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a debate has arisen in the magnetospheres community in recent years about whether planetary magnetic fields actually inhibit atmospheric loss from planets. on one hand, atmospheric escape rates are driven in part by the strength of the stellar wind, and unmagnetized venus and mars both have atmospheric isotope ratios that suggest they have undergone greater atmospheric loss over their history than unmagnetized earth. on the other hand, the measured ion escape rates from these three planets is the same, to within an order of magnitude. to resolve this debate, an interdisciplinary team of scientists has come together to construct an interdisciplinary living framework that enables the evaluation of atmospheric loss from an arbitrary rocky planet given information about the planet and the space climate of its host star. the team consists of observers, modelers, and theoreticians from the terrestrial magnetospheric, planetary magnetospheric, and exoplanetary communities. in this presentation we will summarize the progress the team has made after one year of interaction with each other. this progress includes placing escape observations from different planets on a common footing, improving global simulations capable of estimating escape from both magnetized and unmagnetized planets, better understanding the extreme ultraviolet output of exoplanet hosting stars, and estimating the atmospheric escape rate from mars if it orbited an m dwarf star. | progress on understanding the influence of planetary magnetic fields on atmospheric escape rates |
the origin of the radius distribution gap for short-period kepler planets has become a priority investigation in exoplanet science. the current leading explanation is the photoevaporation of h/he envelopes during the first 100 million years of a planet's lifetime, where the gap is created when planets below some initial envelope mass fraction lose their atmospheres entirely. the star's high-energy spectrum is the main driver of atmospheric loss in short-period planets. spectral information on the x-ray, euv, and fuv flux is crucial for accurate modeling of the photoevaporation process in specific systems. the recently-discovered multi-transiting planet system v1298 tau offers a unique opportunity to investigate atmospheric evaporation in young systems. v1298 tau is a ~24 million year old star with multiple gas giants in short period orbits. furthermore, the brightness of v1298 tau and the transiting nature of it's close-in planets make this system ripe for atmospheric characterization via transmission spectroscopy. we propose to use hst observations to characterize the fuv and nuv flux of v1298 tau which, combined with scheduled nicer x-ray observations, will allow us to perform a differential emission analysis and approximate the euv spectrum. we will also perform transit observations to search for absorption by metals in escaping planetary material of the innermost planet and estimate its mass loss rate. our observations of this incredible star will provide a high-energy spectrum for one of the few <100 myr old systems with a transiting short-period planet that is amenable to mass loss measurements with current facilities. | planetary mass loss and the high-energy spectrum of v1298 tau |
a debate has arisen in the magnetospheres community in recent years about whether planetary magnetic fields actually inhibit atmospheric loss from planets. on one hand, atmospheric escape rates are driven in part by the strength of the stellar wind, and unmagnetized venus and mars both have atmospheric isotope ratios that suggest they have undergone greater atmospheric loss over their history than unmagnetized earth. on the other hand, the measured ion escape rates from these three planets is the same, to within an order of magnitude. to resolve this debate, an interdisciplinary team of scientists has come together to construct an interdisciplinary living framework that enables the evaluation of atmospheric loss from an arbitrary rocky planet given information about the planet and the space climate of its host star. the team consists of observers, modelers, and theoreticians from the terrestrial magnetospheric, planetary magnetospheric, and exoplanetary communities. in this presentation we will summarize the progress the team has made after one year of interaction with each other. this progress includes placing escape observations from different planets on a common footing, improving global simulations capable of estimating escape from both magnetized and unmagnetized planets, better understanding the extreme ultraviolet output of exoplanet hosting stars, and estimating the atmospheric escape rate from mars if it orbited an m dwarf star. | progress on understanding the influence of planetary magnetic fields on atmospheric escape rates |
the hd 97658b super-earth was recently detected in transit across one of the brightest star (v=7.7) known to host a transiting planet. the density of the planet suggests it must contain a large mass fraction of water. although the water vapor has not been detected in the lower atmosphere by hst/wfc3 due to the small atmospheric scale height, the moderate orbital distance of this warm (700-1000 k) planet favors the atmospheric escape of this water, which should be promptly dissociated at high altitude and become observable as hydrogen flowing within and beyond the roche lobe. the parent star properties are similar to those of hd 189733 (k dwarf, v=7.8, d=20 pc) and numerical simulations show that the halo of atomic hydrogen resulting from the dissociation of water is observable with hst/stis at lyman-alpha (121 nm). the detection of this atomic hydrogen will be the first signature of an evolved evaporating ocean on an extrasolar planet, as well as the first validation of internal structure models of exoplanets in this mass regime. a non-detection of escaping hydrogen, as in the case of 55 cnc e, would also bring useful constraints on the nature of the planetary atmosphere (co2-rich vs. h2o-rich?), the fate of super-earths, and the progenitors of the rocky evaporation remnants detected by corot and kepler. | search for an evaporating ocean on the super-earth hd 97658b |
