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starshades suppress on-axis starlight to enable the direct imaging of exoplanets with non-specialized space telescopes of variable size. even relatively small, commercially available telescopes are capable of imaging earth-like planets in the habitable zone, and larger telescopes provide the capability to characterize planet atmospheres with increasing spectral resolution. we detail two options developed by the stdt for probe-class starshade missions (exo-s): a co-launch of a starshade with a dedicated 1.1m telescope, and a standalone starshade to augment an existing large telescope such as wfirst-afta. for these concepts, we describe the optical and mechanical design, the formation flying system, and the augmentations required to make a telescope 'starshade-ready'. we also lay out typical design reference missions for each and their scientific yield, and show both concepts have the capability to image terrestrial exoplanets orbiting nearby stars. | imaging exoplanets with the exo-s starshade mission: baseline design |
the modern view of planets goes far beyond the usual concept of the planets as bodies of the solar system. the discovery of exoplanets has immeasurably expanded the understanding of the architecture and properties of planetary systems. major advances have been made in the study of the planets and minor bodies of the solar system. however, no answers have been received to fundamental questions about the causes of various paths of evolution and formation of planetary natural complexes. to give answers to these questions, research on exoplanets is called upon, of which more than 5000 have been discovered to date. exoplanet studies provide an approach to solving the key problems of stellar-planetary cosmogony — the genesis and evolution of planets as byproduct of star formation. the most urgent problems concern the formation of planetary systems around stars of various spectral classes; the nature of hot jupiters; the reasons for the predominance of super-earths and sub-neptunes, which are absent in the solar system; stability of planetary systems of binary stars, including circumbinary systems. of particular interest are terrestrial planets with orbits in zones of "potential habitability", studies of which open a new page in astrobiology. | planets — a modern view |
over the last decade numerous discoveries of exoplanets significantly enhanced our knowledge about planetary systems and their interaction with the host stars. future missions such as plato and ariel will further shed light on the diversity and frequency of rocky planets within the habitable zones of m, k, and even g stars. the potential detection and characterization of secondary atmospheres around such planets will lead to a much better understanding on habitability in general, but also on the host star's complex role in shaping the atmospheric evolution of any potentially habitable planet. by now, however, our own solar system provides the best laboratory for studying habitability and the therewith connected evolution of planetary atmospheres. if one wants to get an insight into habitability beyond the solar system, it is crucial to understand why earth turned out to be habitable whereas venus and mars both evolved into completely different environments. for this, however, one also has to take a deeper look onto the evolution of the sun and how its early plasma and radiation environment affected the atmospheres of the rocky planets. through the example of our solar system, we will investigate the role the host star plays on the evolution of a secondary atmosphere around a planet in the habitable zone. different related key factors have to play together to build up and maintain a nitrogen-dominated atmosphere, such as the orbital location of the planet or the euv flux evolution of the young star. however, in addition to star-planet interactions, also other factors turn out to be crucial to maintain the long-term stability of a habitable planet. we will briefly discuss some of these, such as plate tectonics and the role of aerobic life, before we will finally extend our view beyond the solar system. this will include a discussion on the frequency of earth-like habitats, i.e. earth-sized planets with nitrogen-dominated atmospheres, in the habitable zones of other stars and the potential detection of exoplanetary biospheres. finally, we will conclude with potential science cases for future space missions. | habitability in the solar system and beyond - the role of star-planet interactions |
the search for water-rich earth-sized exoplanets around low-mass stars is rapidly gaining attention because they represent the best opportunity to characterize habitable planets in the near future. understanding the atmospheres of these planets and determining the optimal strategy for characterizing them through transmission spectroscopy with our upcoming instrumentation is essential in order to constrain their environments. for this study, we present simulated transmission spectra of tidally locked earth-sized ocean-covered planets around late-m to mid-k stellar spectral types, utilizing the results of general circulation models previously published by kopparapu+ (2017apj...845....5k) as inputs for our radiative transfer calculations performed using nasa's planetary spectrum generator (psg.gsfc.nasa.gov). we identify trends in the depth of h2o spectral features as a function of planet surface temperature and rotation rate. these trends allow us to calculate the exposure times necessary to detect water vapor in the atmospheres of aquaplanets through transmission spectroscopy with the upcoming james webb space telescope (jwst) as well as several future flagship space telescope concepts under consideration (the large uv optical infrared surveyor (luvoir) and the origins space telescope (ost)) for a target list constructed from the transiting exoplanet survey satellite (tess) input catalog (tic). our calculations reveal that transmission spectra for water-rich earth-sized planets around low-mass stars will be dominated by clouds, with spectral features <20ppm, and only a small subset of tic stars would allow for the characterization of an ocean planet in the habitable zone. we thus present a careful prioritization of targets that are most amenable to follow-up characterizations with next-generation instrumentation, in order to assist the community in efficiently utilizing precious telescope time. (1 data file). | vizier online data catalog: tic star exposure times for jwst, luvoir and ost (suissa+, 2020) |
plato (planetary transits and oscillations of stars) is a european space agency medium class mission, whose launch is foreseen for 2026. its primary goal is to discover and characterise terrestrial exoplanets orbiting the habitable zone of their host stars. this goal will be reached with a set of 26 wide field-of-view cameras mounted on a common optical bench. here we show some results of the first cryogenic vacuum test campaign made on the engineering model (em) of one plato camera, performed at the netherlands institute for space research (sron). in particular we present the search for the best focus temperature, which was done first by using a hartmann mask, and then by maximizing the ensquared energy fractions of the point spread functions (psfs) on the entire field of view taken at different temperature plateaus. furthermore we present the psf properties of the em at the nominal focus temperature over all the field of view, focusing on the ensquared energy fractions. the engineering model camera was successfully integrated and validated under cryo-vacuum tests, allowing the mission to pass esa's critical milestone, and confirming the mission is on track for launch in 2026. | plato em first cryogenic vacuum test campaign psf results |
exoplanets around low mass stars are the focus of the search for habitable exoplanets. previous general circulation models (gcm) studied the locations of the ihz around stars with effective temperature from 3300 to 4500k (yang et al. 2014, kopparapu et al. 2016). however, water vapor mixing ratios at 3 hpa pressure level do not satisfy what is required for scenarios of rapid water loss in the "last converged solution" for stars cooler than 4000 k. in this work we use the community earth system model (cesm) to investigate the ihz problem for low mass stars. the model includes atmospheres with 1 bar of n2, 1 ppm of co2, and slab oceans with thermodynamic sea ice. rotation period is determined by the mass and luminosity of the star and planet orbital distance. black body spectra of low mass stars are used to obtain top-of-atmosphere incident short wavelength radiation. our model results are qualitatively consistent but quantitatively different from those in earlier works. specifically, water vapor mixing ratios required by rapid water loss are found at 3 hpa for hosts star warmer than 3650 k. | climate and water contents on rocky planets near the inner boundary of habitable zones (ihz) around low mass star |
one of the main goals of exoplanet science is to characterize the atmosphere of rocky exoplanets in the habitable zone of nearby stars. a space-based nulling interferometry, observing in the mid-infrared (3-20 μm), is considered to be one of the most promising solutions to tackle this observing challenge. the life project, a free-flying spacebased mid-infrared nulling interferometer, would have this capability. however, several key technologies need to be demonstrated before launching such an ambitious mission. a small space-based mission can be considered as a useful prerequisite. in this paper, we consider three small satellite architectures, two cubesats, and a proba-like satellite. based on a bracewell architecture and without free-flying, these monolithic satellites can demonstrate some key components like the null capability and its stability on real targets. the achromatic phase shifter needs also to be demonstrated in space. based on the scientific capabilities and exoplanet detection yield of these architectures, optical constraints are derived (pointing stability, and optical path difference correction). orbital simulations, exploring a range of classical orbits for such a satellite, are also discussed. | feasibility study of an interferometric small-sat to study exoplanets |
it is recently proposed that early stellar luminosity evolution of m dwarfs leads to severe water loss and the buildup of massive o2 atmospheres on rocky exoplanets in the habitable zone of these stars if interactions of such o2 atmospheres with planetary surfaces are inefficient. here we show that the existence of a massive o2 atmosphere on such exoplanets depends on stellar xuv properties, the mass of the exoplanets, and most importantly the initial planetary water inventories. | history of water loss and atmospheric o2 buildup on rocky exoplanets near m dwarfs |
most exoplanets lying in the habitable zones around stars are in fact inhospitable to plant life as we know it, according to a new study. | 'goldilocks zone' may not be a good metric for life |
in this thesis i use global plasma simulations to explore the star-planet interaction for mars and mars-like exoplanets, with an emphasis on the relationship to ion escape.i compare the results of five global martian plasma models run with identical input conditions to each other and corresponding maven data, in order to assess the effect of the different physical assumptions and numerical implementations. i show that no one model outperforms all others in every data comparison, necessitating the careful selection of model for the type of physics one is analyzing. there are clear morphological differences in ion behavior in the tail and southern hemisphere, as well as in the location of various plasma boundaries.i then apply a hybrid plasma model to the study of a generic mars-like planet in the habitable zone of a typical m-dwarf star. i systematically vary the stellar input conditions and examine the changing plasma environment and ion escape. both ion loss morphology and overall rates vary significantly, and in cases where the stellar wind pressure was increased, the ion loss begins to be diffusion or production limited. a quasi-parallel interplanetary magnetic field drives asymmetrically draped field lines and correspondingly asymmetric ion escape.i use the same hybrid model to explore the effects of intrinsic planetary magnetic field strength on ion outflow, for both current mars and a mars-like exoplanet. the presence of an intrinsic magnetic field enhances escape to a certain point before beginning to inhibit it, depending on the polar cone angle and the magnetic standoff distance. i argue that ion escape reflects a balance between the competing effects of magnetic shielding at the equator and enhanced escape at the poles. | planetary plasma modeling and ion escape |
venus has long been of interest to humanity given its prominence in the night sky. with the development of the telescope and later multi-wavelength instrumentation and in-situ/orbiting spacecraft we have a detailed record of venusian observations stretching back hundreds of years. this record of observations is only second to that of mars and our own world earth. these planets form most of the basis for our understanding of the formation and evolution of terrestrial worlds. on the other hand, extrasolar planetary exploration is in its infancy. theoretical studies involving concepts like the liquid water habitable zone [1,2,3,4] pre-date the first exoplanetary discoveries (1992 for planets around pulsars [5], 1995 for planets around main sequence stars [6]), yet even these are only a few decades old. we discuss the synergies that exist between venus and exoplanetary studies today and how they may inform each other to better understand the evolution of venus and its exoplanetary cousins. this work is part of an international space science institute sponsored collection of papers on the evolution of venus submitted to space science reviews. references: [1] huang (1959) am sci. 47, 397. [2] dole (1964) habitable planets for man. blaisdell, new york. [3] shklovski & sagan (1966) intelligent life in the univers, holden-day, san francisco [4] hart (1978) icarus 33, 23. [5] wolszczan & frail (1992) nature 355, 145. [6] mayor & queloz (1995) nature 378, 355. | searching for synergies between venus and exoplanetary worlds |
through the success of missions like nasa's transiting exoplanet survey satellite (tess) mission, the number of planetary systems outside of our own solar system has significantly increased. studying the dynamics of multi-planet systems may give insight into the origin and dynamics of our solar system. for example, sub-neptunes (<4 earth radii) which are the most common planets in the universe, have no analogue in our own solar system, limiting our ability to understand these unique objects. while vetting the ~500 multi planet tess objects of interests (tois), we considered the size and temperature of the candidate host stars, the transmission spectroscopy metric (tsm), and radius of the planets in each system. for this work, we are especially focused on systems that contain a candidate small planet that resides in their star's optimistic habitable zone. from this analysis, we have identified a few keystone candidates, like toi-2404, a system that contains a hot neptune and two saturn sized planets that bracket the habitable zone for their host star. i will present our process of vetting the multi planet tois and discuss toi-2404 in comparison to our own solar system. | identification of keystone tess multiplanets systems |
radial velocity precision at and below the 1 m/s level is necessary for the detection of earth-mass exoplanets within the habitable zones of m-dwarfs. laser frequency combs provide a dense and absolute array of frequencies ideal for the in-situ calibration of high-resolution astronomical spectrographs. however, the challenges of generating large mode spacings, wide optical bandwidth, and the operational robustness suitable for remote facilities have prevented their widespread use. overcoming these obstacles, we built a 30 ghz frequency comb spanning 800 nm to 1300 nm and achieved long term, on-sky, 1.5 m/s rms radial velocity precision with the near-infrared habitable-zone planet finder spectrograph at the 10 m hobby-eberly telescope in texas. our frequency comb is generated through robust, fiber integrated electro-optic modulation of a single frequency laser and subsequent supercontinuum generation in a highly nonlinear fiber and a nano-photonic device. all frequencies are traceable to a gps disciplined atomic clock. the frequency comb has been running continuously since may 2018. we detail the architecture, performance, and long-term operation of the comb. this work was supported by nsf under grant ast-1310875 and the nist-on-a-chip program. | a 30 ghz laser frequency comb for high-precision radial velocity calibration and exoplanet searches |
exoplanets are revolutionizing planetary science by enabling statistical studies of a large number of planets. empirical measurements of planet occurrence rates inform our understanding of the ubiquity and efficiency of planet formation, while the identification of sub-populations and trends in the distribution of observed exoplanet properties provides insights into the formation and evolution processes that are sculpting distant solar systems. in this paper, we review the current best estimates of planet populations. we focus in particular on η⊕, the occurrence rate of habitable zone rocky planets, since this factor strongly influences the design of future space based exoplanet direct detection missions. | current best estimates of planet populations |
