grating excludes all sources outside confined angles encompassing the target system alone. within the target, direct spectrographic observation of individual albedos is feasible. we choose the near-uv not simply for its utility in running the proposed differential flux test for an earth analog. we show that coronagraphy in this band is facilitated by higher contrast absorption lines compared to longer wavelengths. the troughs of the star absorption lines give access to peaks of albedo lines, dropping the contrast ratio by two to nearly three orders of magnitude across selected fraunhofer lines. further coronagraphy is facilitated by a unique circular line of foci coming from the gzp primary. we show how this permits angular differential imaging without physically rotating the telescope. a third coronagraph called bloc (bifurcated light optical coronagraph) is being investigated. this last design uses the nulling interferometric method with the improvement that the null occurs only over an inner diameter extinguishing the host star but does not touch the region where exoplanets can be observed. the proposed telescope called duet (dual use exoplanet telescope) is intrinsically spectrographic and has the alternative use of taking indirect radial velocity measurements by doppler shift. it has been tested on an optical bench. we present our laboratory results. | discovering earth analog candidates in the near-uv |
1. introductionhydrogen emisssion by degassing magma is likely to have a significant impact on the h2 mixing ratio in early atmospheres of terrestrial planets depending on the surface pressure, volcanic flux and oxygen fugacity of the magma [1]. this process can account for the presence of a few percent of hydrogen in the early atmospheres of terrestrial exoplanets. the formation of water using a n2-co2-h2 mixture was identified in a previous experimental study focusing on the early earth with nitrogen as a dominant gas and an initial h2 mixing ratio fixed at 4% [2]. we now explore various amounts of hydrogen in the mixture to cover the unknown parameters regarding surface emission and to highlight the role played by hydrogen in the chemistry of these exoplanetary atmospheres.2. experimental methodthe pampre (french for "aerosol production in microgravity using a reactive plasma") experimental setup [3] is a plasma reactor used to simulate the photochemistry at low pressure (around 1 hpa) in planetary atmospheres. the co2-n2-h2 mixture is injected in the reactor with various n2 to h2 abundance ratio and a fixed abundance of 70% for carbon dioxide. the molecular hydrogen mixing ratio is varied from 5% down to 0.5% in volume. the gas phase in the pampre reactor is analyzed using a quadrupole mass spectrometer (hiden analyticals) to identify the species formed once the plasma is turned on. 3. experimental resultsfigure 1. production of water and oxygen with 0.5% h2figure 2. production of water and oxygen with 5% h2 the significant oxygen production associated to a hydrogen ratio of 0.5% is reduced drastically when increasing the hydrogen mixing ratio by a factor of 10. the formation of oxygen via the hox cycle well known in the case of mars becomes secondary when the hydrogen mixing ratio increases. as a result, the production rate of the oh radical formed by (1) increases.o(1d) + h2 --> oh + h (1)the radical oh then reacts by (2) and (3) which explains the significant formation of water with an initial hydrogen mixing ratio of 5%.h2 + oh --> h2o + h (2)2 oh --> h2o + o (3)these new experiments put forward the presence of a hydrogen mixing ratio threshold over which the formation of water dominates the formation of oxygen. two regimes can therefore be identified : o2-dominant production regime at low amounts of h2 and h2o-dominant production regime at higher amounts of h2.4. conclusions and perspectivesfor terrestrial exoplanets known to be in the habitable zone of their host star, the present study shows that molecular hydrogen in the atmosphere is critical to characterize their habitability as this particular molecule leads to an important formation of water in oxydized conditions. it is also shown that based on the hydrogen abundance in the atmosphere, two regimes can be identified and this particular process suggests that high amounts of hydrogen could lead to a significant abundance of water in these atmospheres even without liquid water at the surface. quantification of relative concentrations using quadrupole mass spectrometry by a recently developped tool [4] is in progress and would allow to quantify the h2 mixing ratio which seperates the two regimes in our experimental conditions. based on these experimental results, the impact of this process is now being studied using the chemical kinetics model vulcan [5] taking into account vertical mixing and using a known star spectrum. the predictions obtained from the model will be used to predict future observations of these specific environments in the frame of jwst and ariel space missions.acknowledgements nc acknowledges the financial support of the european research council (erc starting grant primchem, grant agreement no. 636829).references[1] liggins, p., shorttle, o., rimmer, p., 2020. can volcanism build hydrogen-rich early atmospheres ? earth and planetary science letters 550[2] fleury, b., carrasco, n., marcq, e., vettier, l., määttänen, a., 2015. the astrophysical journal letters 807[3] szopa, c., cernogora, g., boufendi, l., correia, j., coll, p., 2006. planetary and space science 54[4] gautier, t., serigano, j., bourgalais, j., hörst, s.m., trainer, m.g., 2020. decomposition of electron ionization mass spectra for space application using a monte-carlo approach. rapid communications in mass spectrometry[5] tsai, s-m., lyons, j.r., grosheintz, l, rimmer, p.b., kitzmann, d., heng, k.,2017. vulcan : an open-source, validated chemical kinetics python code for exoplanetary atmospheres. the astrophysical journal supplement series 228 | molecular hydrogen in oxidized atmospheres of terrestrial exoplanets : implications for water and oxygen formation |
energetic photon and particle radiation resulting from stellar magnetic activity evaporates and erodes planetary atmospheres and controls upper atmospheric chemistry. most rocky exoplanets detected in the "habitable zone" are in close-in orbits around very low-mass stars that are typically very active and expose their planets to much stronger high-energy radiation than we experience on earth. unfortunately, we still do not have a comprehensive theory of stellar magnetic activity and energetic radiation needed to understand exoplanet atmospheric evolution and habitability. stellar magnetic activity, visible in x-ray emission, is known to be both driven by and control stellar rotation rates. rotation evolution of cool stars has proven difficult to explain under the existing magnetic braking models. it has increasingly been realized that the morphology of the stellar surface magnetic field plays a vital role in controlling stellar spin-down rates, and provides a natural physical basis that explains stellar rotation period observations. in this talk, i will discuss (a) how magnetic activity scales with rotation and, therefore, with age, (b) the resulting energetic particle and photon irradiation dose and its effects on exoplanetary environments, (c) how to improve stellar age estimates for large samples, which is important for understanding the secular integrated radiation dose of exoplanets, and (d) the need for a mission to observe stellar variability in the euv. | stellar high-energy radiation and its effects on exoplanets. |
a threshold was crossed in the late 20th century with the discovery of planets around other stars (e.g., mayor & queloz 1995). at this key point in history, humanity knows that exoplanets are both abundant and diverse — and the possible discovery of life-bearing worlds is within our grasp. this monumental objective demands powerful and flexible new tools, as well as application of multi-disciplinary scientific skills. the next frontier is to extend our characterization capabilities to rocky exoplanets, including finding the "pale blue dots" in the solar neighborhood. with the right tools, we can determine whether those worlds have earth-like surface conditions and probe them for signs of life. focusing on the planetary systems most like the solar system, those with earth-size exoplanets orbiting in the habitable zones of sun-like stars, increases the chances of finding and recognizing biosignatures. concurrently, we will nurture a new discipline — comparative exoplanetology — by studying a huge range of exoplanets and comparing them with the vastly better studied solar system planets. as preparation for the 2020 astrophysics decadal survey (astro2020), in 2016, nasa initiated studies of four large space telescope concepts. two of these concepts — habex and luvoir — have the driving goal of finding and studying potentially habitable exoplanets around sun-like stars, as well as enabling a wide range of revolutionary astrophysics and solar system studies. here we will outline the observational requirements for achieving these goals, describe the technical solutions proposed by the two concepts, and compare their capabilities and challenges. the results of these detailed mission concept studies, which spanned nearly four years, were presented to astro2020 last year and are currently being considered for prioritization as nasa's next great observatory. | tools to find living worlds: the habex and luvoir mission concepts |