the helium (he) 1083 nm line offers insight into the atmospheric mass loss of close-in exoplanets, which is likely to be significant in sculpting their population. most studies of atmospheric escape have been done at uv wavelengths using the hydrogen lyman-alpha line, but in the last few years the metastable he 1083 nm line has emerged as a more observationally accessible alternative. by measuring the amount of excess absorption in this line during a transit, we can characterize the spatial extent of the planet's exosphere and its corresponding present-day mass loss rate. we used an ultra-narrow band filter to observe two transits of the gas giant hat-p-18b, using the 200" hale telescope at palomar observatory, and report the first-ever detection of outflowing gas from its upper atmosphere. with a j-band magnitude of 10.8, this is the faintest system for which such a measurement has been made, demonstrating the effectiveness of this approach for surveying mass loss on a diverse sample of close-in gas giant planets. | metastable helium reveals ongoing mass loss for the gas giant hat-p-18b |
in the past months we have obtained evidence that an unusual phenomenon is happening in the atmosphere of one of the hot jupiters with shortest period. high-resolution spectroscopy from the ground reveals a transit spectrum where the sodium absorption signal from the planet peaks at 2-3%, which is larger than the planet transit depth in white light and 100 times larger than the well hst-established detection of sodium in hd 209458b (charbonneau et al. 2002). only in the uv have such large signatures been observed, for lighter hydrogen, carbon and oxygen atoms being blown-off by hydrodynamical atmospheric escape. so far, sodium atoms have never been observed higher than the thermosphere, where they should get promptly ionized. analysis of ground-based data is challenging because the spectroscopic signatures can be mimicked by the earth atmosphere, and a sophisticated removal of telluric contamination is necessary. our observations show that an efficient telluric correction for this target, particularly faint in the sodium region, is impossible, making a space-based confirmation necessary. in a single transit, hst/stis could obtain a 5-sigma confirmation of the signal. this detection would unambiguously show that the planetary atmosphere is in a state of extreme blow-off, with large exospheric densities allowing for a high recombination rate able to maintain sodium in a neutral state even high up in the atmosphere. this would represent the first constraint on atmospheric evaporation obtained in the optical, and would thus open a new, uv-independent path to the characterization of evaporating atmospheres, crucial in the post-hst era. | is the atmosphere of the extremely irradiated exoplanet wasp-43b in a blow-off state? |
exoplanets in short-period orbits provide a unique opportunity to observe phenomena critical to the development and evolution of our own solar system, including atmospheric escape, interaction with the host star, and the potential to study exoplanetary magnetism. at present, the theories explaining upper atmosphere observations exceed the number of relevant transit observations because these processes cannot be observed in broad-band visible/nir light curves. owing to their large sizes and short-periods, the physics of atmospheric mass loss can be studied on hot jovian and neptunian-mass planets by a dedicated small instrument operating at ultraviolet wavelengths, such as the colorado ultraviolet transit experiment (cute). cute will monitor planetary transits at near-ultraviolet wavelengths to study the physics of atmospheric escape and possibly detect the presence of magnetic fields on exoplanets. | cute: the colorado ultraviolet transit experiment |
progress on addressing two important exoplanet science topics can be made in the near-uv, where transit spectroscopy can measure aerosol scattering from the lower atmosphere, while large atmospheric escape signatures can be detectable in narrow-bands centred around strong mg and fe lines. wfc3/uvis has never been used nor verified for high photometric precision transit observations, though it has the highest throughput among all hst near-uv instruments, making it an important instrument to develop for uv exoplanet science. this program targets the ultra-hot jupiter hat-p-41b to measure both the broadband near-uv transmission spectra, and the narrowband escaping atmosphere signatures of mg. recent wfc3/ir observations show strong evidence for aerosol haze in hat-p-41b covering a near-ir h2o feature, making it an ideal target to study near-uv aerosol properties at high altitude. by measuring how high in the atmosphere near-uv scattering signatures can be seen, important constraints can be made on both cloud aerosol sizes and atmospheric vertical mixing rates; both critical parameters needed to explain how exoplanet clouds are suspended to high altitudes. in addition, the strong estimated atmospheric escape rates in hat-p-41b also make it an ideal target to detect hydrodynamical escape signatures of mg. our pilot program will demonstrate the exoplanet capabilities of wfc3/uvis, expanding hubble's unique uv access with a potential to probe hundreds of exoplanets. | how small and how high? enabling uv exoplanet cloud and exosphere science with wfc3/uvis |