this research project focuses on mathematical and scientific analysis of exoplanet data to determine their physical characteristics and potential habitability through comparison and contrast with earth. the researchers made use of the data compiled by the kepler space telescope to study the characteristics of k2-3 d, situated in the k2-3 planetary system within the kepler k2 campaign field 3. through data optimization, data visualization and other trial and error processes, the researchers scrutinized the light curve of the selected exoplanet's host star to apply kepler's laws of planetary motion. using mixed methods of investigation, the researchers were successful at calculating estimated results of the following values of k2-3 d: radius, orbital period, eccentricity, aphelion, perihelion, semi-major axis, orbital velocity, and force of gravity exerted at the exoplanet. furthermore, the constraints for life on earth were defined and compared with k2-3 d observations through the earth similarity index equation (schulze-makuch et al., 2011) and astrobiology climate models. the orbital characteristics were used to create a visual model of the planetary system according to scale. all things considered, the initial hypothesis was indeed validated: mathematical equations derived from kepler's laws of planetary motion are able to determine an estimated radius and orbital period of exoplanets by analyzing the host star's light curve; k2-3 d is 80% similar to earth, is in the habitable zone, has an adequate expected temperature for extremophiles to develop, and may contain water. | exploring the characteristics of exoplanet k2-3 d with the high earth similarity index |
the determination of extrasolar planet masses with the radial velocity (rv) technique requires spectroscopic doppler information from the planet's host star, which varies with stellar brightness and temperature. we analyze the doppler information in spectra from dwarfs of spectral types f-m utilizing empirical information from harps and carmenes data and model spectra. we revisit the question of whether optical or near-infrared instruments are more efficient for rv observations in low-mass stars, and we come to the conclusion that an optical setup (bvr bands) is more efficient than a near-infrared one (yjhk) in dwarf stars hotter than 3200k. we publish a catalog of 46480 well-studied f-m dwarfs in the solar neighborhood, and we compare its distribution to more than 1 million stars from gaia dr2. for all stars, we estimate the rv photon noise achievable in typical observations under the assumption of no activity jitter and slow rotation. we find that with an espresso-like instrument at an 8m telescope, a photon noise limit of 10cm/s or lower can be reached in more than 280 stars in a 5 minute observation. at 4m telescopes, a photon noise limit of 1m/s can be reached in a 10 minute exposure in approximately 10000 predominantly sun-like stars with a harps-like (optical) instrument. the same applies to ~3000 stars for a red optical setup that covers the r and i bands and ~700 stars for a near-infrared instrument. for the latter two, many of the targets are nearby m dwarfs. finally, we identify targets in which earth-mass planets within the liquid water habitable zone can cause rv amplitudes comparable to the rv photon noise. assuming the same exposure times as above, we find that an espresso-like instrument can reach this limit for 1m{earth} planets in more than 1000 stars. the optical, red optical, and near-infrared configurations reach the limit for 2m{earth} planets in approximately 500, 700, and 200 stars, respectively. an online tool is provided to estimate the rv photon noise as a function of stellar temperature and brightness and wavelength coverage. (1 data file). | vizier online data catalog: rv photon limits of well-characterized f-m stars (reiners+, 2020) |
the transiting exoplanet survey satellite (tess) will discover thousands of exoplanets in orbit around the brightest stars in the sky. in its two-year prime survey mission, tess will monitor more than 200,000 bright stars in the solar neighborhood for temporary drops in brightness caused by planetary transits. this first-ever spaceborne all-sky transit survey will identify planets ranging from earth-sized to gas giants, around a wide range of stellar types and orbital distances.tess stars will typically be 30-100 times brighter than those surveyed by the kepler satellite; thus, tess planets will be far easier to characterize with follow-up observations. for the first time it will be possible to study the masses, sizes, densities, orbits, and atmospheres of a large cohort of small planets, including a sample of rocky worlds in the habitable zones of their host stars. an additional data product from the tess mission will be full frame images (ffi) with a cadence of 30 minutes. these ffi will provide precise photometric information for every object within the 2300 square degree instantaneous field of view of the tess cameras. these objects will include more than 1 million stars and bright galaxies observed during sessions of several weeks. in total, more than 30 million objects brighter than magnitude i=16 will be precisely photometered during the two-year prime mission. in principle, the lunar-resonant tess orbit could provide opportunities for an extended mission lasting more than a decade, with data rates in excess of 100 mbits/s.an extended survey by tess of regions surrounding the north and south ecliptic poles will provide prime exoplanet targets for characterization with the james webb space telescope (jwst), as well as other large ground-based and space-based telescopes of the future.a nasa guest investigator program is planned for tess. the tess legacy will be a catalog of the nearest and brightest main-sequence stars hosting transiting exoplanets, which should endure as the most favorable targets for detailed future investigations.tess is targeted for launch in 2017 as a nasa astrophysics explorer mission. | the transiting exoplanet survey satellite (tess): discovering exoplanets in the solar neighborhood |
the distinctive nature of tidal-locking exoplanets is the very uneven heating by stellar radiation between the dayside and nightside. thus, the permanent nightside can be extremely cold. it had been worried about that atmosphere and water could be condensed on the nightside of habitable exoplanets around m dwarfs. previous studies have demonstrated that atmospheric circulations are able to transport sufficient heat to warm the nightside and prevent atmosphere collapse there. however, it remains a question of how ocean heat transports and sea-ice feedbacks play important roles in determining climates and habitability of such kind of exoplanets and whether water could be completely frozen on the nightside. here, we apply a coupled atmospheric and oceanic general circulation model and a three-dimensional ice-sheet model to this problem. it is found that oceanic zonal heat transport plays important roles in determining climate states of habitable aqua-exoplanets orbiting m-type stars. for sufficiently high greenhouse gas levels, the nightside can be completely ice free due to ocean heat transport. futhermore, we show that for an ocean planet surface winds drive sea ice toward the dayside and the ocean carries heat toward the nightside, both of which keep the nightside sea ice thin. our results show that the thickness of nightside sea ice is only about 10 m or less. thus, nightside water trapping on a water-world should not be significant. we also test whether a large ice sheet could grow on a nightside super-continent using an ice sheet model driven by the climate model output. we find that for weak precipitation generated by the climate model the ice-sheet thickness is strongly dependent on the geothermal heat flux, and could reach 1-2 km if the geothermal heat is similar to earth's or lower. these suggest that complete nightside water trapping would not happen if exoplanets have fairly deep and extensive oceans. | exo-oceanography, climate, and habitability of tidal-locking exoplanets in the habitable zone of m dwarfs |
it may be possible to detect biosignatures of photosynthesis in an exoplanet's atmosphere. however, such a detection would likely require a dedicated study, occupying a large amount of telescope time. it is therefore prudent, while searching for signs of life that we may recognise, to pick the best target possible. we present a new region, the "photosynthetic habitable zone" —the distance from a star where both liquid water and oxygenic photosynthesis can occur. it is therefore the region where detectable biosignatures of oxygenic photosynthesis are most likely to occur. our analysis indicates that in the most ideal conditions for life and no atmospheric effects, the photosynthetic habitable zone is almost as broad as the habitable zone. on the other hand, if conditions for life are anything less than excellent and atmospheric effects are even moderate, the photosynthetic habitable zone is concentrated at larger separations around more massive stars. such cases are also not tidally locked to their host star, which could result in planetary rotation. we conclude that the parameter space in which we should search for signs of life is much narrower than the standard habitable zone. | a new definition of exoplanet habitability: introducing the photosynthetic habitable zone |
ozone has a strong absorption band at 9.6 μm. detection of this absorption band in the transmission spectra of transiting terrestrial exoplanets can provide us with the evidence of the existence of oxygen and thus the biosignature of extrasolar photosynthetic life. in this study, we use a 3d climate-chemistry model to simulate the ozone layer on tidally locked exoplanets in the habitable zone of m dwarfs, with four representative stellar spectra - the solar spectrum, the spectra of two ultraviolet (uv) active m dwarfs, and the spectrum of a uv inactive m dwarf. our model simulations show that the total ozone abundance on exoplanets irradiated by uv-active m star spectra is comparable to the earth's, assuming the oxygen level is the same as that on the present earth. if the oxygen level is lowered by two orders of magnitude, the exoplanets can still sustain significant ozone layers and a safe surface uv environment. these ozone layers can optimally give rise to a 40-km effective height of the atmosphere, which will be detectable with a photometric precision of 10-5 at 9.6 μm. however, for the uv-inactive m star spectrum, the ozone layer is extremely thin. it is spectroscopically undetectable, and the effect on the surface uv environment is negligible. | the ozone layer over tidally-locked terrestrial exoplanets and biosignature detection |
the transiting exoplanets survey satellite (tess) habitable zone star catalog initially derived a list of stars where tess was expected to be able to detect transiting planets with earth-analog irradiation based on expected observation times. however, tess only observed a fraction of the stars long enough to be able to find planets like earth. we use the primary mission data-the first two years of observations-and identify 4239 stars within 210pc that tess observed long enough to see three transits of an exoplanet that receives similar irradiation to earth. (3 data files). | vizier online data catalog: the revised tess habitable zone catalog (kaltenegger+, 2021) |
impact craters, volcanic constructs and compressional and extensional fracture zones are typical landforms found on terrestrial planets. features characteristic of plate-tectonics like on earth are generally lacking. an exception are the surfaces of ganymede and perhaps europa where indications of lateral displacements of surface units have been observed. planetary geologists modeling tectonic features often resort to studies of features on earth such as graben formation of which valles marineris on mars is a giant example or shield volcanoes where olympus mons is another giant example. scaling laws can be used, for instance for shield volcanoes to relate their height and base diameter to the value of the planet's gravity. more specific for numerical modeling of landforms on terrestrial planets is the formation of impact craters, which can be more easily studied on airless bodies or planets of low atmospheric pressure such as mercury, the moon, and mars. impact modeling relates crater diameters and morphology to the size and mass of the impactor. very large impacts may even be affecting the mantles and cores and have been modeled, for instance, to study the heating and melting of a planet's deep interior. the giant impact hypothesis for the formation of earth's moon has been tested by modeling. such studies have only been possible due to significant improvements of so-called hydrocodes over the last couple of decades, specifically in terms of material modeling. the consideration of elastic-plastic material behavior taking ductile and brittle deformation, fracturing, and the compaction and opening of pore space into account was recognized to be key for a better understanding of impact crater formation. as a consequence of the advancements in material modeling the classic term "hydrocode" is actually no longer justified and is now often replaced by the term "shock physics code". another tectonic feature that is more specific to terrestrial planets are lobate scarps that result from cooling of a one-plate planet. they can be used to constrain the cooling history and even the amount of contraction associated with e.g., inner core growth. the formation of lobate scarps as well as the buckling of the lithosphere is related to the thickness of the lithosphere and its growth on a single plate planet. a significant amount of numerical work has been related to understanding the formation of continuous lithospheres or stagnant lids on terrestrial planets. these studies have shown under which conditions stagnant lids will form and how these lids will grow, thus, to a large extent, shaping the tectonic and thermal history of a planet. a substantial progress was possible in the 90's when modelers started to consider a strongly temperature dependent viscosity. the transition to plate-tectonics requires a more complex modeling of the mantle rheology, a problem that has become of particular interest to the growing field of exoplanet studies for which plate tectonics is important as an element of habitability. | numerical models in planetary geology - specifics of one-plate planets |
the main goal of this contribution is to study the habitable zone (ha) of binary systems. therefore, as explained by barbosa1, we will initially use the habitable zone in multiple star systems website | possible habitable planets in habitability zone of binary star systems |
alpha centauri (acen a&b) is not only the closest star system to us at 4.36 ly away, but also both stars are similar to the sun with spectral types g2 and k1 respectively. among the exoplanet detection techniques, direct imaging and astrometry detection limits, to first order, are inversely proportional to the star distance a given telescope size and coronagraph performance. specifically,the inner edge of alpha cen a&b habitable zone is found at exceptionally large angular separations of 0.7" and 0.4" respectively. this enables direct imaging of the system with a 30cm class telescope. a mission concept called acesat has been proposed to directly image the habitable zone of acen a&b. on the other hand, the astrometry signal of an earth-like planet around acena&b is approximately 3mas, which is the largest possible around any fgk star. three astrometry mission concepst have been proposed; toliman, stare and mass. during this talk we will review the configuration of the acen a&b system from the perspective of direct imaging and astrometry, and will describe the missions previously mentioned. | review of space missions concepts to detect earth-like planets around alpha centauri |
a major goal of exoplanet studies is to identify terrestrial exoplanets with similar (or otherwise distinct) bulk and interior properties to our earth. recent results have highlighted the urgent need to include devolatilization to reduce exoplanetary interior modelling degeneracies and to thereby achieve more accurate and reliable results. devolatilization - i.e. depletion of volatiles from protoplanetary disks to planets - is crucial for obtaining accurate bulk compositions for terrestrial planets from their host stellar abundances, particularly for non-refractory elements (e.g. hydrogen, carbon, and oxygen). our pioneering devolatilization approach was benchmarked with the proto-sun and bulk earth and other solar system rocky bodies. in this work, we studied a sample of 15 small-planet-hosting stars and obtained high-precision stellar parameters and chemical abundances for up to 18 elements using observations from the 10m keck telescope. for each planet host, we applied our devolatilization approach to obtain the bulk elemental composition of the hypothetical, habitable-zone terrestrial planets ("exo-earths"). our modelling shows that most of these exo-earths would have an earth-like structure while their mantle compositions would be rather diverse in terms of mgo:sio$ _{2}$:feo ratios. we then applied the same approach to the galactic chemical evolution (gce)-corrected host stellar abundances and find that the exo-earth mantles are more oxidized, resulting in relatively smaller core sizes compared to the previous modelling. we attribute this modelling discrepancy mainly to the lower c/o ratios in these stars when they (and the systems) were born. this work represents essential steps - high-precision and homogenous analysis of stellar chemical abundances, the application of gce corrections, as well as the devolatilization process - towards studying the properties of terrestrial exoplanets. | from accurate stellar abundances to exoplanet structures and compositions |
to date, there are only a handful of advanced modeling studies for atmospheric mass-loss, and only a simple estimation is available for describing the transition between two end-member mechanisms of thermal escape, which play an important role in shaping atmospheres of planets in or near the habitable zone. therefore, a more physically motivated modeling efforts are indispensable for significantly improving the quality and quantity of exoplanetary research that answer important science questions and help interpretation of current and future observations. in this study, we conduct a modeling study of atmospheric escape processes over the wide planetary parameter space covered by exoplanets. we carry out a cutting-edge study by integrating a fluid model to describe the dense region of the atmosphere and a kinetic particle model to describe the region where collisions are not frequent to help build a theoretical basis of escape mechanisms and more specifically to help assess the variability of the transition and interaction between hydrodynamic and non-hydrodynamic escape regimes. | modeling the boundary between hydrodynamic and non-hydrodynamic escape |