stellar rotation and its associated activity signals can mimic the radial velocity signatures of orbiting planets. rotation is both mass- and age-dependent; and despite the prevalence of low-mass stars, few have measured rotation periods. the mearth project is a transiting planet survey looking for earths and super earths around mid-to-late m dwarfs (<0.33 rsun) within 33 pc of the sun, with observatories in the northern and southern hemispheres. using the mearth planet-search data, we measured photometric periods ranging from 0.1 to 150 days for approximately 500 mid-to-late m dwarfs. we used galactic kinematics to estimate the ages of the stars with detected rotation periods. we find that mid m dwarfs in the field are slowly-rotating, with periods of approximately 100 days at 5 gyr. we consider late-type stars at 5 gyr and show where stellar rotation may impact the discovery of habitable planets. the slow rotation rates and close-in habitable zones of mid m dwarfs indicate that rotation-induced signals are unlikely to interfere with the detection of habitable planets around these stars. we present a catalog of photometric rotation periods and non-detections for the 2300 stars that have been observed by mearth. the mearth target list comprises the brightest and nearest low-mass stars and is the sample of m dwarfs best suited for the discovery and atmospheric characterization of habitable planets. we highlight the subset that, based on their photometric rotation, is well-suited as targets for the upcoming generation of radial velocity surveys dedicated to low-mass stars, including spirou, carmenes, and hpfs. | the rotation of nearby m dwarfs and implications for exoplanet discovery |
atmospheric characterization of exoplanets in habitable zones is one of the greatest challenge of astrophysics. in fact, all known potential targets either do not transit, or they transit stars too faint or distant, making them impossible to probe with transit spectroscopy. a recently announced k2 planet candidate found in the habitable zone of a nearby m dwarf, could be a game changer as the first habitable-zone super-earth (2.2 r_earth) amenable to characterization. we propose to use hst to (1) validate the planet candidate by observing a high-precision near-infrared transit with wfc3 and (2) characterize its atmosphere by detecting an extended hydrogen exosphere in the far ultraviolet with stis. hydrogen escape is indeed a telltale sign of terrestrial planets enduring a runaway greenhouse effect. further considerations on the habitable potential of the planet thus need to be vet against a detection of hydrogen escape. our recent stis lyman-alpha observations of a moderately irradiated neptune show that extended upper atmospheres can reach much larger sizes around such planets than around very hot exoplanets. we could thus obtain a significant detection with a modest amount of hst orbits. in parallel, we started a ground-based campaign to constrain the yet unknown mass of this planet with doppler measurements. combining the lyman-alpha transit depth with the measurement of the planet bulk density (from the accurate near-infrared transit and the doppler mass), will reveal for the first time whether an exoplanet can be telluric and actually habitable, or if it is losing its water because of a runaway greenhouse effect. | hst confirmation and characterization of a potentially habitable world |
dont plan to knock on a door at alpha centauri asking to borrow a cup of sugar just yet but with a new look at this star system using a powerful telescope, we now know a bit more about our neighbors.secrets among nearby starsthis artists interpretation shows the planet proxima centauri b around its host star. you can see the binary cen ab in between the planet and star, as two faint white dots in the background. [eso]when it comes to exploring sun-like stars that might host planets, the alpha centauri ( cen) star system is an ideal target. at just over 4 light-years away, this triple system the binary pair cen ab and an additional companion, proxima centauri contains the closest stars to the sun.recent news has hyped the discovery of two exoplanets around the red dwarf proxima centauri but what other surprises might the larger stellar system harbor? given that cen a and b are both very similar to the sun, it would be particularly valuable if we could find earth-like, potentially habitable worlds around these near neighbors.choosing a methodbut how to detect them? searching for transits works only for very specific orbit orientations. direct imaging might be an option, since cen is so close. but even these nearby stars are challenging: cen as habitable zone lies at about 1.2 au, or just 0.9 in angular separation, from our point of view. its hard to confidently detect a small, dim object at that separation!calibrated images of cen a and b taken with alma in october 2018 (left two panels) and in august 2019 (right two panels). [adapted from akeson et al. 2021]another method may prove useful in this case, however: astrometry. in a new study, a team of scientists led by rachel akeson (nasa exoplanet science institute, caltech-ipac) has used the high resolving power of the atacama large millimeter/submillimeter array (alma) to make some of the most precise astrometric measurements of cen ab yet.hints of influenceastrometry relies on the idea that the slight gravitational tug of an orbiting planet causes a star to wobble in place. if this effect is large enough, we can detect it via meticulous imaging that very precisely tracks the location and motion of the star on the sky over time.taking advantage of the high-resolution observations provided by almas long baseline, akeson and collaborators captured measurements of cen a and b during 2018 and 2019. their results provide the first high-accuracy absolute measurements of the stars positions on the sky since 1991, as well as the highest-accuracy differential astrometry yet, comparing their relative separation and searching for the tiny influence of planets around the two stars.top left: astrometric measurements (red: hipparcos and alma data, blue: archival data) and best-fit orbit of cen b relative to cen a. top right: enlargement of the 2019 alma measurements (the total orbit takes ~80 years). bottom: residuals of the fit as a function of time. [akeson et al. 2021]the authors then combine these results with archival data to better constrain cen abs orbit and properties.a promising futureakeson and collaborators show that alma can produce remarkably precise astrometric measurements for the cen system, demonstrating the exciting potential of using alma for this technique. though the observations dont reveal signs of a planet yet, continued monitoring should allow us to ultimately be able to detect planets of a few tens of earth masses in stable orbits between 1 and 3 au around cen a.but these results go beyond our search for planets they also refine our measurements of cens motions. this allows us to make more accurate estimates of the physical properties of cen a and b, filling in our understanding of our nearest neighbors.citationprecision millimeter astrometry of the centauri ab system, rachel akeson et al 2021 aj 162 14. doi:10.3847/1538-3881/abfaff | getting to know our nearest neighbors with alma |
in this talk we assess how differences in the composition of exoplanet host stars might affect the availability of water in their systems, particularly the role of carbon and oxygen abundances. water, one of the key chemical ingredients for habitability, may be in short supply in carbon-rich, oxygen-poor systems even if planets exist in the 'habitable zone'. | stellar c/o: effects on habitability of exoplanet systems |