studying the magnetic fields of exoplanets will allow for the investigation of their formation history, evolution, interior structure, rotation period, atmospheric dynamics, moons, and potential habitability. we previously observed the transits of 16 exoplanets as they crossed the face of their host-star in the near-uv in an attempt to detect their magnetic fields (turner et al. 2013; pearson et al. 2014; turner et al. in press). it was postulated that the magnetic fields of all our targets could be constrained if their near-uv light curves start earlier than in their optical light curves (vidotto et al. 2011). this effect can be explained by the presence of a bow shock in front of the planet formed by interactions between the stellar coronal material and the planet's magnetosphere. furthermore, if the shocked material in the magnetosheath is optically thick, it will absorb starlight and cause an early ingress in the near- uv light curve. we do not observe an early ingress in any of our targets (see figure 1 for an example light curve in our study), but determine upper limits on their magnetic field strengths. all our magnetic field upper limits are well below the predicted magnetic field strengths for hot jupiters (reiners & christensen 2010; sanchez-lavega 2004). the upper limits we derived assume that there is an absorbing species in the near-uv. therefore, our upper limits cannot be trusted if there is no species to cause the absorption. in this study we simulate the atomic physics, chemistry, radiation transport, and dynamics of the plasma characteristics in the vicinity of a hot jupiter using the widely used radiative transfer code cloudy (ferland et al. 2013). using cloudy we have investigated whether there is an absorption species in the near-uv that can exist to cause an observable early ingress. the number density of hydrogen in the bow shock was varied from 104 - -108 cm-3 and the output spectrum was calculated (figure 2) and compared to the input spectrum to mimic a transit like event (figure 3). we find that there isn't a species in the near-uv that can cause an absorption under the conditions (t = 1×106 k, semi-major axis of 0.02 au, solar input spectrum, solar metallicity) of a transiting hot jupiter (figure 3). therefore, our upper limits can not be trusted. we can eventually use cloudy to explore the escaping atmospheres from hot jupiters. we can still use our data to constrain the atmospheric proprieties of the exoplanets. | the robustness of using near-uv observations to detect and study exoplanet magnetic fields |
the 2006 guinsaugon landslide in st. bernard, southern leyte is one of the largest known landslides in the philippines in recent history. it consists of a 15-20 million m3 rockslide-debris avalanche from an approximately 675 m high mountain weakened by continuous movement of the philippine fault. the catastrophic guinsaugon landslide killed 1221 people and displaced 19 000 residents over its 4.5 km path. to investigate the present day morphology of the scar and potential failure that may occur, analysis of a 5 m resolution ifsar-derived digital elevation model was conducted using coltop3d and matterocking software, leading to the generation of a landslide hazard map for the province of southern leyte in central philippines. the dip and dip-direction of discontinuity sets that contribute to gravitational failure in mountainous areas of the province were identified and measured using a lower schmidt-lambert color scheme. after measurement of the morpho-structural orientations, potential sites of failure were analyzed. conefall was then utilized to compute the extent of rock mass runout. results of the analysis show instability in the scarp area of the 2006 guinsaugon landslide and in adjacent slopes because of the presence of steep discontinuities that range from 45-60°. apart from the 2006 guinsaugon landslide site, runout models simulated farther rock mass extent in its adjacent slopes, revealing a high potential for fatal landslides to happen in the municipality of st. bernard. concerned agencies may use maps produced in the same manner as this study to identify possible sites where structurally-controlled landslides can occur. in a country like the philippines, where fractures and faults are common, this type of simulated hazard maps would be useful for disaster prevention and facilitate disaster risk reduction efforts for landslide-susceptible areas. | structurally controlled hazard mapping of southern leyte, philippines |
in this work, we present preliminary results of the simulation of the atmospheric escape in a hot-jupiter like exoplanet using the radiative- hydrodynamic code 3d guacho. our model is able to reproduce, in a self-consistent way, the formation of a planetary wind generated by the absortion of extreme ultraviolet (euv) photons in the planetary atmosphere and shaped by its interactions with the stellar wind. from this model, we compute the synthetic spectra produced during transit in lyman-α and h-α lines to study this interaction. | simulación numérica 3d del escape atmosférico de un hot-júpiter |