mars is the only currently accessible geologic record that can provide an independent test of earth-derived models of planetary habitability. mars is the only planet whose surface is known to have become uninhabitable. we are studying this environmental disaster as a benchmark for rocky exoplanets. the most important climate-regulating greenhouse gas on earth and mars today is co2. to constrain past pco2 on mars (on mars, pco2 ≈ total atmospheric pressure), we used mars' geologic record of the changing distribution of paleochannels and paleolakes, in combination with global climate models. it is usually assumed that co2 warming was near-maximum at times in mars history when rivers flowed. to the contrary, we found evidence that even after average co2 had dropped, rivers still flowed. specifically, we found a decline over time in the preferred elevation for rivers, and a shift from early-stage elevation control to late-stage latitude control. these changes are simply explained, based on comparison to global climate models (gcms), by a reduction in the total strength of the atmospheric greenhouse effect, with a decline in average pco2 from ≫102 mbar for early river-forming climates, to ≲102 mbar for later river-forming climates. to explain late-stage rivers, strong non-co2 greenhouse warming must have continued intermittently through ~3 ga. river-forming climates at low average pco2 on mars constrain mars warming mechanisms and atmospheric evolution, challenge habitable zone theory, and raise the likelihood of false negatives in the search for habitable exoplanets. | mars3 ga had river-forming climates at low average pco2, raising the likelihood of false negatives in the search for habitable exoplanets |
the focus on m dwarfs as planet hosts has led to a number of high-precision spectroscopic surveys monitoring m dwarfs to detect the doppler radial-velocity signature of planets, including with the near-infrared habitable zone planet finder (hpf) instrument. in this work, we study the variability of the k i 12435.67å line in spectra of two m dwarfs — gj 699 (barnard's star) and teegarden's star — and we attribute these changes to zeeman splitting. using the non-lte inversion code using the lorien engine (nicole), we modeled the shape of the k i line under the influence of the zeeman effect for various configurations of the magnetic field on the surfaces of barnard's star and teegarden's star. comparing measurements of the k i line's equivalent width and full width at half maximum in our nicole simulations to measurements of the same line in habitable zone planet finder (hpf) spectra of the two stars over multiple years, we find that our models encompassed the range of variation seen in the data. the temporal behavior of these variations matches line width changes across the spectrum, as would be expected with rotationally modulated photospheric magnetic field changes. our results demonstrate that detailed line profile measurements and modeling the zeeman effect in near infrared lines are useful for analyzing stellar magnetic variability even for some of the slowest rotating stars. the techniques demonstrated here hold promise for future efforts to separate exoplanet radial velocity signals from noise introduced by magnetic activity of their host stars. | rotationally modulated zeeman spectral signatures in two slowly rotating m dwarfs |
the imminent arrival of the extremely large telescopes (elts) will finally deliver the observational power capable of assessing the habitability of nearby rocky exoplanets. the elt presents us with the exciting opportunity of being able to spatially resolve the terrestrial exoplanet proxima b, which lies in the habitable zone of proxima centuri. this would allow molecule mapping, a technique that uses the spatial separation plus cross correlation high resolution spectroscopy to disentangle the planet's spectrum from the host star and characterise its atmosphere. here we present simulations in reflected light for the high contrast adaptive optics (hcao) mode of harmoni/elt, using model planet spectra from the carl sagan institute designed specifically for proxima b and its m-dwarf host star. harmoni's resolution (r = 17,385) is well suited to molecule mapping, with access to wavelengths covering multiple biosignatures. our simulator shows that this first light elt instrument can characterise the atmosphere of proxima b, within a very reasonable time frame, but requires intervention on the focal plane masks in harmoni's current instrument design. if changed, harmoni has the potential to identify co2, ch4, and h2o in proxima b. our simulator is highly versatile and we are extending to other instruments for the elts, including metis/elt and gmagao-x+ifs/gmt. | detecting biosignatures of nearby rocky exoplanets: simulations of high spectral resolution observations with the elts |
exoplanets orbiting in the habitable zone (hz) of m dwarfs can lose significant amounts of water during the stellar pre-main sequence, a major obstacle to habitability (luger & barnes, 2015). trappist-1, a late m dwarf located 12 pc away, harbors 7 transiting terrestrial exoplanets that span the hz, offering an unprecedented laboratory to examine how water loss impacts potentially-habitable exoplanets (gillon et al., 2017). we perform a markov chain monte carlo (mcmc) analysis to constrain the xuv luminosity evolution of trappist-1, the key driver of water loss, while accounting for observational uncertainties. we use these constraints to derive plausible distributions for the amount of water the trappist-1 exoplanets could have lost and define a metric, the exoplanet habitability index (ehi), to quantify which planet is most likely to retain water. we find that trappist-1e likely resided interior to the hz for ~370 myr, potentially losing 8-12 earth oceans and releasing ~1000 bars of o2, a false biosignature. trappist-1 f and g likely entered the hz 200-250 myr earlier than e, losing 4-5 earth oceans each. trappist-1 b, c, and d probably are still in a runaway phase and could have each lost over 100 earth oceans. | constraining the water loss histories of the trappist-1 exoplanets: comparative habitability using the exoplanet habitability index |
to further our understanding of how forming stars and planetary atmospheres can survive large inputs of radiative and kinetic energy, i have undertaken observational studies of the orion bn/kl outflow and a sample of nearby m dwarfs with exoplanets. the orion bn/kl explosive outflow is a spectacular, wide-angle ensemble of hundreds of protostellar-like jets. with near-ir integrated intensity maps of 13 ro-vibrational h2 lines, i determined the excitation conditions across the arcminute-scale, wide-angle bn/kl outflow at 1" resolution. the warm h2 populations ( 2000-2500 k) are consistent with collisional excitation by shocks, but small areas are likely excited radiatively. as a potentially common phenomenon in massive star forming regions and a test bed for shock models due to its brightness, the bn/kl is important to characterize in detail. exoplanets orbiting stars less massive than the sun are prime targets for current and near-future atmospheric characterization efforts, because they are nearby and offer larger signals. however, it is an open question whether a habitable-zone rocky planet could maintain an atmosphere through all of the expected mass-loss processes driven by the parent star, especially from high-energy photons and particles. i have characterized the far-uv spectra of 11 k and m dwarfs, determined a correlation between the far- and near-uv spectra and more easily-obtained optical spectra, and developed a method for estimating stellar particle output associated with flares. these are useful tools for evaluating the likelihood that a m dwarf terrestrial planet has a substantial atmosphere. | star formation and planets in harsh environments |
in the near-future, new space telescopes like jwst, luvoir, and habex will begin attempting to explore the properties of atmospheres of potentially habitable planets. this will require a significant amount of time and resources for even a single planet, which makes it essential to prioritize observations by those most-likely to have detectable life. here we present a statistical method to estimate the probabilities that specific exoplanets have been continuously in the habitable zone of their host stars for more than 2 billion years, the approximate time it took life on earth to significantly increase the oxygen content of the atmosphere. we introduce the use of statistics of an ensemble of 3d planetary general circulation models to estimate these probabilities, replacing prior 1d model estimates. | continuous habitable zones: pairing a gcm and bayesian framework to predict habitable zone evolution |
observations from the kepler and k2 missions have provided the astronomical community with unprecedented amounts of data to search for transiting exoplanets and other astrophysical phenomena. here, we present k2-288, a low-mass binary system (m2.0±1.0; m3.0±1.0) hosting a small (rp = 1.9 rearth), temperate (teq = 226 k) planet observed in k2 campaign 4. the candidate was first identified by citizen scientists using exoplanet explorers hosted on the zooniverse platform. follow-up observations and detailed analyses validate the planet and indicate that it likely orbits the secondary star on a 31.39-day period. this orbit places k2-288bb in or near the habitable zone of its low-mass host star. k2-288bb resides in a system with a unique architecture, as it orbits at >0.1 au from one component in a moderate separation binary (aproj approximately 55 au). additionally, its estimated size straddles the observed gap in the planet radius distribution; planets of this size occur less frequently and may be in a transient phase of radius evolution. k2-288 is the third transiting planet system identified by the exoplanet explorers program and its discovery exemplifies the value of citizen science in the era of kepler, k2, and the transiting exoplanet survey satellite. | k2-288bb: a small temperate planet in a low-mass binary system discovered by citizen scientists |
to search for life in the galaxy, nasa's exoplanet exploration program (exep) guides the development of technology that enables or enhances the direct imaging and spectral characterization of earth-like exoplanets in the habitable zone of sun-like stars with future space observatories. among the technologies needing advancement will include coronagraphs and starshades, detectors, mirror technology, and wavefront sense and control systems. we present an overview and recent results from exep's technology activities. | direct imaging of exo-earths: technology investments by the nasa exoplanet exploration program |
many future missions are planned to monitor, discover, and characterize extrasolar planets orbiting the habitable zones of nearby stars. thus there is a need to understand the habitability limits and observability of these planets using atmospheric and climate models of various heritages. prior work using 3d general circulation models (gcms) to explore habitable zone planets typically do not include chemistry-climate interactions. in this study, we use a 3d global chemistry-climate model (ccm) to investigate the atmospheres of rocky exoplanets across a range of host star spectral-types, total stellar fluxes, and planetary rotation periods. we find that our results depend critically on the stellar uv activity and atmospheric circulation, indicating the need to constrain these parameters using future ground- and space-based satellite surveys. | coupled 3d chemistry-climate simulations of moist greenhouse terrestrial planets: water-loss and spectroscopic observability |
gj 667cc is a nearby (23.6 lt-yr) super-earth that orbits within the habitable zone (hz) of its ~m2v host star and has earth-like properties: (m- $\sin (i)\sim 4.1\,{m}_{\oplus };$ r ~ 1.7 r ⊕; t eq (a = 0.3) ~ 277 k). the age of the star/planet is poorly constrained at 2 ≳ gyr. age is crucial in evaluating the potential for complex life. we determine an age of ~6.10 ± 2.2 gyr from our recent age-rotation relations. this is compatible with its low metallicity ([fe/h] ~ -0.59 ± 0.1 dex), weak coronal/chromospheric x-ray/fuv emissions, and space motions. we also determine the photodissociating/photoionizing x-ray-uv irradiances from chandra and rosat x-ray and hst/fuv observations. like most hz-planets hosted by m-dwarfs, these irradiances are much higher than earth's and are detrimental to the planet's atmosphere retention, water inventories, and habitability. a strong geomagnetic field could mitigate atmospheric erosion, permitting life to develop. | super-earth gj 667cc: age and xuv irradiances of the temperate-zone planet with potential for advanced life |
oxygen-rich atmospheres cannot arise without hydrogen escape. free oxygen is necessary for advanced life. this presentation maps out the region of exoplanet parameter space inside the conventional habitable zone in which oxygen and advanced life will not be found. these predictions should be testable sometime in the next 100 years. | limits to creation of oxygen-rich atmospheres on planets in the outer conventional habitable zone |
artificial mega-structures (ams) constructed by advanced civilizations can be imaged in the reflected light using an inversion technique called exoplanet surface imaging (epsi). ams can be of some "recognizable" shape and/or have a homogeneous albedo. examples of possible ams include low albedo photovoltaic systems for harvesting stellar energy and high-albedo installations for redirecting stellar light. they can be constructed on the planetary surface or in the near-planetary space. in addition, various geoengineering projects may be constructed on a global scale for optimizing planetary resources. such ams are most probably "geostationary" and, therefore, can be imaged using reflected light curve inversions. when data are acquired in several spectral bands, the nature of ams can be deduced from composite images and spectral information of resolved structures. we have simulated examples of exoplanets with high and low albedo ams and geoengineering structures imbedded in a natural environment similar to the earth and demonstrated the possibility to detect them in resolved images of exoplanets. this technique requires high-precision photometric data which can be delivered by currently planned hybrid telescopes-interferometers exolife finder (elf) and colossus. they will allow obtaining resolved albedo maps of earth-size planets in habitable zones of thousands stars in the solar neighborhood. | imaging of technosignatures on exoplanets |
the advent of the next-generation space-based telescopes jwst and luvoir/habex/ost and the recent detections of nearby stellar systems with terrestrial planets in or near the habitable zone (e.g., trappist-1, proxima centauri, gj 1132, lhs 1140) motivate detailed study of the observability of atmospheric signatures of terrestrial exoplanets. in this work, we analyze a large suite of numerical atmospheric circulation models to simulate observations with these future space telescopes. we focus on the effect of clouds on these simulated observations, in order to quantify the effect clouds have on the observability of molecular features. we find that clouds are abundant in the atmospheres of planets orbiting small, cool m dwarf stars. as a result, clouds greatly mute the signature of water vapor in the atmospheres of slowly rotating planets orbiting m dwarf stars. the effect of clouds is especially large in observations of the atmospheric transmission of stellar radiation. we predict that it will be challenging for jwst to observe water features in the atmospheres of planets orbiting m dwarf stars in transmission. | the effects of clouds on observable properties of terrestrial exoplanets: results from a large suite of gcms |
since the first discovery in 1995, data for over 5300 exoplanets have been documented in the nasa archive, revealing a vast diversity. identifying life-enabling analogs of the earth among this rapidly expanding catalog is of major interest. the stability of liquid water at the planetary surface defining the concept of the habitable zone (hz) around the host star, may be necessary for the emergence of life as we know it but not sufficient. the practically constant atomic ratio nitrogen:phosphorous = 16:1 in oceanic surface layers of our planet earth was discovered by redfield in 1934. it corresponds to phytoplanktonic biomass in suspension and appears optimal to fertilize phytoplankton development and therefore the food pyramid of marine life. loladze and elser have shown that it corresponds to a homeostatic protein:rna ratio and is therefore "rooted in the stoichiometry of the foundational structures of life." i show that according to the recent theory of the chemical differentiation of planets, this optimal ratio is also an intrinsic chemical property of our planet earth uniquely determined in the solar system by its average orbital radius. on that basis, i propose a criterion of fertility within the hz of a stellar system, which when applied to screen the public database allows us to sort out an extended list of up to 74 earth analogs. the latter and its future extensions could provide priority targets for focused detection techniques. | screening earth analog exoplanets on the basis of a predicted nitrogen over phosphorus ratio |
in this study, we perform a parametric analysis of how planetary obliquity (or axial tilt) affects the atmospheric escape rates from magnetized exoplanets. we focus on two distinct examples owing to their astrobiological relevance: an earth-like planet around a sun-like star and an earth-like planet around a late m-dwarf (such as the well-known trappist-1). we found that planetary obliquity has small effect on the atmospheric loss rate of earth-like planets orbiting around sun-like stars, but it is not the case for earth-like planets in the habitable zone of m-dwarfs. | roles of (exo)planetary obliquity on atmospheric escape |
the catalog of stellar evolution tracks discussed in previous work is meant to characterize exoplanet host-stars for follow-up observations with missions like jwst. however, the utility of the catalog has been predicated on the assumption that we would precisely know the age of a specific host-star; in reality, it is unlikely that we would be able to accurately measure the real age of a given system. ultimately, we must rely on the predictions of accurate stellar evolution models, as well a consideration of measured observable data (stellar mass, composition, orbital radius of an exoplanet) in order to create a statistical framework, to identify candidate systems for follow-up characterization. we describe development of a statistical approach to broadly constrain long-term planetary habitability by evaluating the likelihood that at time of observation, a star would have a planet in the 2-billion-year (2 gy) continuously habitable zone (chz2). additionally, we discuss how to use existing observational data (i.e. data assembled in the hypatia catalog and the kepler exoplanet host star database) for comparison to the catalog of stellar models. | a flexible bayesian framework for assessing habitability with joint observational and model constraints |