we report on the discovery and validation a transiting planet identified by a search through the four years of data collected by nasa’s kepler mission. this possibly rocky 1.63-re planet orbits its g2 host star every 384.843 days, one of the longest orbital periods for a terrestrial exoplanet to date. the likelihood that this planet has a rocky composition lies between 43% and 58%. the star has an effective temperature of 5757 ± 85 k and a log g of 4.32 ± 0.09. at a mean orbital separation of 1.046 au, this small planet is within the habitable zone of its star, experiencing only 10% more flux than earth receives from the sun today. the star is slightly larger and older than the sun, with a present radius of 1.11 rsun and an estimated age of ~6 gyr. thus, this planet has always been in the habitable zone and will remain there for another ∼3 gyr. | a 1.6 earth radius planet in the habitable zone of a g2 star |
large scale infrared imaging surveys have facilitated the discovery of sub-stellar objects in the field and as wide companions, with mass down to a few jupiters and teff as low as ~250k. this population may have diverse origins with formation in both circumstellar and interstellar environments, with much work still needed to properly understand the "brown dwarf-exoplanet connection". the wide-field infrared survey explorer (wise) is currently providing the greatest sensitivity to free-floating planetary mass objects, and has revealed a new classification that covers "habitable zone" temperatures - the y dwarfs. wise scans the sky in a way that yields time-domain as well as colour/brightness/morphology information, and offers an expanded opportunity to discriminate between genuine sources and noise signatures near the survey limits, as well as scope to reveal very high proper motion objects in the solar neighbourhood. i have developed a bayesian search methodology to identify the coolest faintest objects in wise, from within the reservoir of faint contamination and noise signals. i define multi-parameter probability distributions using controlled sampling of the allwise database. the coolest sub-stellar objects are detected in the wise w2 band, but are un-detected at w1, so my analysis prioritises sources that display source-like and noise-like properties respectively in these two bands. i will review the followup observations that allow me to confirm or reject candidate y dwarfs, and present recent discoveries from the programme. | sifting planetary mass objects at the limits of the wise survey |
the plato mission, part of esa's cosmic vision program, will launch in 2024 and will revolutionize the field of transiting exoplanets. by observing a large sample of bright stars, plato will discover thousands of terrestrial planets, including hundreds in the habitable zones of their host stars. the brightness of plato targets allows full characterization of both the planets and their host stars, including asteroseismic analysis to precisely determine masses, radii, and ages. moreover, plato host stars will be bright enough to allow atmospheric spectroscopy. confirmation and characterization of plato planets will require a coordinated, ground-based follow-up program to both eliminate false-positives, and derive planetary masses. i will present an introduction to plato, discussing the scientific motivation behind the mission, its aims and goals, and the significant contribution that plato will make to the search for a second earth. i will also talk about the requirements and formulation of the follow-up program, showing that the demands are not as onerous as might be feared. | habitable zone planets: plato, and the search for earth 2.0 |
the kepler mission by nasa is motivated to find more earth-size planets in or near the habitable zone using the transit method. till now, more than 4,100 planet candidates are released, of which about 40 percent planet candidates with the radius less than 4 r⊕orbiting the host stars in the single planet system are located within 0.1 au. generally, these planet sare called close-in terrestrial or neptune-like planets, one special class of exoplanets observed by kepler mission, and they are good examples to investigate planet information, since it is not clear for earth-like planet formation in the current formation models. for such close-in planets, tidal effects are important to reshape the final orbits following a series of orbital migration such as disk migration, planetary scattering and in situ formation etc. in this work, we use various initial conditions, for example, random unit distribution and rayleigh distribution of orbital semi-major axes and eccentricities will be investigated. all orbital elements are assumed to be close relative of possible certain migrations. especially the in-situ formation mechanism is to be focused in this work. numerical simulation results compared with the observations indicate that such planets are possible form near current positions as the rayleigh distribution with the peak of semi-major axis between 0.05 and 0.1 au. results caused by other migration scenarios and various eccentricities, planetary or stellar masses and tidal dissipation parameters are also discussed. | tidal evolution indicating the possible formation of the kepler candidate samples with planetary radius less than 4 earth radii |
the field of exoplanetary science has seen a dramatic improvement in sensitivity to terrestrial planets over recent years. such discoveries have been a key feature of results from the kepler mission which utilizes the transit method to determine the size of the planet. these discoveries have resulted in a corresponding interest in the topic of the habitable zone (hz) and the search for potential earth analogs. within the solar system, there is a clear dichotomy between venus and earth in terms of atmospheric evolution, likely the result of the large difference in incident flux from the sun. since venus is 95% of the earth's radius in size, it is impossible to distinguish between these two planets based only on size. in this talk i will discuss planetary insolation in the context of atmospheric erosion and runaway greenhouse limits for planets similar to venus. using the ``venus zone'' (vz), i will present identified potential venus analogs from kepler data and subsequent occurance rates of such planets. | the venus zone: seeking the twin of earth's twin |
the kepler mission has shown that small planets are extremely common. it is likely that nearly every star in the sky hosts at least one rocky planet. we just need to look hard enough-but this requires vast amounts of telescope time. minerva (miniature exoplanet radial velocity array) is a dedicated exoplanet observatory with the primary goal of discovering rocky, earth-like planets orbiting in the habitable zone of bright, nearby stars. the minerva team is a collaboration among unsw australia, harvard-smithsonian center for astrophysics, penn state university, university of montana, and the california institute of technology. the four-telescope minerva array will be sited at the f.l. whipple observatory on mt hopkins in arizona, usa. full science operations will begin in mid-2015 with all four telescopes and a stabilised spectrograph capable of high-precision doppler velocity measurements. we will observe ~100 of the nearest, brightest, sun-like stars every night for at least five years. detailed simulations of the target list and survey strategy lead us to expect new low-mass planets. | minerva: small planets from small telescopes |
in order to properly assess the potential for habitability and prioritize target selection for the characterization of exoplanets, we need to understand the limits of orbital and rotational dynamics. large satellites may be rare and very difficult to detect. consequently, it is necessary to quantify the likelihood of a planet's having extreme obliquity cycles in the absence of a moon and to model the potential impact on the planet's climate. we explore the obliquity evolution of (1) known exoplanet systems that could contain earth-like planets in the habitable zone and (2) hypothetical planets in mutually inclined, chaotic resonant configurations that experience some of the most extreme orbital evolution possible. we use a secular obliquity model coupled to either an n-body models or a 4th order secular orbital model.we find that in some known systems, planets' obliquity variations are small and unlikely to have a major effect on climate, unless undetected planets are present. systems with three or more planets are significantly more dynamically rich, with planets that undergo obliquity changes of ~10° over 50,000 years and >30° over a few million years. in resonant configurations, earth-like exoplanets can undergo dramatic and chaotic evolution in eccentricity and inclination while remaining stable for over 10 gyr. in configurations in which eccentricities and inclinations stay below ~0.1 and ~10°, respectively, obliquities oscillate quasi-periodically with amplitudes similar to the non-resonant, three-planet configurations. in more dynamically active configurations, in which eccentricities and inclinations evolve to e > 0.3 and i > 15°, obliquities can extend from ~0° to well past 90°. in extreme cases eccentricities can reach >0.9999 and inclinations >179.9 degrees, driving precession rates in excess of degrees per year. however, these planets can graze or impact the stellar surface and are probably not habitable. | obliquity evolution of earth-like exoplanets in systems with large inclinations |