the hydrogen exosphere constitutes the uppermost atmospheric layer of the earth, and its shape may reflect the last stage of the atmospheric escape process. the distribution of hydrogen in the outer exosphere remains unobserved because outer geocoronal emissions are difficult to observe from within the exosphere. in this study, we used the lyman alpha imaging camera (laica) onboard the proximate object close flyby with optical navigation (procyon) spacecraft, located outside the exosphere, to obtain the first image of the entire geocorona that extends to more than 38 earth radii. the observed emission intensity distribution can be reproduced using our analytical model that has three parameters: exobase temperature, exobase density, and solar radiation pressure, which implies that hot hydrogen production in the magnetized plasmasphere is not the dominant process shaping the outer hydrogen exosphere. however, the role of the magnetic effect in determining the total escape flux cannot be ruled out. many earth-size exoplanets have already been discovered, but it is difficult to distinguish an earth-like planet with an ocean from a venus-like planet. among venus, earth, and mars, only earth has a large hydrogen exosphere. here we show the earth's far-extended hydrogen exosphere is caused by the high temperature of the upper atmosphere, which is consistent with the existence of an ocean through co2removal. in this presentation, we report our observation result and instrument development | hydrogen lyman alpha imaging camera onboard procyon |
in the present chapter we present the results of evolutionary studies of exoplanetary atmospheres. we mostly focus on the sub- to super-earth domain, although these methods are applicable to all types of exoplanets. we consider both thermal and nonthermal loss processes. the type of thermal loss mechanism depends on so-called escape parameter $\beta$, which is the ratio of the gravitational energy of a particle to its thermal energy. while $\beta$ is decreasing, an exoplanet switches from classical jeans to modified jeans and finally to blow-off escape mechanisms. during blow-off the majority of the atmospheric particles dispose of enough energy to escape the planet's gravity field. this leads to extreme gas losses. although nonthermal losses never exceed blow-off escape, they are of significant importance for planets with relatively weak jeans-type escape. from the diversity of nonthermal escape mechanisms, in the present chapter we focus on ion pickup and discuss the importance of other loss mechanisms. the general conclusion of the chapter is, that escape processes strongly shape the evolution of the exoplanets and determine, if the planet loses its atmosphere due to erosion processes or, on the contrary, stays as mini-neptune type body, which can probably not be considered as a potential habitat as we know it. | stellar driven evolution of hydrogen-dominated atmospheres from earth-like to super-earth-type exoplanets |
how many rocky planets in the galaxy have an atmosphere? the answer to this question is urgent: models suggest that many exoplanets suffer prolonged atmospheric escape, implying that most rocky planets could actually have little chance of sustaining a surface biosphere. the james webb space telescope (jwst) might be able to settle this question empirically, but estimates show that finding atmospheres on rocky exoplanets with current techniques will require large observational effort. here i present the results from a multi-institution collaboration in which we propose a much faster test for the presence of an atmosphere. the test relies on secondary eclipse photometry with jwst, which measures a planet's dayside brightness temperature. we argue that an atmosphere will reduce the dayside temperature of a tidally locked planet relative to that of a bare rock, either because the atmosphere transports heat to the planet's night side or because atmospheric scatterers such as clouds will increase the planet's bond albedo. if a planet's dayside was then observed to be significantly cooler than a bare rock, this implies that the planet has a substantial atmosphere. we investigate the promise and limits of this technique with simulated jwst observations of three high-priority super-earths: trappist-1b, gj1132b, and lhs3844b. our simulations are based on general circulation models, atmospheric-dynamical theory, and state-of-the-art radiative-convective models. we also investigate geological false positives and negatives using solar system analogs and laboratory data. we find that for all three planets jwst can identify an o(1) bar atmosphere in as little as one to two eclipses. in contrast, atmospheric detection with other techniques typically requires more effort. neither are the three super-earths we consider special: nasa's tess mission has already found ~10 additional rocky planets that are promising targets for this technique, and this number is expected to grow to more than 100 by the time jwst launches.a jwst search for atmospheres on these planets promises to have major implications for our understanding of planetary evolution and astrobiology. in particular, if atmospheres were common on hot m dwarf planets, then cooler, potentially habitable planets around these stars will also likely have atmospheres. | how to quickly search rocky exoplanets for signs of atmospheres |