the james webb space telescope (jwst) is predicted to make pioneering observations of terrestrial exoplanet atmospheres, such as the system of seven earth-sized planets transiting trappist-1, which will include transmission spectroscopy of potentially habitable worlds. uncovering whether or not exoplanets in the habitable zone (hz) of these late m dwarf stars offer suitable environments for sustained surface liquid water is a crucial question, tied to the intrinsic prevalence of life in the universe. however, m dwarf hz exoplanets may face barriers to habitability, and if these hz planets are currently habitable, it remains unclear if jwst will have the precision to confirm this, or if robust habitability assessment will be the domain of next generation telescopes. we explore this question and find that although it should be possible to detect terrestrial atmospheres for all seven trappist-1 planets with jwst, water absorption features in the transmission spectrum of trappist-1e, which would hint at habitability, are difficult to detect due to the presence of clouds and the concentration of water vapor in the lower atmosphere where transmission spectroscopy cannot readily probe. we demonstrate how next generation space telescopes that can directly image reflected light or thermal emission can better constrain exoplanet habitability. | trappist-1 and beyond: strategies for characterizing terrestrial exoplanets and their habitability |
this table lists the orbital parameters of 230 companions detected in our work. the systems are sorted by the host names listed in the first column, while the companions in each system are sorted by their orbital periods. for companions with orbital periods of less than 1000-days, the inclination is not given and the mass reported in this table is mcsini. for companions with mass higher than 75 mjup, we use capital letters to label them. among the parameters, the directly inferred parameters are p (orbital period), k (rv semi-amplitude), e (eccentricity), ω* (argument of periastron), i (inclination), and ω (longitude of ascending node). the derived parameters are tp (periastron epoch), mc (companion mass), and a (semimajor axis). the dynamical stabilities of earth-like planets in the habitable zones (hzs) of their host stars are determined numerically, as described in section 7. the median and 1σ quantiles (i.e., the 16% and 84% quantiles) are used to measure the uncertainty of each parameter. (2 data files). | vizier online data catalog: 3-d selection of 167 sub-stellar companions (feng+, 2022) |
thousands of exoplanets have been uncovered by observations made by the kepler space telescope, where a subset of these worlds is shown to have additional, more distant stellar companions. only recently has the habitable zone, region where liquid water could exist on a rocky planetary surface, been modified to include the impact an earthlike planet would experience under the reign of two suns, like α centauri ab. the first order concern for such planets lies within orbital stability, where planets can evolve onto trajectories that quickly lead to either destruction or isolation relative to astrobiological timescales. an earthlike planet that can stably orbit within the habitable zone can face an additional challenge through the evolution of its spin axis or obliquity variation. we investigate the obliquity evolution of an earthlike planet within the habitable zone of α centauri b, where the spin axis can be perturbed by neighboring terrestrial analogs and α centauri a on timescales relevant to climate stability. | obliquity variations and habitability in alpha centauri ab |
the discovery of exoplanets has altered our understanding of the universe. but, for the planets to show the possibility to harbour life in it or have biosignatures, it must have optimum physical, biological, geological and chemical conditions. there are two types of indicators of habitability: direct and indirect. the former indication is the presence of water and its stability on the surface of the planet. thus, the reflection from the waterbody will lead to 'glint'. polarization of light is another alternative method to find water. the reflection, emission of radiation help us to characterize habitable zones. indirect methods include the presence of co2 and water vapour in the atmosphere, size of the planet and extent of axial tilt. the presence of magnetic fields and satellites revolving around the planet also play an important role. in this review article, we aim to provide a comprehensive explanation to the researches done till date to characterize habitable zones for exoplanets. the methods devised to retrieve results will also be discussed. future prospects, the voids which could be amended are also elaborated. this could give cosmological research a new dimension, demonstrating that life is not limited to our planet. | identifying exoplanets' potentiality for life in habitable zones: giving new dimension to cosmological studies |
we establish a theoretical framework for the direct detectability of exoplanets in the mid-infrared based on currently available models. the spectra of temperate (t~300k) exoplanets peak in the mid-infrared, resulting in the maximized contrast to their host star. compared to previous observations in the near-infrared, this wavelength range enables colder (and hence older) exoplanets to be directly imaged, including habitable-zone planets around nearby stars. of the 81 closest bafgk type stars, we select 72 with well-established ages. we then identify the most promising candidates for near-term ground-based observations. we prioritize targets based on a star's luminosity, age, and the properties of known exoplanets in the system. we then predict the most likely planet types to be detected based on anticipated instrumental sensitivity limits combined with known exoplanet demographics. putting this together, we recommend observation times needed for the detection of different types of habitable-zone exoplanets, including both mature habitable-zone exoplanets around nearby stars, as well as moderately young giant planets around more distant a-type stars. | the direct mid-infrared detectability of habitable-zone planets |
rocky exoplanets are indicated to be common in the galaxy. future instruments including the james webb space telescope, high resolution ground-based spectrographs, and direct imaging missions are poised to unlock the nature of planets orbiting the circumstellar habitable zones of nearby stars. however, interpretation of remotely sensed atmospheric data will require understanding a planet's signals in context, i.e., understanding interactions of atmospheric chemistry, dynamics, and thermodynamics, within the space weather environment and electromagnetic radiation regime. in this talk, i will describe results from my dissertation work and the first studies that couple 3d photochemistry with climate dynamics to simulate terrestrial exoplanet atmospheres. specifically, my work employs 3d numerical modeling techniques in conjunction with observed data of stellar properties to explore the habitability and observational prospects of oxygen-rich exoplanets around g, k, and m-stars. these results confine the degree of spatial heterogeneity of key biosignature trace gases on slow-rotators, investigate the dynamics and onset of moist greenhouse stratospheres of inner edge habitable zone planets, present the flare-driven atmospheres on magnetized and unmagnetized worlds. these results will be applicable to potentially habitable systems such as proxima centauri, lhs 1140, trappist-1, and toi-700. | chemistry-climate dynamics of warm rapidly and slowly-rotating exoplanets |
water is critical to life as we know it on earth. so naturally, finding evidence of liquid water on a planet in its stars habitable zone is extremely relevant to searches for extraterrestrial life. thus far, weve only discovered water vapor in the atmospheres of massive, short-period gas giants but new observations of sub-neptune k2-18b have now changed that.hubble has its eye on youthe kepler spacecraft was a planet-finding expert, using the transit method to identify thousands of exoplanets and even more exoplanet candidates. k2-18b is one of keplers more notable finds, a sub-neptune orbiting within the habitable zone of its star. fits to k2-18bs white-light curve (top) and an example spectral light curve (bottom) with data points from the hubble observations overplotted. the white-light curve considers points across all available wavelengths while the spectral light curve considers points across a specific wavelength range. the wavelength range for each plot can be seen in the lower right. [adapted from benneke et al. 2019]with an orbital period of 33 days and an m dwarf (k2-18) as its host, k2-18b receives about as much radiation as the earth does. it occupies an odd space in exoplanet demographics, being fairly large and massive but still close to its star. this makes k2-18b a good target for atmospheric studies.for exactly that purpose, a team of scientists led by bjrn benneke (universit de montral) used the hubble space telescope to observe nine transits of k2-18b over three years and they found something interesting!characterize, characterize, characterizeto begin, benneke and collaborators re-determined the properties of k2-18b and its host. aside from hubble data, the team also used new and archived observations of k2-18b taken by kepler and the spitzer space telescope (rest well, good instrument) along with gaia data of k2-18.one result of this analysis was that the measured radius of k2-18b was bumped up from 2.29 earth-radii to 2.61 earth-radii. a previous study gives the planets mass as nearly nine earth masses which means that k2-18b is very much a sub-neptune and not a super-earth as had been thought earlier.a high chance of cloudsbenneke and collaborators then analyzed the features of the observed transits in k2-18s spectra. the spectra in this study are known as transmission spectra, since they are spectra of light that is emitted by the host star and passes through the planets atmosphere. benneke and collaborators produced a single spectrum for k2-18b based on eight observed transits.the transmission spectrum of k2-18b based on observations (the data points) and modeling. the red line indicates the best fit to the model, the blue line indicates the median fit, and the shaded blue areas indicate confidence intervals. the water feature is located at 1.4 m. [benneke et al. 2019]the most exciting thing to come out of k2-18bs spectrum is statistically significant evidence of water! models suggest that this water exists in a hydrogen-rich atmosphere and that its presence in the spectrum is caused by clouds. other gases detected in the spectrum include nitrogen, carbon dioxide, and methane.so if there are clouds, could there be rain? possibly! due to k2-18bs close orbit around a cool star, the incoming sunlight and atmospheric temperature for this planet are similar to earths. benneke and collaborators demonstrate that its likely that water vapor could therefore condense into liquid droplets in k2-18bs atmosphere.the james webb space telescope will help us probe k2-18bs atmosphere further, answering some of the questions this study has raised.citationwater vapor and clouds on the habitable-zone sub-neptune exoplanet k2-18b, bjrn benneke et al 2019 apjl 887 l14. https://doi.org/10.3847/2041-8213/ab59dc | today's forecast for k2-18b: cloudy with a chance of rain? |
habitable zone planet finder (hpf) is a high-resolution (r~55000) nir spectrograph on the 10m hobby-eberly telescope (het), covering the doppler-information-rich z, y, and j bands from 810 to 1280nm. hpf has an nir laser-frequency comb (lfc) calibrator, which has been shown to enable ~20cm/s calibration precision in 10minute bins and 1.53m/s rv precision on-sky on barnard's star over an 3 month baseline. in this paper, we extend this baseline to 856 days. (1 data file). | vizier online data catalog: radial velocity follow up of barnard's star with hpf (lubin+, 2021) |
one of the fundamental properties of a-type and f-type stars is that they rotate rapidly throughout their lifetimes. unlike g/k/m dwarfs, these stars do not slow their spin rates over time, but rather maintain their high primordial rotation rates over the course of their main sequence lifetimes. rapid stellar rotation can dramatically change a star's luminosity and spectral energy distribution and, therefore, can impact the habitability of any surrounding planets. a-type and f-type stars commonly rotate near their breakup speeds, which causes two effects relevant to planet habitability. first, these stars flatten into oblate spheroids with shorter polar radii and elongated equatorial radii. the stars' distorted shapes change their total sizes and luminosities, which directly changes the total energy received by a planet. second, rapid rotation induces a pole-to-equator temperature gradient on the surface of these stars (known as gravity darkening). their polar regions can be up to several thousand kelvin hotter than their equatorial regions, which impacts the stars' total luminosities and their spectral energy distributions. in this project, we investigate how rapid rotation in a/f stars affects the locations of their habitable zones. we find that in general, rapid rotation causes the habitable zone to reside closer in than for a non-rotating equivalent star. we also find that gravity darkening dramatically reduces a/f stars' uv emission, which combats the common assumption that a/f stars emit too much uv light for habitable worlds. overall, we determine that rapid stellar rotation has important consequences for the overall habitability of a system and must be accounted for both when modeling exoplanet environments and in observation of planets around high-mass stars. | the habitable zones of rapidly rotating main sequence a/f stars |
to date, we have discovered roughly 5000 exoplanets, 14 of which orbit around two stars. both single-star and binary-star systems can have planets in their habitable zones: the planets simply need to be at the correct distance from the star(s) to have a temperate climate. however, there is a distinct difference between a single-star and binary-star scenario which severely limits the possibility of circumbinary planets being located in the habitable zone: the critical instability radius. interior to this radius, the time-varying gravitational perturbations from the two stars prohibit a planet from having a stable orbit. while both the habitable zone and the critical instability radius depend on the stars' masses, separation, and eccentricity, their dependencies on these parameters are different, and thus grow at different rates as these parameters increase. therefore, it is possible for the critical radius to exceed the outer (cold) edge of the habitable zone. if this happens, then it is impossible for the binary to host any planets in the habitable zone. we use the holman and wiegert (1998) formulation for the instability radius, the eggl et al. (2020) calculation of the habitable zone of a binary star (based on the kopparapu et al. (2014) determination of the habitable zone boundaries), and stellar isochrones to get the stellar radii, temperatures, and luminosities for a given stellar mass. with these, we show that the instability radius does indeed prohibit the existence of planets in the habitable zone around many short-period binary star configurations. this "uninhabitability" is particularly important for low mass stars, and this potentially puts a significant constraint on the number of habitable-zone circumbinary planets in the galaxy. | limits on circumbinary planet habitable zones |
<strong>the habitability of the exoplanet has a strong dependence on the stellar activity of the host star. m-dwarf stars constitute a large fraction of the stellar population, therefore they are of primary interest in searching potentially habitable exoplanets. it is easier to detect planets around low-mass stars since they have effectively smaller radii and low temperatures due to which we can easily detect planet transits. the stellar flares from the host star may play a crucial role in triggering prebiotic chemistry on the exoplanet, whereas large frequency flares and cmes would lead to atmospheric erosion and ozone depletion. the stellar flares sometimes are associated with coronal mass ejections, which may directly impact the exoplanetary atmospheres and affect habitability. here, we present a case - study of the flaring activity of an m-dwarf star and its impact on its orbiting exoplanet in the habitable zone. we utilized the light-curve data from tess. we used the altaipony python-based package to identify flares. we further analyze and quantify the flare activity to understand its possible impact on the exoplanet environment around the habitable zone.</strong> | investigating the impact of space weather on the habitability of exoplanet around m-dwarf star as a case study using tess observations |
i present an analysis of starspot modulation and flares in the tess light curves of 112 m dwarfs with a tess magnitude <=11.5 that are listed in the tess habitable zone star catalog (hzcat, kaltenegger et al. 2019). understanding the magnetic activity of these potential exoplanet host stars is crucial for planet characterizations since an active, flaring host star might impact planetary habitability. our sample is of particular interest as it comprises stars in the hzcat with sufficiently long tess observation times for planets to be detected in the entire habitable zone. we detected more than 2000 optical flares in this sample and i discuss the relation of flares with spectral type and rotation period. i also present flare frequency distributions and the flux reaching the habitable zone during flare events. | rotation and flares of m dwarfs with habitable zones accessible to tess |