we explore the potential of astrometry method in finding habitable planets orbiting nearby solar-like stars. we consider different types of planet systems with an undiscovered habitable earth-like exoplanets in them. we simulate their astrometry signals and use our code to fit the masses and orbits of the habitable planets. when comparing our fitting results with the real ones, we find the threshold of different noise levels and the detection potential in different planet systems. we find astrometry is nearly suitable for all kinds of planet systems we discover today. only in the case when the other planets in the systems have very short periods(less than 3 days) or long period about 30 years, the fitting errors of the giant planets lead to large fitting errors of the habitbale earth-like planets. after adding noise in the astrometry data, the fitting of the orbital elemants, such as the eccentricity and inclination, becomes difficult. under the presicion of 0.3 uas, we suggest 10 earth mass is the boundary of good-fitting, compared to 100 earth mass under the precision of 10 uas. at last, we simulate the discovered planet systems in 50 pc, assuming that 10 earth mass is in the habitable zone at about 1 au, we use the fitting results of their masses, semi-major axes, eccentricities and inclinations to give a rank to suggest the possibility of discovering habitable exoplanets in these systems. | detecting habitable planets via astrometry in current planetary systems |
the "radius valley" is a feature in the short-period, small exoplanet population in kepler and k2 data showing an abundance of super-earths (1-2 re) and mini-neptunes (2-3.5 re), with a relatively scarce population of intermediate-sized planets between the two. by employing updated stellar properties and implementing refined measures of completeness and reliability, we discover that the occurrence of super-earths over that of mini-neptune has a period and stellar mass dependence. we use these dependencies to extrapolate the occurrence of super-earths in the habitable zone of mkgf stars. finally, we discuss our results in the context of plato's search for long-period small planets and their dependence on stellar mass. | kepler's small planets and their dependence on stellar mass |
the "radius valley" is a feature in the short-period, small exoplanet population in kepler and k2 data showing an abundance of super-earths and mini-neptunes, with a relatively scarce population of intermediate-sized planets between the two. several studies explore the radius valley's dependence on host star properties, specifically stellar age and mass, with a wide range of treatments to the population of small close-in planets. we employ updated stellar properties and implement refined measures of completeness and reliability to observe how the kepler small planet population varies as a function of stellar mass. these results are extrapolated into the habitable zone, placing an estimate on the occurrence rate of habitable, earth-like planets that still retain their atmospheres. we further attempt to constrain the degree of stripped cores "contaminating" the super-earth population as a function of period, which will help constrain models of photoevaporation and core powered mass loss. we discuss these results in the context of tess and compare preliminary demographics between the kepler field and the solar neighborhood. | demographics of small kepler planets and their dependence on stellar mass |
to be habitable by conventional terrestrial definitions, a planet must possess a solid or liquid surface, a source of free energy, sufficient supplies of carbon and other bioessential elements, and liquid water. the habitable zone is defined as the region around a star where a planet can maintain liquid water on its surface. however, other factors may play an important role in planetary habitability. this chapter discusses the state of the art and major questions on exoplanetary habitability and the opportunities and challenges on this frontier. | exoplanetary habitability |
the astro2020 decadal survey recommended as the next strategic astrophysics mission a 6 m class space telescope capable of high-contrast direct imaging of earth-size exoplanets in about one hundred habitable zones of nearby sun-like stars. the expected number of imageable exoplanets for such a telescope depends on the architecture and the metrics used to evaluate those architectures. in this paper, we assess the yield of notional coronagraph-only, starshade-only, and hybrid starshade/coronagraph architectures for several metrics. we evaluate the exoplanet yield for a 20% bandwidth, snr=5, r=70 water search metric; a 20% bandwidth, snr=8.5, r=140 oxygen search metric; and a 4x20% bandwidth metric, snr=8.5, r=7 for 450-700 nm and r=140 for 700-1000 nm, which is tailored for a coronagraph's sequence of 20% bandwidth sub-spectra. we bound the number of expected exoplanets by considering three cases of a priori knowledge: the case of no prior knowledge that requires a photometric blind search for exoplanets; the theoretical case of perfect prior knowledge that skips the photometric blind search and performs only spectral characterizations using realistic mission scheduling constraints (this approach shows the upper bound and when target exhaustion is reached); and a case of partial prior knowledge via a hypothetical, future extreme precision radial velocity instrument with 3 cm/s sensitivity. this work is an initial study of the potential exoplanet science return for the decadal-recommended large infrared/optical/uv great observatory (irouv). | an exploration of expected number of exoplanets for a 6m class direct imaging observatory |
we present the catalog of earth-like exoplanet survey targets (celesta), a database of habitable zones around 37000 nearby stars. the first step in creating celesta was assembling the input data. the revised hipparcos catalog (van leeuwen 2007, cat. i/311) is a stellar catalog based on the original hipparcos mission (perryman et al. 1997, cat. i/239) data set. hipparcos, launched in 1989, recorded with great precision the parallax of nearby stars, ultimately leading to a database of 118218 stars. mcdonald et al. 2012 (cat. j/mnras/427/343) calculated effective temperatures and luminosities for the hipparcos stars. the next step was selecting appropriate stars for the construction of celesta. the stellar parameter catalog of 103663 stars included many stars that were not suitable for our purposes, especially stars off the main-sequence (ms) branch, e.g., giants. please refer to section 3.2 in the paper for additional details about the star selection. the final celesta catalog contains 37354 stars (see table2), each with a set of associated attributes, e.g., estimated mass, measured distance. the complete database can also be found online at a dedicated host (http://www.celesta.info/). (2 data files). | vizier online data catalog: catalog of earth-like exoplanet survey targets (chandler+, 2016) |
we present the kepler object of interest (koi) catalog of transiting exoplanets based on searching 4yr of kepler time series photometry (data release 25, q1-q17: twicken+, 2016, j/aj/152/158). the catalog contains 8054 kois, of which 4034 are planet candidates with periods between 0.25 and 632 days. of these candidates, 219 are new, including two in multiplanet systems (koi-82.06 and koi-2926.05) and 10 high-reliability, terrestrial-size, habitable zone candidates. this catalog was created using a tool called the robovetter, which automatically vets the dr25 threshold crossing events (tces). the robovetter also vetted simulated data sets and measured how well it was able to separate tces caused by noise from those caused by low signal-to-noise transits. we discuss the robovetter and the metrics it uses to sort tces. for orbital periods less than 100 days the robovetter completeness (the fraction of simulated transits that are determined to be planet candidates) across all observed stars is greater than 85%. for the same period range, the catalog reliability (the fraction of candidates that are not due to instrumental or stellar noise) is greater than 98%. however, for low signal-to-noise candidates between 200 and 500 days around fgk-dwarf stars, the robovetter is 76.7% complete and the catalog is 50.5% reliable. (6 data files). | vizier online data catalog: kepler planetary cand. viii. dr25 reliability (thompson+, 2018) |
many earth-sized planets have already been discovered and some seem located in the habitable zone. moreover, several earth-sized planets were recently detected around low temperature stars near the solar system. however, it is difficult to characterize them as earth-like or venus-like, even though they are relatively close to us. transit spectroscopy for exoplanetary atmosphere has been performed to characterize larger exoplanets but it requires very high accuracy for much smaller earth-like planets. recently, earth's hydrogen exosphere was re-investigated and it was revealed that the earth's exosphere is extended to ~ 38 earth radii. on the other hand, venus' and mars' hydrogen exosphere is not so much extended because of its low temperature of upper atmosphere. this is caused by the difference of mixing ratio of co2 in the upper atmosphere. on earth, co2 was removed from its atmosphere by a carbon cycle with its ocean and tectonics. translating these arguments to exoplanets in a habitable zone presents a possible marker to distinguish an earth-like planet from a mars-like or venus-like planet. the expanded exospheres can be observed in uv, during the exoplanet transit event in a primary eclipse. it reduces the stellar flux, when an exoplanet orbiting in front of the host star. we performed a conceptual design of ultraviolet spectrograph for exoplanet (uvspex) for world space observatory ultraviolet (wso-uv), which is 1.7-m uv space telescope being prepared by russia. the dominant engineering requirements for the uvspex are following. the spectral resolution is better 0.5 nm to separate o i line from other spectral lines. the spectral range is to exceed the wavelengths from 115 nm to 135 nm to detect at least h lyman alpha 121.6nm to o i 130 nm. the throughput is better 0.3%. to achieve these requirements, a simple spectrograph design is proposed, containing the slit, the concave (toroidal) grating as a disperse element and the imaging photo-detector. this optical concept is conventional and used in the other space missions for uv spectroscopy. uvspex is planned to be a part of field camera unit (fcu). in this presentation, we show the configuration of uvspex instrument and its science objectives. | uvspex onboard wso-uv for characterization of earth-like exoplanets by exosphere observation |
the kepler & k2 missions have revolutionized our understanding of exoplanet demographics. however, for most kepler/k2 discoveries our knowledge of planet radii has been limited by the uncertainties in the radii of the host stars. additionally, radius uncertainties for the full sample of ~500,000+ targets have been a dominant source of systematic errors for measuring planet occurrence rates. the release of gaia dr2 parallaxes in april 2018 has spectacularly solved this bottleneck by allowing a precise radius determination for nearly every kepler/k2 target. in this talk i will review the latest results on kepler/k2 exoplanet radius demographics based on gaia dr2 parallaxes, including an investigation of the intriguing radius gap for close-in super-earths and a revised catalog of small, habitable-zone exoplanets. i will also discuss the remaining challenges for characterizing exoplanet host stars in the gaia era, including strategies towards accurate effective temperature and radius scales for solar-type stars through the combination of constraints from interferometry, asteroseismology and astrometry. | what is left to learn about kepler/k2 planet host stars? |
spirou is a near-infrared spectropolarimeter and a high-precision velocimeter optimized for both the detection and characterization of terrestrial planets orbiting nearby low-mass stars, and the study of the impact of magnetic field on the star-planet formation. the spectrograph is designed to record the whole near-infrared spectrum simultaneously in either circular or linear polarization and to reach a rv precision of 1 m/s at a resolving power of 75,000. it will be use to carry out the "spirou legacy survey" targeting two science objectives (habitable terrestrial planet detection & magnetic field impact on star-planet formation) and is intended to provide the community with an extensive, homogenous, well characterized and high-quality data. spirou is expected to make a major breakthrough in the field of telluric planets in the habitable zone of cool stars. once implemented at cfht in 2017, spirou is expected to be used extensively by the astronomical community - supporting in particular space missions such as tess, jwst and plato. in this presentation, i will focus on the impact of the spirou future observing programs in the field of exoplanets: 1) the radial-velocity survey, its target selection of cool dwarfs, strategy and expectations; 2) the follow-up characterization of transiting candidates; 3) the search for giant planets around very young stars; 4) the importance of spectropolarimetry to filter out the intrinsic jitter of target stars at the sub m/s level; 5) the anticipated role in preparing further exoplanet characterization missions. | spirou -a near-infrared spectropolarimeter @ cfht |
due to their cosmic abundance and long lifetimes, m-dwarfs are by far the most abundant stars in the universe. planets orbiting in the habitable zone of these stars are tidally locked, most likely in synchronous rotation, with crucial consequences for their climate and habitability. however, there is no analogue in the solar system of such configurations and observations of terrestrial exoplanets remain challenging, leaving their climate and habitability to speculation. we coupled a gcm of an earth-like planet to a simple ice sheet elevation and bedrock deformation model, and assessed how much, and where, water ice is trapped on the permanent nightside; such ice trapping has previously been identified as a threat to the habitability of synchronously-rotating temperate terrestrial planets. we identified a strong feedback between snowfall rates and global ice elevation, and found that ice sheet elevation depends on the whole tropospheric temperature structure. we also revisited previous results of water trapping by estimating where ice sheets are limited by the amount of water available in the atmosphere rather than by basal melting. these results show that assessing the habitability of synchronously-rotating terrestrial planets requires sophisticated geophysical modeling, an endeavor still in its early days. | coupled atmospheric-ice sheet model for tidally locked exoplanets |
the science promised by high-contrast imaging missions will result in great leaps in our understanding of exoplanetary and the detection and spectral characterization of earth-like planets in the habitable zone. however, none of these missions can image multi-star systems with current technology except when the leak and glare of the companion star(s) is negligible or small enough to be removed by post-processing. therefore, many systems with multiple stars (or optical multiples) are often excluded from target lists of such missions. our solution to binary star suppression is a wavefront control algorithm called multi-star wavefront control (mswc). in general, any region of interest where we wish to detect planets contains a mix of speckles from both stars, which are mutually incoherent. when mswc is used it is possible to null speckles from both stars if the region of interest is appropriately chosen, and non-redundant modes of the deformable mirror are used. feasibility of mswc was demonstrated in computer simulations and at the ames coronagraph experiment experiment laboratory. in this paper, we report the demonstration of mswc using the subaru coronagraph extreme adaptive optics (scexao) as part of our technology development effort. our goal is to show that mswc can achieve an order of magnitude better contrast on binary stars than conventional (i.e., single-star) techniques on binaries. demonstrating this technique on scexao will validate mswc on a real system (with a calibration source). thus, proving that the technique can be applied on deployed instrumentation without major modifications. | demonstration of multi-star wavefront control using scexao |