the search for life in the universe has primarily been limited to the surface of terrestrial bodies and the oceans of icy worlds. however, in the age of the james webb space telescope and the astro2020 decadal survey recommendations, where exoplanets are increasingly becoming an important area of research, it is necessary to further characterize the types of environments where extraterrestrial life may be found. on earth, we see life in many forms and locations, including microbial life deep in the crust. the deep biosphere is estimated to be one-tenth to one-third of earth's total current biomass and is dominated by autotrophic and heterotrophic metabolisms including methanogenesis and sulfur cycling. in terrestrial bodies far from their host star, stellar flux is lower, and after the first few billion years, the heat of accretion begins to dwindle. this leaves radioactive decay as a major source of long-lived thermal energy that exists independently from a planet's orbit. the decay process can also act as a catalyst for radiolysis, which can provide electron donors essential for metabolic processes to occur. we therefore propose subsurface pore space on rocky planets as another potentially habitable zone to consider in the search for life. we utilize a pre-existing thermal evolution model to constrain the history of a hypothetical earth-sized exoplanet in a mars-like orbital radius over the course of 10 gyr given different endmember budget scenarios of radioactive heat-producing elements and construct a box model to consider potentially habitable zones in the subsurface. we show that the major limiting factor to habitability in the subsurface rests with porosity and explore whether viscous relaxation processes and water-rock interactions close all possible deep pore spaces at a rate faster than fissures and pore spaces can be created or maintained. additionally, we show that if porosity is not a limiting factor, energy from rock and water interactions could produce oxidants and reductants that could support and sustain metabolisms in the deep subsurface of terrestrial planets over long periods of geologic time. | liquid water stability zones on super-mars exoplanets: implications for subsurface astrobiology |
the question of what causes global glaciations to occur on earth-like planets is of great importance to habitability and climate evolution. earth itself has a complex climate history consisting of long stretches of apparently clement conditions in the archean, a stable proterozoic climate punctuated by major intervals of glaciation at the beginning and end, and fluctuation between warm and cool climates in the phanerozoic without any further global glaciation events. deterministic models of the carbonate-silicate cycle on earth-like planets do not predict such a sequence of transitions, instead yielding either permanently clement conditions, or limit-cycle behavior only for planets receiving low stellar fluxes.in this work, we take a stochastic approach to modeling atmospheric co2 evolution. we present a simple model that assumes an imperfect co2 thermostat, such that pco2 follows a bounded random walk around a mean value that alone would maintain clement climate conditions. because less co2 is required to keep the planet warm as solar luminosity increases, the model predicts an increase in climate variability with time. this implies that unless some mechanism is present to decrease co2 variance as stellar luminosity increases, the climates of earth-like planets should become increasingly unstable as they approach the inner edge of their systems' habitable zones. implications for exoplanets are discussed, and the model is then applied to the specific problem of earth's climate history. in particular, the potential role of the biosphere in forcing and/or inhibiting snowball transitions both in the phanerozoic and earlier in earth history is discussed. | stochastic modeling of co2 fluctuations and snowball transitions on earth and other planets |
a planets climate can be strongly affected by its orbital eccentricity due to variation in incident stellar flux. here we use a time-dependent one-layer energy balance model to resolve the temperature evolution on eccentric planets located near the inner edge of the habitable zone. we consider energy storage by the atmosphere, ocean, and land, as well as latent heat. we find that the ocean plays an essential role in climate stability by suppressing surface temperature at periastron. moreover, atmospheric heat capacity and latent heat can stabilize the climate of eccentric planets even if they have an ocean only about 1m deep. our model provides a simple and fast way to examine the habitability of eccentric exoplanets that move inside and out of the inner edge of the habitable zone. | heat capacity and latent heat can stabilize climate of eccentric planets that pass inside the inner edge of the habitable zone |
the proximity of planets to the host star in habitable zones around m-dwarfs makes them highly susceptible to stellar activity. based on observations from kepler, gaia, and tess, it is now established that m-dwarfs emit flares at extremely high rates. the energy emitted in some of these flares could reach up to 10e38 ergs and could have a significant impact on the planetary atmosphere. enhanced flux of xuv from flares would heat the upper atmosphere and initiate photochemical reactions on close-in planets, leading to increased atmospheric escape rates. stellar energetic particles (seps) can produce secondary particles propagating down to the planetary surface, and enhancing radiation dose, which can disrupt potentially hospitable conditions on planets. we model the impact of flaring on atmospheric escape and surface radiation dose of potentially habitable planets around m-dwarfs on myr-gyr timescales. taking particle spectra from 70 major solar events (observed since 1956) as proxy, we use the geant4 monte carlo model to simulate sep interactions with exoplanetary atmospheres, and we compute surface radiation dose. we have demonstrated that radiation dose varies significantly with charged particle spectra and an event of a given fluence can have a drastically different effect depending on the spectrum. our results show that radiation dose can vary by about 5 orders of magnitude for a given fluence. in terms of shielding, we found that atmospheric depth (column density) is a major factor in determining radiation dose on the planetary surface. radiation dose is reduced by 3 orders of magnitude corresponding to an increase in the atmospheric depth by an order of magnitude. we found that the planetary magnetic field is an important but a less significant factor compared to atmospheric depth. the dose is reduced by a factor of about 30 corresponding to an increase in the magnetospheric strength by an order of magnitude. we also find that xuv-induced hydrodynamic escape can cause significant atmospheric loss on planets around m-dwarfs, especially in the early stages of their evolution. we discuss the implications of these findings in constraining the habitability of terrestrial exoplanets around m-dwarfs. | stellar flares and planetary habitability around m-dwarfs |
we present the occurrence rates for rocky planets in the habitable zones (hzs) of main-sequence dwarf stars based on the kepler dr25 planet candidate catalog and gaia-based stellar properties. we provide the first analysis in terms of star-dependent instellation flux, which allows us to track hz planets. we define {eta}{earth} as the hz occurrence of planets with radii between 0.5 and 1.5 r{earth} orbiting stars with effective temperatures between 4800 and 6300k. we find that {eta}{earth} for the conservative hz is between 0.37-0.21+0.48 (errors reflect 68% credible intervals) and 0.60-0.36+0.90 planets per star, while the optimistic hz occurrence is between 0.58-0.33+0.73 and 0.88-0.51+1.28 planets per star. these bounds reflect two extreme assumptions about the extrapolation of completeness beyond orbital periods where dr25 completeness data are available. the large uncertainties are due to the small number of detected small hz planets. we find similar occurrence rates between using poisson likelihood bayesian analysis and using approximate bayesian computation. our results are corrected for catalog completeness and reliability. both completeness and the planet occurrence rate are dependent on stellar effective temperature. we also present occurrence rates for various stellar populations and planet size ranges. we estimate with 95% confidence that, on average, the nearest hz planet around g and k dwarfs is ~6pc away and there are ~4 hz rocky planets around g and k dwarfs within 10pc of the sun. (1 data file). | vizier online data catalog: 117 exoplanets in habitable zone with kepler dr25 (bryson+, 2021) |
many will say that other stars' sizes are about the same as the sun's. that is not quite true. the sun is known to be a yellow dwarf. however, the most common stars are red dwarfs, the smallest and coolest stars on the main sequence, a continuous band of stars that appears on plots of stellar color versus brightness. compared to the sun, they are lower in luminosity, making them harder to detect, as the tess gathers the flux of the stars, directly proportional to the luminosity, the fixed brightness of a star. however, their small size offers key advantages to searching for earth-like planets. for instance, the habitable zone is closer to the star, making transits easier to detect due to frequency. this work will explore the red dwarfs that are in jen winter5's catalog that we identify to have transiting planets from and present the calculated parameters of the star systems. | exploration of exoplanets transiting red dwarfs within 50 ly of the sun |
trappist-1 has 7 earth-size exoplanets in close proximity that are in resonance. n-body simulations show that these planets can experience relatively large eccentricity and mean motion variations. most studies and discussions of these planets' climates assume these planets are always tidally locked, as are many other m-dwarf, habitable-zone planets with short orbital periods. however, large eccentricity and mean motion variations in trappist-1 can lead to sporadic rotation of the planets with synodic rotation periods on the order of years. i conduct n-body simulations using rebound to retrieve the evolution of orbital parameters in the trappist-1 system. the orbital parameters are used to produce a continuous equation of motion which describes the evolution of the substellar longitude and, thus, the hemisphere illuminated by the star. i will discuss the nature of these planets that transition between tidally-locked and slow rotation states, the origin of these transitions in trappist-1's unique orbital dynamics, and the impact on climate using ebm models. | day 'n' nite: habitability of tidally-locked planets with sporadic rotation |
at the end of the k2 mission, just before it ran out of fuel, campaign 19 was conducted but cut short which resulted in an extremely short set of observations. the small amount of data recorded makes discovering exoplanets within the campaign 19 dataset difficult. to our knowledge, no other teams have been able to find exoplanets in the campaign 19 dataset yet. we are using this dataset to search for single transits and characterize new planetary systems. despite the short duration of the dataset provided from k2, we are still able to determine important characteristics of the star and planet, such as radius, mass, period, and the likelihood of the planet residing in the habitable zone. we are complementing the k2 data with observations from ground-based telescopes. our work will help ensure that all k2 data is utilized to its full potential so that no planets are left behind. these techniques can be adapted and applied to single transit systems recorded by other telescopes such as tess. | no planets left behind: single transit candidates from k2 campaign 19 |
we present the validation of two exoplanets orbiting m dwarfs identified by tess. both planets are mini-neptunes orbiting nearby m dwarfs, making them promising prospects for atmospheric characterization with the james webb space telescope (jwst). we validated the planetary nature of both using adaptive optics (ao) imaging, ground based photometry, and radial velocities. radii and orbital ephemerides were refined using a combination of tess, the 3.5 m apache point telescope's optical ccd camera arctic, and the 0.6 m telescope at the red buttes observatory (rbo). radial velocities were obtained using the near-infrared habitable zone planet finder (hpf) spectrometer on the 10 m hobby-eberly telescope, which enabled us to place upper limits on the masses of both transiting planets. the first planet has a radius between that of earth and neptune, placing it in a sparsely-populated region of parameter space considering its host star's temperature, as planets of its size are quite rare around late m dwarfs. the second planet is an excellent candidate for the possible detection of a unique type of biosignature made by combining hydrogen and nitrogen to make ammonia. | constraining the masses of two mini-neptunes orbiting m dwarfs using hpf |
the discovery and characterisation of new extra-solar planets (exoplanets) is ongoing, but to date only a handful of low-mass planets have been found orbiting in the habitable zone of sun-like stars. the next generation of major facilities (e.g. tess and plato) aimed at the systematic search for earth-like planets orbiting solar-like stars will be operational in the coming years, and some of the planets they will find may orbit stars close enough for atmospheric characterisation, including the possible detection of bio-signature gases. studies on the formation and evolution of the earth reveal that an earth-like habitat is characterised by a n-dominated atmosphere and could be detected by measuring the relative atmospheric abundances of n, o, c, and h (noch). however, n, which is the main fingerprint of an earth-like habitat, is extremely difficult to detect and may be possible only in the ultraviolet, a wavelength range that has not been studied for low-mass exoplanets. before starting the search for bio-signatures with future facilities (e.g. elts, luvoir), we need to explore our capabilities to detect earth-like habitats. here, we present several synthetic transmission spectra for the earth's atmosphere, for the wavelength range 915 to 11000 å, at a spectral resolution of r = 100,000. we focus on both atomic and molecular features, and discuss the detectability of n. | teth: towards extra-terrestrial habitats |
spaceborne nulling interferometry in the mid-infrared waveband is one of the most promising techniques for characterizing the atmospheres of extra-solar planets orbiting in the habitable zone of their parent star and possibly discovering life markers. one of the most difficult challenges is the control of a free-flying telescope spacecraft moving around a central combiner to modulate the planet signal. moreover, the entire array must be reconfigured regularly to observe different celestial targets, thus increasing the risk of losing one or more spacecraft and aborting the mission before its normal end. this paper describes a simplified optical configuration in which the telescopes do not need to be rotated and the number of necessary array reconfigurations is minimized. it allows efficient modulation of the planet signal, using only making use of rotating prisms or mirrors located in the central combiner. the general principle of a nulling interferometer with a fixed telescope array is explained. mathematical relationships were established to determine the planet modulation signal. numerical simulations were performed for three different arrangements of collecting telescopes. they confirmed that nulling interferometry in space did not require a rotating telescope array. | space borne nulling interferometry with non-rotating telescope arrays |
one-dimensional (1d) modeling from a horizontally averaged perspective can oftentimes greatly simplify problems in atmospheric and oceanic sciences and thus capture leading-order physics. meanwhile, 1d numerical models have great advantages such as numerical stability and time efficiency, hence they are widely used to gain insights into complex problems. however, oversimplification by 1d models may cause failures in finding solutions, revealing novel phenomena, and discovering scaling laws in the three-dimensional (3d) real world, and those are when 3d thinking proves its value. also, the rise in computational power has allowed investigations using 3d numerical models. this thesis discusses three examples of how 3d modeling transcends the limitations of 1d modeling and reveals new solutions, phenomena, and scalings in planetary atmospheres and earth's ocean.chapter 2 is focused on the dispersion of methane plumes on mars and how it can reconcile the discrepancy between observations. in the face of ostensibly inconsistent observational results of methane on mars, we adopt a novel approach—inverse lagrangian modeling in 3d space—to find the scenarios in which the inconsistency in the observations can be reconciled and locate the methane source. we find that the inconsistency between the results of the near-surface in situ methane measurements and the satellite remote sensing measurements can be reconciled if and only if an active methane emission hot spot is located in the immediate vicinity of the curiosity rover in northwestern gale crater, or unknown physical or chemical processes are rapidly removing methane.chapter 3 presents a novel phenomenon that could exist on exoplanets—self-sustained photochemical oscillations, which is only produced by 3d atmospheric models. we use a 3d, fully coupled, chemistry-radiation-dynamics model to simulate the ozone-nox-hox photochemistry in the atmosphere of a tidally locked earth-like exoplanet in the circumstellar habitable zone, and calculate the transmission spectra during transits. we find that under certain conditions, biological nitrogen fixation like the one on the earth can drive large-magnitude, self-sustained photochemical oscillations in the atmospheres of terrestrial exoplanets. the resulting large temporal variability in ozone abundance on exoplanets, if observed, may suggest a strong surface nox emission source, which could signal extrasolar life participating in the nitrogen cycle on exoplanets. fully coupled, three-dimensional atmospheric chemistry-radiation-dynamics models can reveal new phenomena that may not exist in one-dimensional models, and hence they are powerful tools for future planetary atmospheric research.chapter 4 uses a 3d fluid dynamics model to study the vertical exchange in the upper part of earth's ocean that potentially has great implications for the marine ecosystem. we develop scaling laws for the exchange rate between the surface ocean and the ocean interior which is critical to the rate of nutrient supply to phytoplankton near the ocean surface. these scaling laws could substitute the crude 1d parameterizations that are currently widely used in ocean models. we find that submesoscale turbulence energized by baroclinic instability in the ocean mixed layer can induce tracer exchange between the surface ocean and the ocean interior. various environmental physical parameters affect the exchange rate. the exchange is stronger where the ocean mixed layer is thicker, the richardson number (defined as the ratio of the squared buoyancy frequency to the squared vertical shear of the horizontal flow) of the thermocline is smaller, and the richardson number of ocean mixed layer is larger. the associated nutrient supply from the ocean interior to the surface ocean is also expected to be stronger under these conditions. | tracer transport in three dimensions: dispersion of methane on mars, coupled chemistry and dynamics on exoplanets, and submesoscale mixing in the ocean |
exoplanets residing close to their stars can experience evolution of both their physical structures and their orbits due to the influence of their host stars. for planets in or near the habitable zone, the evolving star's x-ray and ultraviolet (xuv) luminosity can drive atmospheric mass loss. simultaneously, the orbital properties of these planets may evolve over time due to the dissipation of energy via tides. together, these factors can impact the habitability and atmospheric structures of planets over time. in this presentation, we discuss how these factors affect the planets in the trappist-1 planetary system, using a combination of observational data and theoretical modeling. | atmospheric mass loss and tidal evolution in xuv irradiated exoplanets: the trappist-1 case study |
i will present the results of 3-d global climate model (gcm) simulations of steam (water-dominated) atmospheres, performed with the 'generic pcm' (formerly known as the lmd generic gcm). these simulations are aimed at reproducing the conditions of primordial surface water condensation and thus of the formation of oceans on rocky planets. the simulations reveal that clouds preferentially form on the nightside of young and hot telluric planets endowed with a water-rich atmosphere. these clouds thus have a net warming effect that effectively prevents the formation of surface liquid water oceans. this result has important implications for the past and present habitability of earth and venus (turbet et al. 2021, nature vol. 598) ; but not for mars, which has always received too little insolation for this process to actually play a role (turbet et al. 2020, icarus vol. 335). this result also has important implications for the habitability of exoplanets, and in particular tidally-locked planets orbiting around cool stars. our simulations predict that a large fraction of the rocky planets in the habitable zone (hz) have never been able to form oceans and therefore cannot be habitable today. this is because forming oceans is more difficult than keeping them liquid due to opposite cloud feedbacks finally, i will discuss ways to test this result observationally, in particular with the support of the upcoming jwst (james webb space telescope) observations of trappist-1 inner planets. | primordial ocean condensation on rocky planets |
warm jupiters provide a unique opportunity to better understand the formation and evolution of planetary systems. their atmospheric properties remain largely unaltered by the impact of the host star, and their orbital arrangement reflects a different, and less extreme, migrational history compared to close-in objects. warm jupiters are known to cover a wide range of eccentricities but it is unclear which are the dominant formation pathways to explain this observation. increasing the sample of long-period exoplanets with known radii is thus crucial. in this talk, i report the results of a survey set out to find transiting giants with orbital periods between 20 and 200 days. we selected 50 stars which show a single transit in one tess sector (27 day baseline) and followed them with ground-based photometric and radial velocity facilities (e.g. ngts, harps). after one year of observations, we report the detection and characterization of ten new transiting warm jupiters, increasing by 50% the number of known warm jupiters with precise masses and radii. we infer the metal enrichment of the newly discovered warm jupiters and explore their influence on the mass-metallicity correlation of giant planets. the growing sample of warm jupiters allows us to interpret these systems in terms of planet formation models. finally, these targets orbit bright stars and thus are ideal for follow-up studies of the planetary atmosphere and the system' spin-orbit alignment. this work is a stepping stone for plato, as identification and follow up of single transit events will be key in order to detect transiting earth-sized planets in the habitable zone of sun-like stars. | from tess single transits to well-characterized warm jupiters |
in the search for potentially habitable planets, terrestrial planets orbiting in stable, nearly circular orbits in the habitable zone (hz) of a sun-like star is the gold standard from which we derive comparative scenarios. the search for habitable planets, however, has revealed a plethora of different types of planets that can vary greatly from this basic template. from highly eccentric planets, giant planets, planets with low or high density, planets orbiting very active stars, or evolved stars, there is a wide variety of planets that have been found within the hz that have forced us to reconsider what a habitable world may be. here we will point out some of the highlights from our recent hz catalog paper (hill et al., in prep). this paper explores the demographics of the hz planets compared with the full catalog of planets from the nasa exoplanet archive and investigates how the unique properties of some of the hz planets may affect the habitability of such planets. | extraordinary outliers of the habitable zone catalog |
transiting exoplanets orbiting white dwarf stars, though rare, are incredibly valuable to probe the long evolution of planets, potential secondary planet formation, and as targets for atmospheric characterization. we present work to use the new 24-inch automated telescope at the van vleck observatory to search for such systems. our sample is drawn from the extensive gaia white dwarf list and modified to identify those stars that have heavy metals observed in their spectra. such "polluted" systems are evidence of recent accretion that abundance analyses show is planetary in origin. indeed, several such systems have recently been found to show evidence of transiting signatures of the tidally disrupted planetary material itself. we identify those that are visible from the van vleck observatory and have not already been observed in depth by other survey telescopes. as of now, 93% of the white dwarf stars in the latest gaia release have been observed for substantial periods of time by tess or kepler/k2. that leaves approximately 50 unobserved stars that also have polluted spectra. as part of the pilot study and the commissioning phase of the automated capabilities for the telescope, we focus on the star lb 1188 and can conclude with 100% confidence that lb 1188 does not have a transit with a period less than 5.3 hours and with 70% confidence does not have a transit with a period of <12 hours. we explore the future prospects of searching the remaining targets in our sample, and fully sampling periods in the 4-40 hour timeframe, which corresponds to the habitable zone for these low luminosity stars. we gratefully acknowledge the national science foundation's support of the keck northeast astronomy consortium's reu program through grant ast-1950797. | constructing a survey to find transits of polluted white dwarf stars using the new van vleck 24-inch telescope |
the abiogenesis zone is the zone in which a yield of 50% for the photochemical products is obtained, adopting the current uv activity as representative of the uv activity during the stellar lifetime and assuming a young earth atmosphere. although there are eight small exoplanets in this zone, none of them are very good candidates due to their size, except for maybe tau cet e. so far, not a single earth-sized planet has been discovered to be in both the habitable zone and the abiogenesis zone. | the occurrence of planets in the abiogenesis zone (screen 7) |
nasa's kepler mission is the source of more exoplanets than any other instrument, but the discovery depends on complex statistical analysis procedures embedded in the kepler pipeline. a particular challenge is mitigating irregular stellar variability without loss of sensitivity to faint periodic planetary transits. this proposal presents a two-stage alternative analysis procedure. first, parametric autoregressive arfima models, commonly used in econometrics, remove most of the stellar variations. second, a novel matched filter is used to create a periodogram from which transit-like periodicities are identified. this analysis procedure, the kepler autoregressive planet search (karps), is confirming most of the kepler objects of interest and is expected to identify additional planetary candidates. the proposed research will complete application of the karps methodology to the prime kepler mission light curves of 200,000: stars, and compare the results with kepler objects of interest obtained with the kepler pipeline. we will then conduct a variety of astronomical studies based on the karps results. important subsamples will be extracted including habitable zone planets, hot super-earths, grazing-transit hot jupiters, and multi-planet systems. groundbased spectroscopy of poorly studied candidates will be performed to better characterize the host stars. studies of stellar variability will then be pursued based on karps analysis. the autocorrelation function and nonstationarity measures will be used to identify spotted stars at different stages of autoregressive modeling. periodic variables with folded light curves inconsistent with planetary transits will be identified; they may be eclipsing or mutually-illuminating binary star systems. classification of stellar variables with karps-derived statistical properties will be attempted. karps procedures will then be applied to archived k2 data to identify planetary transits and characterize stellar variability. | kepler autoregressive planet search |
we propose to carry out a deep 50 ksec at teegarden's star with the chandra hrc-i. teegarden's star is a very close (d = 3.8pc) and a very late star of spectral type mv7, which is orbited by two earth-like planets in the habitable zone. the proposed hrc-i observation is deep enough to detect x-ray emission even for the minimally expected x-ray flux levels. if we fail to detect x-ray emission, a counter example to the minimal coronal flux paradigm would have been found, otherwise we expect to characterize the effects of stellar x-ray irradiation in two very nearby and close-in extrasolar planets (d = 0.026 au, d = 0.045 au). | x-ray emission from teegarden's star |
the esa plato mission will provide unprecedented data to study the diversity of planets orbiting up to the habitable zone of bright sun-like stars. plato will detect and characterise exoplanets using the transit method combined with ground-based radial velocity measurements, and study the host stars with asteroseismology. plato"s core observing sample consists of sun-like stars of v < 11. for statistical studies, plato will also monitor a large sample of sun-like stars with v < 13 and cool late-type dwarfs with v < 16. to benefit from plato"s advanced photometric capabilities, the general community will be invited to submit proposals on complementary science topics in the framework of a guest observer"s programme. the plato payload consists of four groups of six cameras each that overlap covering a total field of about 2150 deg2 with four different sensitivities. two additional cameras will observe the brightest stars (v < 8.5) in two-colours, and will be used as fine guidance sensor. plato is the third medium-class mission in esa"s cosmic vision programme, with a planned launch date in 2026. the satellite will operate in an orbit around the second lagrange point, l2. we will present the status of the mission science definition and performance, and of the satellite and ground-segment developments. | the plato mission: studying the diversity of exoplanets orbiting up to the habitable zone of sun-like stars |
we propose observations of 5 nearby (< 22 pc) stars: yz cet, eps eri, men, trappist-1, and hd 215152. together, they host a combined 20 earth-mass planets and 2 massive planets, with 6 planets located in the presumed habitable zones (hzs). the targets cover a stellar mass range from g to m-stars. while a star's bolometric luminosity is important, the high energy uv/x-ray stellar irradiation is a major driver of photochemistry, upper atmospheric heating, and atmospheric mass loss in exoplanets. for the 3 brightest, we will obtain temperatures and fluxes using acis-s. for t he faintest two, we will observe using hrc-s using both the "thin" and "thick" filters to measure the soft x-ray flux and to estimate the euv fluence. | x-rays in the prime of life: measuring the high energy dose to middle aged exo-earths |
editors note:in these last two weeks of 2020, 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.the interior and atmosphere of the habitable-zone exoplanet k2-18bpublished february 2020main takeaway:a team of scientists led by nikku madhusudhan (university of cambridge, uk) investigated the atmospheric and interior conditions of k2-18b, an exoplanet located ~124 light-years away that is roughly 2.6 earth radii in size and 8.6 earth masses in weight. they found that this planet could have the right conditions to support liquid water beneath its atmosphere on its surface.why its interesting:artists impression of a super-earth exoplanet orbiting its host. [jack madden/cornell university]most studies of potentially habitable worlds have focused on earth-like, rocky planets. k2-18b represents a different category: a super-earth thats between earth and neptune in its properties. madhusudhan and collaborators analysis demonstrates that the potential for habitable conditions arent restricted to earth-like planets larger, less dense bodies might also present the right conditions to support life.what kind of world k2-18b might be:madhusudhan and collaborators use observations of k2-18bsatmosphere and its bulk properties to constrain this planets structure. the resulting possible scenarios for k2-18b range from arocky world surrounded by an enormous hydrogen and helium atmospheric envelope, to a water world with a very thin atmosphere. future observations with observatories like the upcoming james webb space telescope should help us to learn more about this potentially habitable world.citationnikku madhusudhan et al 2020 apjl 891 l7. doi:10.3847/2041-8213/ab7229 | selections from 2020: habitability of a super-earth |
our solar system is just one of billions in the milky way, situated about half way from the galaxy's core to its edge, and nestled safely between a pair of spiral arms. out of those billions of planets, ours is the only one that we know to support life. this begs two questions. first, is our location in the galaxy especially suitable for life? second, if we want to find other life out there, where should we focus our search? in this dissertation, i contribute answers to both questions by seeking to better understand the boundaries of the milky way's galactic habitable zone (ghz), the place in the galaxy where habitable worlds are most likely to be found. we start in chapter 2 by introducing a novel method for finding the average height of surface features on exoplanets, a characteristic that influences a planet's habitability but was heretofore unknowable. we use elevation data for the rocky bodies in our solar system to simulate their transits in front of stars of different sizes. we provide a relationship between the scatter at the bottom of the resulting light curves and the so-called "bumpiness" of the transiting planet. in chapter 3, we zoom out from planets to get a better understanding of the dynamical and chemical evolution of the milky way, which are both crucial for constraining the galaxy's ghz. we use the extreme deconvolution gaussian mixture model to identify overdensities of stars in both velocity and action space, called moving groups and orbit groups, respectively. velocities and actions are calculated using data from the early third data release of the gaia mission. when we analyze the chemical abundance distributions of these moving and orbit groups with galah dr3 data, we find that using velocities alone to define moving groups, or even using velocities and actions together, yields an incomplete view of the underlying density distributions and their origins. our chemical analysis also confirms expected chemical evolution trends in the solar neighborhood. next, we explore the effects of stellar motion and galactic dynamics on the habitability of planets in different regions of the galaxy. in chapter 4, we use gaia dr2 data to calculate 3d galactocentric velocities for stars observed by the kepler spacecraft. we compare the velocities of confirmed kepler host stars to those of their non-host stellar twins and find that there's no relationship between stellar velocity and planet occurrence in the solar neighborhood. in chapter 5, we shift our attention to the milky way bulge, where stars are closer together and moving more quickly on more elliptical orbits than in the disk. we simulate the orbits of bulge stars and use a semi-analytical method to derive the rate of close stellar encounters. we find that roughly 8 in 10 bulge stars will come within 1000 au of at least 1 other star every billion years. half of these stars experience dozens of these encounters every gigayear. these encounters can have dramatic consequences for planets, and our findings strongly suggest that the milky way bulge is not the most suitable environment for life. in chapter 6, i share an overview of the science communication and outreach work i've done while in graduate school and explain how it's so closely tied to my research on ghzs. | why are we here?: constraining the milky way's galactic habitable zone |