since the first discovery, more than 800 exoplanets have been detected through the radial velocity method, the majority orbiting solar-like stars. although m-stars are the most frequent stars, very few planets have yet been found around m-stars of late spectral type. carmenes, operated since 2016, is a high-resolution visible-near-ir spectrograph dedicated to search for such low-mass planets around low-mass stars and already doubled the number of known planets with host stars below 0.2 msun. not surprisingly, also this stellar parameter range has its surprises in terms of planetary system architectures. we will give an overview of exoplanet detections (published and unpublished) from the carmenes survey and then concentrate on the low-mass planets, including the very recent detection of two earth-mass planets around teegarden's star highlighting the capability of carmenes. the planetary system is special since teegarden's star is only one out of three planet host stars with an effective temperature below 3000k. its two planets are within the optimistic and conservative habitable zone, respectively. notably, the earth, as well as other solar system planets are currently or in near future in the transit visibility zone see from teegarden's star. | rocky planets from the carmenes survey |
the future of exoplanet detection lies in the mid-infrared (mir). the mir region contains the blackbody peak of both hot and habitable zone exoplanets, making the contrast between starlight and planet light less extreme. it is also the region where prominent chemical signatures indicative of life exist, such as ozone at 9.7 μm. at a wavelength of 4 μm the difference in emission between an earth-like planet and a star like our own is 80 db. however a jovian planet, at the same separation exhibits 60 db of contrast, or only 20 db if it is hot due to its formation energy or being close to its host star. a two dimensional nulling interferometer, made with chalcogenide glass, has been measured to produce a null of 20 db depth, limited by scattered light. measures to increase the null depth to the theoretical limit of 60 db are discussed. | photonic mid-infrared nulling for exoplanet detection on a planar chalcogenide platform |
in this talk, i will discuss the recently announced exoplanet kepler-1649 c — an earth-sized planet in the habitable zone of a nearby low-mass star. this discovery was over a decade in the making, involving automated analyses and dozens of dedicated scientists sifting through thousands of complex signals. the same process that unearthed kepler-1649 c also revealed other overlooked planets and astrophysically interesting objects that remain to be studied in detail. in addition to kepler-1649 c, i will also review these other finds, the data products available from the kepler and k2 missions, advances in the field to mine this data, and what may remain to be found in these rich datasets. | discovery of the earth-sized habitable-zone planet kepler-1649 c: what other treasures remain to be unearthed in kepler and k2 data? |
we request a short (10ks) chandra-hrc-s observation of a recently discovered exoplanet which resides in the habitable zone around a near-by m dwarf and has a density consistent with a thick water envelope. an x-ray observation would allow us to measure the stellar x-ray flux irradiating the planet, which is the driver of atmospheric evaporation of exoplanets. current upper limits for the host star are only available from rosat and are uninformatively high (with ca. 5e27 erg/s). the anticipated measurement (allowing a detection down to lx = 3e26 erg/s, practically guaranteeing a detection for an m dwarf) would tell us whether this exoplanet is subject to a similar x-ray flux like the small exoplanet gj 436 b, which shows a huge atmospheric evaporation tail. if so, this planet could be followed up meaningfully at other wavelengths to test for the presence and density of its evaporation plume and constrain habitability. | evaporation of a habitable-zone exoplanet with a water envelope |
t has been observed, mostly from the findings of the kepler mission, that the vast majority of earth-like exoplanets, and small planets in general, are formed very close to their host stars. this comes into contradiction with the cases of planets in the size of neptune and bigger that most likely form at longer distances and migrate closer to the star over time. based on the findings of the kepler telescope, it is suggested that there is a relationship between the radius and the formation distance. the bigger the radius, the greater the distance at which the planet will form, and the smaller the radius, the smaller the distance from the host star. in this work, we present an empirical approach for the derivation of the density for habitable-zone exoplanets with radii 0.6-1.5 earth radii, without the knowledge of the mass of the planet. the 1.5 earth radii limit was selected as objects up to this size are believed to be rocky. in general, it seems that, up to this limit, gravity continues to accumulate and compress matter, and as a result, density increases. this can also be observed for planets within the fulton gap (1.5-2.0 earth radii). the densities of such objects most likely is the result of accreting heavier elements and of the existence of a core consisted of iron or other ferromagnetic compounds. for bigger planets the possibility of sustaining a solid surface decreases, but there is still some uncertainty on whether they are rocky or gaseous. following the above, we present additional empirical approaches for the derivation of density for all exoplanets with radii 0.6-3.0 earth radii. we split these planets in three categories: 0.6-1.5, 1.5-2.0 (fulton gap), and 2.0-3.0, regardless of the planets' orbital position. 1. introductionwe experimented with a mathematical formula to derive the density of rocky exoplanets that are transiting, focusing to:1) the absence of doppler spectroscopy data. knowing the density helps to directly derive the mass as we already know the radius (and as a result, the volume) from the observed transit dips. our solution proved to be effective for the majority of transiting rocky exoplanets of our sample (table, page 2) with a maximum radius of 1.5 earth radii, given that such planets lie within the conservative habitable zone of their host stars.2) cover the range of densities 2.9-8.5 g/cm3. the observables we use for our solution is the orbital period expressed in days, the mean motion of the exoplanet expressed in radians, and a factor that is related to, and dependent upon, the exoplanet/earth radius ratio, expressed in earth radii. we applied our mathematical expression for rocky-type exoplanets in the conservative habitable zone, and we saw the results to be close to the published values for many transiting exoplanets and candidates that are orbiting their parent stars inside their circumstellar, conservative habitable zones. 2. application of the expression for densitieswe experimented with known predicted values for objects added to the hec (habitable exoplanets catalog) catalogue and we found our results to be in good agreement with the published values. as we are focused on the exoplanet discoveries made by kepler, we developed our mathematical expression in absence of doppler spectroscopy measurements for the vast majority of these exoplanets. the density formula for rocky exoplanets inside the habitable zone is:where n=(2π)/τ, with t expressed in days. for stars with effective temperatures lower than 0.7 times that of the sun"s, the formula includes the ratio tsun/tstar as a denominator.it is important, though, in case of earth-like planets that orbit m stars, to solve with both expressions for density. the reason is that m stars tend to allow the formation of earth-like planets in a wide range of densities, e.g from planets like trappist-1f (3.5 g/cm3) to planets like lhs-1140 b.we applied our expression for densities for a list of 20 habitable-zone transiting exoplanets, and the results are shown in the table below. we also attempted to apply the first expression to the inner planets of the solar system, despite mercury and venus are not inside the habitable zone of the sun. the deviations from the real values were ~12% for mercury, ~3% for venus, ~2% for the earth and ~7% for mars. 3. general expressions for exoplanets between 0.6 and 3.0 earth radiia) terran planets (0.6-1.5 earth radii)the general expression we use for the sub-category of earths covers the range of densities 5.5-7.5 g/cm3. the expression makes use only of orbital period t (days), one of the two main observational parameters (together with the radius), extracted from the light curves. in this expression, as the orbital period increases, density tends to decrease, and vice versa. this is normal and correlates to other naturally inversely proportional ratios (ρ/r and m/r).b) exoplanets in the fulton gap (1.5-2.0 earth radii)the expression we use to calculate the densities for exoplanets inside the fulton gap makes use of both the radius rexo (earth radii) and the orbital period t (days). it covers the range 6.0-8.5 g/cm3.c) exoplanets above the fulton gap (2.0-3.0 earth radii)the expression for the exoplanets of this sub-category makes use of orbital period, t (days) and the radius rexo (earth radii). it covers the range 2.9-5.5 g/cm3.the diagram below shows a comparison between the measured densities for 89 exoplanets with known masses (with 30% maximum uncertainty) and the results of our formulas. for 62% of those planets the formulas give results with a deviation less than 30% (with data from exoplanet.eu). acknowledgementsthe author expresses his gratitude to his friend anastasia, especially, for all the encouragement to transform this idea into a work, as well as for her support during this effort; his brother harry, also for being supportive; dr. angelos tsiaras for his interest on this work, and the frequent communication with the author. | on an empirical mathematical approach to the densities of small transiting exoplanets |