more planets are known to orbit m dwarf stars than any other type of star in our galaxy, in part because they are the most numerous stellar type. habitable planets are of particular interest around these stars. however, the much closer-in habitable zone for an m dwarf creates a harsh plasma and magnetic environment for the planets, potentially causing massive atmospheric escape through various channels. an important channel for oxygen is ion escape, where charged particles in the ionosphere are accelerated to their escape velocities. a global planetary magnetic field plays a crucial role in the interactions between stellar wind plasma and the atmosphere, but its long-assumed shielding effect on the atmosphere has been challenged in recent years, raising the question of its necessity for habitability. in this study, we explore the habitability of a mars-like planet around the ultracool m dwarf, trappist-1, by investigating the ion escape from its atmosphere. we perform a steady-state simulation using a multispecies single-fluid magnetohydrodynamic (mhd) model with the photoionization frequency of each dominant neutral species in the martian atmosphere and upstream stellar wind conditions at trappist-1g. in addition to unmagnetized and weakly magnetized cases, we equip mars with a strong planetary dipole field of 5000 nt at eight different tilt angles with respect to the +z axis to study the change in the ion loss rate. the simulation results suggest a total escape rate that is two to three orders of magnitude higher than for present-day mars in our solar system. the escape rate shows a symmetric relation about the dipole tilt angle of 90 degrees. the magnetic fields, atmospheres, and the connection to habitability (mach) drive science center; laboratory for atmospheric and space physics (lasp); university of colorado boulder. | mhd modeling atmospheric ion escape from a mars-like exoplanet orbiting trappist-1 |
i will discuss the short-comings in the current definition of the habitable zone as well as how to move forward, taking into account both the physical and chemical relationships between a star and its planet. | avoiding 'the boy who cried wolf' in exoplanet habitability |
i have investigated how stars of different mass and composition evolve, and how stellar evolution impacts the location of the habitable zone around a star. current research into habitability of exoplanets focuses mostly on the concept of a “classical” hz, the range of distances from a star over which liquid water could exist on a planet's surface. this is determined by the host star's luminosity and spectral characteristics; in order to gauge the habitability potential of a given system, both the evolutionary history and the detailed chemical characterization of the host star must be considered. with the ever-accelerating discovery of new exoplanets, it is imperative to develop a better understanding of what factors play a role in creating “habitable” conditions of a planet. i will discuss how stellar evolution is integral to how we define the hz, and how this work will apply to the search for earth-like planets in the future.i have developed a catalog of stellar evolution models for sun-like stars with variable compositions; masses range from 0.1-1.2 msol (spectral types m4-f4) at scaled metallicities (z) of 0.1-1.5 zsol, and o/fe, c/fe, and mg/fe values of 0.44-2.28, 0.58-1.72, and 0.54-1.84, respectively. i use a spread in abundance values based on observations of variability in nearby stars. it is important to understand how specific elements, not just total z, impacts stellar lifetime. time-dependent hz boundaries are calculated for each track. i have also created a grid of m-dwarfs, and i am currently working to estimate stellar activity vs. age for each model.this catalog is meant to characterize potential host stars of interest. i have explored how to use existing observational data (i.e. hypatia catalog) for a more robust comparison to my grid of theoretical models, and i will discuss a new statistical analysis of the catalog to further refine our definition of “continuous” habitability. this work is an important step to assess whether a planet discovered in the hz of its star has had sufficient time to develop a biosphere capable of producing detectable biosignatures. the catalog is designed for use by the astrobiology and exoplanet communities to characterize any real planetary systems of interest. | the diversity of chemical composition: the impact of stellar abundances on the evolution of stars and habitable zones |
atmospheric characterisation of temperate, rocky planets is the holy grail of exoplanet studies. these worlds are at the limits of our capabilities with current instrumentation in transmission spectroscopy and challenge our state-of-the-art statistical techniques. i will present the transmission spectrum of the temperate super-earth lhs 1140 b, obtained using the hubble space telescope (hst). the wide field camera 3 (wfc3) g141 grism data of this habitable zone (t = 235 k) super-earth (r = 1.7 re) shows tentative evidence of water. however, the signal-to-noise ratio, and thus the significance of the detection, is low and stellar contamination models can cause modulation over the spectral band probed. we attempted to correct for contamination using these models and found that, while many still lead to evidence for water, some could provide reasonable fits to the data without the need for molecular absorption although most of these cause also features in the visible ground-based data which are nonphysical. future observations with the james webb space telescope (jwst) should be capable of confirming, or refuting, this atmospheric detection. | hubble wfc3 transmission spectroscopy of the habitable zone super-earth lhs 1140 b |
the astro2020 decadal survey recommended a "future large ir/o/uv telescope optimized for observing habitable exoplanets and general astrophysics" to be ready by end of the decade and that mission "to search for biosignatures from a robust number of about ~25 habitable zone [exo]planets." the search for multiple biosignatures requires high quality spectra of broad bandwidth and sufficient signal-to-noise. the combination of spectral resolution, bandwidth, and signal-to-noise-ratio impacts the number of exo-earths that can be spectrally characterized. broader bandwidths and higher signal-to-noise lead to increased integration time and is likely to reduce the number of targets around which an earth-size, habitable zone exoplanet could be sufficiently characterized. an increased integration will also consume a larger share of mission time dedicated to exoplanet direct imaging, possibly resulting in fewer exoplanets with higher quality spectra. in this paper, we evaluate the number of earth-size, habitable zone exoplanets that can be spectrally characterized for a range of spectral resolutions, signal-to-noise ratios, and bandwidths for a 6-m diameter exoplanet direct imaging mission. we evaluate the spectral characterization yield for coronagraph-only, starshade-only, and hybrid coronagraph-starshade architectures by scaling down the luvoir b 8-m diameter (6.7-m inscribed diameter) architecture and scaling up the habex 4m architectures. we evaluate the nominal case of a blind-search survey which uses broadband photometric detections for discovery and orbit determination and we evaluate the bounding case of perfect-prior knowledge which is useful to determine if target depletion occurs. | sensitivity of exo-earth yield of a 6 m ir/o/uv telescope to bandwidth, snr, and spectral resolution |
whats the news coming from nasas newest planet hunter, the transiting exoplanet survey satellite (tess)? launched in april 2018, tess is expected to discover tens of thousands of exoplanets orbiting the nearest and brightest stars. now that observations are underway, what exciting discoveries have been made? read on for an update from just a few of the latest tess studies published in aas journals.around a far sun-like starraw and corrected tess light curves showing the five transits of men c. the bottom panel shows the folded light curve. [huang et al. 2018]within the first six months of tesss launch, a team led by chelsea huang (mit) reported the first official discovery of a planet by tess a super-earth orbiting the sun-like star men.at a distance of about 59 light-years, men is dimly visible to the naked eye near the south celestial pole. over a decade ago, the anglo-australian planet search discovered a giant planet men b orbiting the star every 5.7 years. what tesss high-cadence observations revealed was a second, smaller planet orbiting the star every 6.27 days. by combining tess photometry with precise radial-velocity measurements, huang and collaborators determined the parameters of the planetary system, including the mass and radius of the newly discovered men c.with a radius twice that of earth and a mass nearly five times greater, men c isnt an entirely rocky planet like the terrestrial planets in our solar system. instead, it probably has a hydrogen-helium or water-methane envelope. we hope to learn more from future observations with the james webb space telescope, gaia, or even tess itself!physical parameters of lhs 3844 b compared to other known exoplanets. click to enlarge. [vanderspek et al. 2019]another first for tesstess wont only find planets around sun-like stars. thanks to its redder observing bandpass (6001000 nm, as opposed to keplers 420900 nm), tess is especially sensitive to planets orbiting the small, reddish stars called m dwarfs. because m dwarfs are so small and cool, planets in their habitable zones complete orbits in days rather than years, making them great observing targets.a team led by roland vanderspek (mit) analyzed data from the first month of tesss science operations, leading to the discovery of the first m-dwarf-orbiting planet detected by tess. the planet, dubbed lhs 3844 b, has a radius 32% larger than earths and orbits its small parent star at a distance of just 0.006 au swinging around lhs 3844 in just 11 hours. although its unclear whether or not lhs 3844 b will have an atmosphere it orbits so close to its host star that any atmosphere may have been torn away by stellar winds long ago its definitely worth investigating the first (of many!) m-dwarf planets discovered by tess.seeking confirmationartists impression of a hot-jupiter exoplanet. [nasa/esa/g. bacon (stsci)]as always with new detections, astronomers are rushing to confirm tesss discoveries with other telescopes and instruments.hd 202772a b is the first tess hot jupiter to have been confirmed by follow-up observations. in a recent study led bysonghu wang (yale university), the authors detail radial-velocity measurements made with thechiron, harps, and tres spectrographs that confirm the planetary nature ofhd 202772a b. this inflated gas giant is orbiting a quickly evolving star thats one of the brightest and most massive stars known to host a hot jupiter. as a result, hd 202772a b is one ofthe most strongly irradiated hot jupiters currently known.light curves all aroundwant a chance to explore some tess data on your own? ryan oelkers and keivan stassun (vanderbilt university) have extracted and made available light curves from all the stars in tess sector 1, which is home to all three of the stars discussed above. their website provides both raw and cleaned (systematic trends removed) light curves for each star, as well as information about each target (mass, luminosity class, magnitude, etc.).as more tess data rolls in, oelkers and stassun plan to update their website with the latest light curves for each observing sector. happy planet hunting!citationtess discovery of a transiting super-earth in the pi mensae system, chelsea x. huang et al. 2018 apjl 868 l39. doi:10.3847/2041-8213/aaef91tess discovery of an ultra-short-period planet around the nearby m dwarf lhs 3844, roland vanderspek et al. 2019 apjl 871 l24. doi:10.3847/2041-8213/aafb7ahd 202772a b: a transiting hot jupiter around a bright, mildly evolved star in a visual binary discovered by tess, songhu wang et al 2019 aj 157 51.doi:10.3847/1538-3881/aaf1b7light curves for all stars observed in tess full-frame images: sector 1 and beyond, ryan j. oelkers and keivan g. stassun 2019, res. notes aas 3 1. doi:10.3847/2515-5172/aafc34 | update on the search for planets with tess |
direct imaging of exoplanets in the thermal infrared would add quantitative details on their temperatures, luminosities, albedos, and level of internal/additional heating, all of which factor into their potential habitability. the eso/breakthrough-sponsored new earths in the alpha centauri region (near) program recently completed the first ultra-deep imaging campaign in the thermal infrared with the upgraded visir-near instrument. in this talk, i will describe the near campaign and the unprecedented sensitivity that it demonstrated for imaging rocky habitable-zone exoplanets. i will also discuss on-going efforts to upgrade the mid-infrared capabilities of the lbt based on the lessons from near, which would enable coordinated deep explorations for low-mass habitable-zone planets in both the northern and southern skies. finally, i will discuss expectations for a near-like instrument on an elt, which could potentially enable imaging earth-like planets in the habitable zones of nearby stars. | deep imaging of nearby habitable zones with visir-near and an upgraded lbt |
the kepler mission revolutionized our knowledge about distant worlds. nonetheless, our understanding of the architecture of most planetary systems remains incomplete. dynamite is a software package driven by the most robust trends observed in the kepler exoplanet population demographic data and answers the question: given the system-specific information, what are the likely locations and parameters of yet-undetected planets in the system? here we present the results of our analysis of two different planetary systems: (1) e eridani, the fifth closest sun-like star, that was announced to host three confirmed planets along with three unconfirmed planet candidates, and (2) speculoos-2, a recently discovered late m-dwarf that hosts two planets. for e eridani, we not only found evidence for the unconfirmed planet candidates, but if all six planets exist in the system, we predict an additional seventh planet in the habitable zone of the system. our n-body simulations also show that the real eccentricities of planets have to be lower than the earlier reported values for the system to be dynamically stable. lastly, we also estimate the likely surface temperatures of these planets using a single-layer leaky greenhouse model and a prior on the bond albedo. for speculoos-2, we find that an additional planet could orbit in the habitable zone. we assessed the time this predicted planet spends outside of the habitable zone using stellar evolution models and found that it most likely completed its formation inside the habitable zone. if confirmed, this planet will offer an earth-sized, habitable zone planet that is likely to have an evolutionary path more similar to earth than those in the trappist-1 system, where due to the star's luminosity evolution trappist-1e likely formed interior to the habitable zone, so its formation and nature are likely to be different from that of earth. while predicting the most likely locations of yet-undetected planets in a system can guide future observations, understanding their temperatures and their evolutionary paths can help us characterize exoplanets based on their similarity to earth and shortlist them for a dedicated biosignature detection mission. | prediction and characterization of currently undetected planets in known planetary systems |
a haiku about this work: k dwarfs are awesome hst has been staring cool science in store! efforts to discover and characterize habitable zone planets have primarily focused on sun-like stars and m dwarfs. yet the intermediate k stars provide an appealing compromise between these two extremes that has been relatively unexplored. compared to solar-type stars, k stars are more abundant, maintain longer main-sequence lifetimes, and their planets are more suitable to observations based on the mass and radius ratios of the planet to star. while m stars also excel in these regards, they have been observed to have frequent flares with ultraviolet (uv) energies over 100 times greater than anything observed in the recent history of our sun. therefore, k stars may provide "super-habitable" environments for exoplanets that are more easily detectable and characterizable. understanding the uv radiation incident on planets is pivotal in determining the habitability of any planets orbiting these stars, as well as our ability to observe any potential biosignatures in their atmospheres. with this aim, we observed a sample of k stars in the near-uv and far-uv at three representative ages using the cosmic origins spectrograph on the hubble space telescope (hst) to spectroscopically analyze the uv evolution of k stars. here, we present the first results of this hst program. we will discuss the evolution of both the continuum and line emission from k stars from ages 40 myr to ~gyrs, representing key periods in planetary atmospheric formation. | a spectroscopic analysis of the ultraviolet evolution of k stars |
the process of taking air (co2) and turning it into living matter (e.g. plants and algae) is the fundamental building block for life on earth. almost all (~99%) of this conversion is catalyzed by one enzyme, rubisco (berg, 2011). rubisco has been the chief architect of earth's atmosphere over 3 billion years, reducing co2 and (through its role in oxygenic photosynthesis) oxygenating the atmosphere (nisbet, and nisbet, 2008). despite its global importance, research on rubisco has predominately focused on terrestrial flowering plants, with little focus on the wide diversity across unique environments and organisms. this unexplored majority is of particular relevance in the ocean, where half of all photosynthesis occurs on modern earth (field et al, 1998). marine habitats encompass a wide range, from nutrient limited open oceans to high nutrient coastal zones, encased within sea ice, in deep ocean anoxic zones and on hydrothermal vents. rubisco containing marine organisms are also highly diverse, encompassing plants, algae, bacteria and archaea. exploring the diversity and adaptation of rubisco in marine environments, particularly those analogous to an early earth or exoplanets, will provide fascinating insights into one of the most critical steps for life on earth. | diversity of marine rubiscos harbor insights into the adaptability of biological co2 fixation. |