stellar flares are stochastic events that occur when a star's magnetic field re-connects, releasing intense radiation across the electromagnetic spectrum. rocky planets in the habitable zones of m-dwarfs are often subjected to superflares, events of at least 1033 erg and 10-1000x the energy of the largest solar flares. frequent superflares can erode the ozone layer of an earth-like atmosphere and allow lethal amounts of uv flux to reach the surface. conversely, too few flares may result in insufficient uv radiation to power pre-biotic chemistry due to the inherent faintness of m-dwarfs in the uv. cool stars are often found to exhibit superflares. cool stars are the most common type of star, and are known to frequently host rocky planets. as a result, they may host most of the universe's earth-size planets orbiting in the habitable zones of main sequence stars. my evryflare survey uses observations from the evryscope array of small telescopes and the transiting exoplanet survey satellite (tess) to answer two questions about superflares and their impacts on the habitability of terrestrial planets orbiting cool stars: (1) how frequently are superflares emitted from the nearby cool stars, both in the present and in the first 200 myr after formation? (2) what impact does superflare uv emission have on planetary atmospheres and surface habitability of planets orbiting cool stars? the evryflare survey has resulted in the detection of 575 superflares from 284 stars. results include a superflare from proxima cen, the nearest host star to a rocky planet in the habitable zone. i used these events to measure a decrease in superflare rates with increasing age, rotation, and starspot coverage. i will discuss the effects of superflares on ozone loss to planetary atmospheres, including one superflare with sufficient energy to photo-dissociate all ozone in an earth-like atmosphere in a single event. i present the largest-ever survey of simultaneous observations of dozens of m-dwarf superflares with evryscope and tess to measure the flare blackbody and estimate uv-c continuum emission. i find superflare temperatures increase with flare energy. the largest and hottest flare briefly reached an estimated 42,000 k. during superflares, i estimate rocky hz planets orbiting <200 myr stars typically receive a top-of-atmosphere uv-c flux of ~120 w m-2 and up to 103 w m-2, 100-1000x the time-averaged xuv flux from proxima cen. finally, i will describe a data analysis project with robo-ao, exploring the performance of laser guide star adaptive optics systems in the absence of tip-tilt correction. | investigating exoplanet habitability and the stellar magnetism of cool stars across half the southern sky via superflares, starspots, and stellar rotation |
nulling interferometry is considered as one of the most promising solutions to spectrally characterize rocky exoplanets in the habitable zone of nearby stars. it provides both high angular resolution and starlight mitigation. it requires however several technologies that need to be demonstrated before a large interferometry space-based mission flies. a small-sat mission is a good technological precursor. based on a bracewell architecture, this unique satellite can demonstrate some key components (null capability, fiber injection, achromatic phase shifter). scientific capabilities of such a mission are presented. an exoplanet detection yield is derived, and we show that the detection of exoplanets around nearby stars is feasible. | performance study of interferometric small-sats to detect exoplanets: updated exoplanet yield and application to nearby exoplanets |
a balanced ocean/land ratio is thought to be essential for the evolution of an earth-like biosphere. emerged continents provide direct access to solar energy while oceans prevent an all-to-dry climate. assessing the habitability of earth-like planets, one may be tempted to assume similar geological properties. consider e.g., the volume of the continental crust: the latter is determined by an equilibrium between subduction related continental production and continental erosion. assuming the interior thermal state of earth-sized exoplanets to be earthlike, one might expect a similar equilibrium between continental production and erosion to establish and, hence, a similar continental land fraction. we will show that this conjecture is not likely to be true and that the present-day earth may be an exceptional planet: positive feedback associated with the coupled mantle water - continental crust cycle enhanced by the role of sediments may lead to a bifurcation of possible evolution outcomes. one of these is a land planet with about 80% continental surface coverage. the other extreme is a planet covered by about 20% with continents. of the two, the land planet has a substantially larger zone of attraction in the space of reasonable initial conditions. about 80% of randomly chosen sets of initial conditions evolve to end there. the ocean planet attracts about 20% of the cases. only around a percent of the evolution models result in an earth-like configuration, which is stable with respect to mantle water but unstable with respect to continental coverage. because the rates of change are small after billions of years of evolution, the unstable equilibrium can be occupied for a long time. including co2 outgassing and the long-term carbonate-silicate cycle, we found that the land and the ocean planet differed by only about 5k in average surface temperature. still, we would expect the land planet to have a substantially dryer, colder and harsher climate possibly with extended cold deserts in comparison with the ocean planet and with the present-day earth. these planets would all be considered habitable but their fauna and flora may be quite different. the earth in its geologic history has experienced climates that could resemble the one expected for the land planet (e.g., the pleistocene) and for the ocean planet (e.g., the paleocene). | land/ocean surface diversity on earth-like (exo)planets: implications for habitability |
atmospheric escape is the primary physical process sculpting the population of short-period, irradiated exoplanets, yet the dominant mass loss mechanism remains unclear. several mechanisms likely contribute to the exosphere loss (e.g., photoevaporation, core-powered erosion, stellar wind stripping, giant impact) and they predict distinct correlations between mass-loss rates and properties of environment and planets. directly measuring the mass loss for a large ensemble of exoplanet-host systems will differentiate these atmospheric escape processes and such measurements have been enabled by the helium 10830a triplet, a robust probe of exospheres. using the habitable-zone planet finder spectrograph (hpf) on the hobby-eberly telescope (het), we are conducting an extensive survey for helium exospheres, targeting two dozens of exoplanets spanning wide ranges in (1) the planets' surface gravities and masses, (2) the host stars' spectral type, and (3) the system age. while k-type host stars are expected to have the ideal extreme-uv to mid-uv flux ratios for the high excitation of helium metastable levels, our survey discovers that the planets orbiting the hottest stars (f dwarfs) possess significant excess helium absorption during transit. these helium excesses are among the strongest detection to date and are repeatable when re-observed. with time baselines covering much broader orbital phases of exoplanets than most similar surveys, our data also reveal the spatially extended exospheres of irradiated planets. in this talk, we will present our survey strategy, target selection, data and modeling analysis, as well as an open-source python package we developed to enable the helium studies. with the unique combination of the broad sample, as well as the large aperture, high spectral resolution, and significant institutional accesses to het, our survey will reliably probe the mass-loss mechanism of irradiated exoplanets. | an extensive survey of helium outflows from irradiated exoplanets with the hobby-eberly telescope |