plato is an exoplanet hunting mission of the european space agency. it is a medium-class mission, with a launch foreseen in 2026. its prime objective is to uncover earth-sized planets residing in their habitable zone. the payload consists of 26 cameras with a very wide field of view. these cameras consist of a telescope optical unit (tou), aligned at ambient and characterized at the operational temperature, and a focal plane array bearing the detectors and delivered after coupling with the front end electronics. in this contribution, we report on the methods used at tou level to characterize focal plane using a hartmann mask, i.e. we illustrate the analysis pipeline after data collection in the cryo-vacuum chamber at leonardo (ldo), the implementation of new algorithms, and an extended uncertainties study for the hartmann analysis. | hartmann data analysis for plato tou em |
most known exoplanets, including earth-like planets, are found to orbit m-dwarfs in their habitable zones. as the upper atmospheres of the exoplanets orbiting m-dwarfs are affected to more extreme environments than what solar system bodies experience, the observations of such exoplanets raise questions about processes through which they lose and sustain their atmospheres. this study uses proxima centauri b (pcb), a super-earth exoplanet orbiting an m-dwarf, as a benchmark planet for understanding atmospheric loss from venus-like exoplanets. we examined for the first time the loss of neutral oxygen atoms via photochemical mechanism and how different stellar wind conditions and magnetization of the planet affect the escape process. | exosphere modeling of proxima b: a case study of photochemical escape with a venus-like atmosphere |
nirps is a near-infrared (yjh bands), fiber-fed, high-resolution precise radial velocity (prv) spectrograph installed at the eso 3.6-m telescope in la silla, chile. using a dichroic, nirps will be operated simultaneously with the optical harps prv spectrograph and will be used to conduct ambitious planet-search and characterization surveys. nirps aims at detecting and characterizing earth-like planets in the habitable zone of low-mass dwarfs and obtain high-accuracy transit spectroscopy of exoplanets. the spectrograph is compact for better thermal stability. using a custom r4 grating in combination with a state-of-the-art hawaii4rg detector, the instrument provides a high resolution and high stability over the range of 950-1800 nm. this paper focuses on the lens and optomechanical design, assembly, and test of nirps's spectrograph. some performance tests conducted at université laval (canada) during the integration and at la silla during commissioning are presented | nirps back-end: design and performance |
oxygen is a promising exoplanet biosignature due to the evolutionary advantage conferred from harnessing starlight for photosynthesis, and the low likelihood of producing and maintaining oxygen-rich atmospheres without life. several hypothetical scenarios have been proposed for non-biological oxygen accumulation on planets around late m-dwarfs, where the uv stellar spectrum and extended pre-main sequence may favor abiotic o2 accumulation. however, for planets around f, g, and k-type stars, few plausible mechanisms for generating abiotic oxygen have been suggested. the relative robustness of oxygen biosignatures around sun-like stars has motivated the development of next-generation telescopes capable of directly imaging habitable zone planets around f/g/k stars and searching for oxygen biosignatures. here, we develop a coupled thermal-geochemical-climate model of terrestrial planet evolution to illustrate several scenarios whereby significant abiotic oxygen can accumulate around sun-like stars. for earth-mass planets we find abiotic oxygen can accumulate to modern levels if the co2:h2o ratio of the initial volatile inventory is high, or if the total volatile inventory is dramatically different to that of the earth. fortunately, these abiotic oxygen scenarios could probably be distinguished from biological oxygen with appropriate contextual information. this highlights the need for broadly capable next-generation telescopes that are equipped to constrain surface conditions via time-resolved photometry and search for temporal biosignatures or disequilibrium combination biosignatures to assess whether oxygen is biogenic. | novel oxygen false positives on habitable zone planets |
current diagnostic velocity resolution limits our ability to search for exoplanets within the habitable zone. we propose a new capability (crossfading-edi or x-edi) that will boost optical spectrograph stability and spectral resolution, enabling 100x-1000x more sensitivity for the exoplanet search. this technique for increasing the stability of any dispersive (grating or prism) spectrograph to unwanted wavelength drifts has been simulated on real data and shown to produce a 1000x or more stability improvement, by use of an interferometer using *pairs* of overlapping delays placed in series with the disperser, rather than singly-used delays. this combines fringe shifts having opposite phase reaction, to cancel drift. this technique, crossfading externally dispersed interferometry" (x-edi) builds upon an earlier singly- delayed doppler technique (edi) demonstrated on a variety of telescopes including the 5-meter hale telescope at mt. palomar. the single-delay edi technique already affords a significant stability enhancement to a spectrograph, and has been used by others to discover exoplanets in 2006 and 2016. we expect that improving edi technique further by the use of crossfading pairs of delays will dramatically improve the instrumental noise floor due to spectrograph focal point drifts or detector pixel misplacement, which can limit the detection of small exoplanets over long (months or years) time scales. the x-edi has been simulated on edi data on a thar lamp line measured at the hale telescope*. a simulated drift insult was applied. the observed reaction to the line position drift was reduced 1000x. all spectrographs suffer drift insults of various kinds, and the x-edi technique reduces the reaction to these by moving the fine wavelength determination from the dispersive spectrograph to the interferometer, which uses the symmetry of delay pairs to eliminate drift. this technique can greatly improve spectral precision and stability for (1) doppler radial velocimetry, and (2) direct planet imaging using adaptive optics (such as the gemini planet imager) that feeds a low resolution integral field multi-object spectrograph. *david j. erskine, j. astr. tele. instrum. sys., 7(2):025006, june 2021. prepared by llnl under contract de- ac52-07na27344. | enabling 1000x more sensitive spectrographs for exoplanet search |
astrobites reports on the "continuous habitable zone" — an important criterion for selecting exoplanets for future atmospheric characterization. | which habitable zone planets are the best candidates for detecting life? |
classically, the "habitable zone" of a star has been defined based on the orbital radius at which water could remain liquid, and therefore usable by life (as we know it to exist on earth) (huang, 1959). however, with the upcoming launch of the james webb space telescope, and potential future missions including luvoir and habex, in order to effectively seek out and analyze data, it will be critical to pursue a more refined definition of what it means for a planet to be considered habitable. herein, we investigate whether stellar insolation or planetary rotation rate has any impact on the percentage of a planet's surface and volume that can be considered habitable. extending upon the rocke-3d gcm simulations of way et al. (2018), habitability fractions (spiegel et al., 2008) were computed based on not only temperature (0 - 100c), but also water availability in the form of soil wetness, lake fraction, and ice-free ocean volume (sparrman, 2020). both volumetric and surface area habitability fractions were derived, and the results have been separated into ocean and land habitabilities. the simulation analysis shows that as stellar insolation increases, so does habitability. further simulations are necessary to determine an upper limit of stellar insolation, due to the current model's radiation scheme breaking down at higher incident stellar radiation levels. additionally, as rotation rate decreases (increasing the planet's sidereal day length), the data points' spread for a given rotation rate increases, leading to an overall decrease in average habitability. huang, s. (1959). occurrence of life in the universe. american scientist, 47(3):397402. sparrman, v. (2020). estimates of fractional habitability for proxima centauri b using a 3d gcm. spiegel, d. s., menou, k., and scharf, c. a. (2008). habitable climates. the astrophysical journal, 681(2):16091623. way, m. j., genio, a. d. d., aleinov, i., clune, t. l., kelley, m., and kiang, n. y. (2018). climates of warm earth-like planets. i. 3d model simulations. the astrophysical journal supplement series, 239(2):24. | quantifying the habitability of rocky, earth-like exoplanets using three-dimensional general circulation models |
in this paper we will discuss exoplanets in the habitable zone of the milky way, the origin of terrestrial life, and what 'intelligent' humanity is doing, in order to complete the excursion on the bridge between the big bang and biology, started with the paper i (giovannelli & sabau-graziati, 2020). | frontier research in astrophysics in the gravitational wave era - ii (exoplanets and emergence of life) |
future space telescopes will provide us with the opportunity to characterize the atmospheres of terrestrial planets orbiting within the habitable zones of their stars. in the near future, the james webb space telescope (jwst) will allow us to spectrally characterize terrestrial planets orbiting m-stars. in the more distant future, telescopes such as the large uv/optical/ir surveyor (luvoir) and the habitable exoplanet observatory (habex) will allow us to direct image earth-sized habitable zone terrestrial planets orbiting sun-like stars (exoearths). in this dissertation, i explore the habitability of both g-star and m-star habitable zone planets. in part 1 of this dissertation, i focus on habitable zone planets around sun-like stars, and explore testing key theories of habitability statistically with direct imaging missions. these tests would exploit statistical marginalization of uncertainty inherent to both terrestrial planets and observations by using large samples of detected exoearths. in chapter 2, i propose a statistical test for the silicate-weathering feedback, which is a stabilizing negative feedback believed to help maintain habitable conditions on habitable zone planets. i find that if the silicate-weathering feedback is common on exoearths, we are likely to detect it with luvoir-a, and may detect it with luvoir-b if the natural spread of planetary parameters is small. we are unlikely to detect it with habex due to its small expected exoearth yield. in chapter 3, i develop a statistical test for whether exoearths tend to be earth-like. specifically, i consider whether observations with luvoir and habex could inform us on the fraction of exoearths that are earth-like (fe) in that they develop earth-like, o2-producing life (oxygenic photosynthesis) that oxygenates their atmospheres roughly following earth's oxygenation history. i explore this specifically in the event of a null detection of o2 or o3 on every exoearth. i find that missions with larger aperture mirrors are more robust to uncertainties in the exoearth occurrence rate (η⊕), but all missions are vulnerable to inconclusive null detections if η⊕ is very low. in chapter 4, i quantify the detectability of o2 and o3 for future direct imaging missions luvoir and habex using a 3d global climate model that includes photochemistry and realistic cloud coverage. i find that realistic cloud coverage increases the detectability of both o2 and o3, despite the fact that clouds obstruct some of the absorbing gas column mass, due to the strong effect of clouds on planetary reflectivity for planets with a low-to-intermediate surface albedo.in part 2 of this dissertation, i focus on habitable zone planets orbiting m-stars, which are expected to be tidally locked. specifically, i explore the snowball bifurcation on tidally locked planets. in chapter 5, i show that tidally locked planets do not exhibit a snowball bifurcation, regardless of stellar spectrum, as a direct result of the spatial pattern of insolation they receive. in chapter 6, i explore whether realistic ocean heat transport can reintroduce the snowball bifurcation for habitable tidally locked planets by destabilizing partially glaciated states. i show that including ocean heat transport does not reintroduce the snowball bifurcation and that the lack of a snowball bifurcation on tidally locked planets is robust to realistic levels of ocean heat transport. in chapter 7, i explore whether limit cycling between warm and snowball states at the outer edge of the habitable zone occurs for tidally locked planets. i show that tidally locked planets with an active carbon cycle will not experience limit cycling as a result of their lack of a snowball bifurcation, and instead will settle into "eyeball" states with a small unglaciated substellar region if they orbit near the outer edge of the habitable zone. | theoretical study of habitable terrestrial planets and statistical tests to inform future observations |
potentially habitable, earth-like planets are most likely to be discovered orbiting m dwarfs. to constrain the evolution and current habitability of these planets, we need to understand the magnetic activity of their parent stars today and in the past. the origins and evolution of magnetism in m dwarfs remain murky, however, and we can only weakly predict levels of activity for m dwarfs of a given mass and age. this is particularly true at the old (few gyr) ages that are typical of field stars. we report initial results from our spectroscopic survey of 122 nearby m dwarfs in the southern continuous viewing zone (cvz) of the transiting exoplanet survey satellite (tess). these stars have the benefit of high-precision, year-long optical light curves with which we can measure their (expected) long rotation periods. we have obtained at least one r ~ 2000 optical spectrum, which we use to characterize chromospheric hα emission, a common proxy for the strength of the stellar magnetic field, for 120 of our targets. more than half of our stars have at least four spectra, allowing us to explore potential connections between hα emission and photometric variability. with the rotational information we extract from tess, these hα data enhance our understanding of magnetic activity and its evolution in low-mass stars—and will eventually help us to characterize the past, present, and future habitability of nearby, earth-like planets. | old, quiet, and slow? a study of field m dwarf activity and rotation in the tess southern continuous viewing zone. |
the kepler mission has shown that earthlike planets orbiting within the habitable zones of their host stars are common. we derive an expression for the abundance of life bearing (biotic) extra-solar-system planets (exoplanets) in terms of the (yet unknown) probability for the evolution of biotic life. this "biotic probability" may be estimated by future missions and observations, e.g. spectral analyses of the atmospheres of exoplanets, looking for biomarkers. we show that a biotic probability in the range 0.001-1 implies that a biotic planet may be expected within ~10-100 light years from earth. of particular interest in the search for exolife are planets orbiting red dwarf (rd) stars, the most frequent stellar type. previous researches suggested that conditions on planets near rds would be inimical to life, e.g. the habitable zone of rds is small, so their habitable planets would be close enough to be tidally locked. recent calculations show that this and other properties of rds, presumed hostile for the evolution of life, are less severe than originally estimated. we conclude that rd planets could be hospitable for the evolution of life as we know it, not less so than planets of solar-type stars. this result, together with the large number of rds and their kepler planet-statistics, makes finding life on rd planets ~10-1000 times more likely than on planets of solar-type stars. our nearest biotic rd-planet is likely to be 2-10 times closer than the nearest solar-type one. | the abundance of biotic exoplanets and life on planets of red dwarf stars |
the search for habitable exoplanets and life beyond the solar system is one of the most compelling scientific opportunities of our time. nevertheless, the high cost of building facilities that can address this topic and the keen public interest in the results of such research requires rigorous development of experiments that can deliver a definitive advancement in our understanding. most work to date in this area has focused on a "systems science" approach of obtaining and interpreting comprehensive data for individual planets to make statements about their habitability and the possibility that they harbor life. this strategy is challenging because of the diversity of exoplanets, both observed and expected, and the limited information that can be obtained with astronomical instruments. here, we propose a complementary approach that is based on performing surveys of key planetary characteristics and using statistical marginalization to answer broader questions than can be addressed with a small sample of objects. the fundamental principle of this comparative planetology approach is maximizing what can be learned from each type of measurement by applying it widely rather than requiring that multiple kinds of observations be brought to bear on a single object. as a proof of concept, we outline a survey of terrestrial exoplanet atmospheric water and carbon dioxide abundances that would test the habitable zone hypothesis and lead to a deeper understanding of the frequency of habitable planets. we also discuss ideas for additional surveys that could be developed to test other foundational hypotheses is this area. | a statistical comparative planetology approach to the hunt for habitable exoplanets and life beyond the solar syste |
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