the solar system terrestrial planets have high mean molecular weight secondary atmospheres, ranging from 92 bars of carbon dioxide on venus to a tenuous layer around mercury. we do not know if such secondary atmospheres are typical of terrestrial planets or are a phenomenon unique to the solar system. where terrestrial worlds outside the solar system orbit small (<0.3 solar radii) nearby (<15 parsecs) stars, we can interrogate their atmospheres (should they retain them) using the technique of transmission spectroscopy. in my thesis i undertook ground-based optical observations of four of the most spectroscopically accessible terrestrial exoplanets using multi-object spectrographs mounted on the magellan telescopes at las campanas observatory in chile. gj 1132b has a radius of 1.6 earth radii and an equilibrium temperature of 580 k. we observed five transits of this planet with magellan clay/ldss3c and achieved a median fractional transit depth uncertainty of 4.2% in the 20-nm chromatic bandpasses we used to construct its transmission spectrum. we rule out a clear, low mean molecular weight atmosphere at 3.1-sigma confidence. lhs 1140b is a habitable zone world with a radius of 1.7 earth radii and an equilibrium temperature of 235 k. we observed two transits of lhs 1140b simultaneously with imacs and ldss3c. we jointly fit the data from both instruments and achieved a median transit depth precision in 20-nm chromatic bandpasses of 2.6%. due to the high surface gravity and low equilibrium temperature of this world, we are not able to address physical models of its atmosphere. lhs 3844b has a radius of 1.3 earth radii and is highly irradiated with an equilibrium temperature of 805 k. we observed 13 transits of lhs 3844b with magellan clay/ldss3c and reached a median fractional uncertainty in our 20-nm chromatic transit depths of 1.7%. we rule out a clear, low mean molecular weight atmosphere on this planet, in agreement with a recent finding that used the thermal phase curve of this world to rule out a thick atmosphere. at the time of writing, ltt 1445ab is part of an observing program that will observe four transits of this world with magellan clay/ldss3c. the main finding of this exploratory work is that terrestrial exoplanets do not posses low mean molecular weight atmospheres. with the greater light-gathering capability of the giant segmented mirror telescopes (gsmts) and the broader wavelength coverage and infrared sensitivity of jwst, it will be possible to investigate higher mean molecular weight atmospheres around these terrestrial exoplanets. this work is supported by a grant from the john templeton foundation. | a first look at the atmospheres of four terrestrial exoplanets with ground-based optical transmission spectroscopy |
the earth likely underwent several periods of planet-wide ice coverage in the past, in whats known as snowball earth events. a new study explores whether snowball events are also a risk for tidally locked, habitable exoplanets.an icy fatecurrent theory suggests that the earth underwent several snowball events in its past history. [nasa]snowball events can arise suddenly on a planet like earth, driven by a rapid feedback loop. a planet that experiences a sudden drop in stellar light reaching its surface say, due to a volcanic eruption or asteroid impact can quickly ice over through a runaway effect: as ice coverage grows, more light is reflected from the planets surface. this drops the temperature of the planet, which causes ice coverage to expand even further.under some conditions, this runaway snowball effect can lead to a fully icy world thats no longer able to defrost itself, even if incoming stellar light returns to original levels.artists impression of a cold, tidally-locked planet. ice covers much of the planets surface, but the point directly facing the planets host star remains ice-free. [nasa/jpl-caltech]looking beyond our solar systemthe paradigm described above depends on specifics of how heat is transferred in the atmosphere of a rapidly rotating planet like earth. but in searching for habitable planets beyond our solar system, we might wonder whether other types of worlds also experience snowball events.in particular, the majority of the potentially habitable planets weve discovered lie around dim m-dwarf stars, and many of these planets are tidally locked, meaning the same side of the planet faces its host star at all times. can worlds like this snowball, too?to investigate this question, a team of scientists led by jade checlair (university of chicago) used an atmospheric global climate model to conduct simulations of a tidally locked, earth-sized planet that circles its m-dwarf host on a 50-day orbit. in particular, the team was curious whether heat transfer within a global ocean would affect the outcome so they covered their simulated planet in a multi-layer ocean that reached a depth of 189 meters.no snowballsthe authors results show that sea-ice coverage follows a smooth relationship with stellar irradiation on tidally locked planets: for each level of stellar irradiation, the planet equilibrates to the same final state regardless of where it started. this is not the case on planets with runaway snowball events. [checlair et al. 2019]checlair and collaborators found that, unlike a rapidly rotating planet, tidally locked planets are stable against runaway snowball events. in their model, as the planet experienced decreasing irradiation, its sea ice extent grew gradually and it defrosted again as the stellar irradiation was brought back to original levels.this means that for a tidally locked planet in its stars habitable zone, snowball states should not be possible for extended periods of time. if a planet were to experience a catastrophic event like a volcanic eruption or asteroid impact, it may ice over briefly. but the stellar radiation concentrated on the side of the planet facing its host would quickly cause the planet to warm back up again and return to its original state.good or bad?is the lack of tendency for tidally locked planets to snowball a good thing or a bad thing? though a global ice age could wipe out preexisting complex life, its also possible that snowball events could help drive life to evolve more rapidly, by providing evolutionary pressure to adapt. the jurys still out on the impact of snowball events, but now we know a bit more about where to expect them!citationno snowball on habitable tidally locked planets with a dynamic ocean, jade h. checlair et al 2019 apjl 884 l46. doi:10.3847/2041-8213/ab487d | snowball events for tidally locked planets? |
ozone, an important gaseous biosignature, is not only critical for surface uv habitability, but also a proxy for the detection of oxygen, another important gas related to photosynthesis. detecting ozone on exoplanets is important for assessing their habitability. this study focuses on tidally-locked earth-like planets around m dwarfs, which outnumber all other stellar types in the galaxy. planets in the habitable zone around m dwarfs tend to be tidally locked due to their short orbital periods, resulting in unusual circulation patterns. recent gcm simulations suggest that around uv-active m dwarfs, earth-like planets can develop ozone layers with abundance close to that of earth. two ozone concentration maxima could form on the nightside, especially within two high-latitude gigantic cyclones. therefore, these two ozone concentration maxima can serve as a proxy for nightside cyclones detection, which can further validate the gcm-simulated atmospheric circulation. moreover, it is also an approach of detecting atmospheric circulations on earth-sized exoplanets. studying the detectability of such a spatial distribution can provide guidance for future missions on wavelength selection and observing strategy. given the fact that the capability of current telescopes and those in the foreseeable future would not be able to resolve exoplanets better than one pixel, our analysis focuses on disk-integrated single-point light curves. reflected and emitted light of the simulated earth-like exoplanet are generated using a radiative transfer model and then integrated over the planetary disk to create a single-point source, which follows with time series analysis. detectability of ozone distribution and climate system are derived from the synthetic light curves combined with viewing geometry. | detectability of ozone on tidally-locked earth-like exoplanets |
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.