super-earth characterization continues to be a particular focus of the field given that it is the interface between terrestrial planets and gas-dominated planets. the implications of this division reverberate in planet formation, planetary interiors, and the origins and evolution of planetary atmospheres. a recently discovered ultra-short period exoplanet, toi-1685b, is the lowest density super-earth to orbit a small, low-mass star. its low density and small host star, make it an ideal candidate for extended atmosphere characterization and a measurement of super-earth mass loss. toi-1685b has a relatively hot atmosphere, which makes it an excellent target for measuring atmospheric hydrogen escape, particularly given its close orbit, deep within the stellar wind of its host star. measurements of hydrogen escape provides constraints on the evolution of the planetary atmosphere. we propose spectroscopic observations of two planetary transits to measure the stellar lyman-alpha emission line and search for signatures of an extended hydrogen atmosphere. the lyman-alpha line is also the dominant source of uv emission for cool stars, and thereby has a tremendous impact on planetary atmospheres. these observations, along with a short near-uv observation of mgii, the second most dominant emission line, will provide a vital characterization of the stellar inputs into all of the planetary atmospheres in this system. observations of hot and largely cloud-free atmospheres, particularly of super-earths like the one requested in this proposal, are an important step in planning future observations of temperate planetary atmospheres. | atmospheric evolution and loss of a recently discovered low-density ultra-short period super-earth |
as we are on the verge of receiving high quality spectra of many exoplanetary atmospheres, the next major challenge is discerning the underlying atmospheric characteristics, such as the surface pressure and temperature profile. forward modeling of rocky exoplanet atmospheres shows that many different states can produce similar absorption spectra, and thus backing out the "true" state from an observation will be a difficult task that may require the influx of additional information. by considering the implications of a given atmospheric profile on processes such as atmospheric escape and interaction with the stellar wind, additional constraints may be used to confine the valid parameter space of atmosphere states. as a case study, we consider the rocky exoplanet trappist-1h and, assuming a titan-like atmosphere, use a 1d diffusion model to explore the parameter space by varying temperature, surface pressure, and eddy diffusion coefficient, to produce over 2000 atmosphere profiles. we then use the gamera 3d magnetohydrodynamic (mhd) model to obtain a steady-state plasma solution, and calculate atmospheric loss rates via charge exchange for a subset of the atmosphere profiles. we find that the calculated loss rate scales most strongly with the atmospheric temperature, and that certain atmosphere profiles can be ruled out due to the loss rate being too high to maintain a stable atmosphere. we also discuss the prospect of observing, via lyman-alpha absorption, the energetic neutral atoms produced via charge exchange. if detectable, this measurement would serve as a powerful tool in estimating the charge exchange rate and therefore the plasma and neutral atmosphere densities and spatial distributions. | plasma-neutral interactions as a method of constraining stable rocky exoplanet atmospheres |
luvoir is a powerful and flexible observatory designed to revolutionize our view of the universe. in addition to searching for signs of life on habitable worlds, luvoir will be capable of detecting and characterizing hundreds of non-habitable exoplanets orbiting nearby stars dramatically advancing the field of "comparative exoplanetology". operating at l2, with a large aperture of 8-15 m and a sophisticated instrument suite, luvoir will allow for fantastic characterization of planets across parameter space. at uv wavelengths the luvoir ultravoilet multi object spectrograph (lumos) will obtain high spectral resolution transmission spectra at extraordinarily high signal to noise, allowing us probe the upper atmospheres of exoplanets to detect photochemical products, map the density and velocity structure of planetary exospheres, measure the abundance of metal species with fuv metal lines, and constrain the physics of atmospheric escape. | diversity of exoplanets with luvoir ii: the uv |
with the continually increasing number of newly discovered exoplanets, the quest to characterize the high energy radiation environment of each individual system becomes a larger and more arduous task. stellar euv flux contributes to atmospheric escape and water loss on close-in planets, therefore having a reliable catalog of euv spectra for all host stars will be a valuable resource to better understand and interpret the atmospheres of exoplanets. in an effort to simplify the endeavor of computing spectra for each host star, we present a grid of 1,500 generalized euv spectra for m and k stars between 2500 - 5000 k. these spectra are computed from phoenix upper atmosphere models and galex fuv+nuv fluxes are used to identify the appropriate euv spectrum. our webtool is searchable by target name or user-inputted parameters and also includes an option to download full uv spectra for generic m and k stars of each subtype. | pegasus: phoenix euv grid and stellar uv spectra |
atmospheric escape has traditionally been observed using hydrogen lyman-α transits, but more recent detections utilise the metastable helium triplet lines at 1083nm. capable of being observed from the ground, this helium signature offers new possibilities for studying atmospheric escape. such detections are dependent however on the specific high-energy flux received by the planet. previous studies show that the extreme-uv band both drives atmospheric escape and populates the triplet state, whereas lower energy mid-uv radiation depopulates the state through photoionisations. this is supported observationally, with the majority of planets with 1083nm detections orbiting a k-type star, which emits a favourably high ratio of euv to mid-uv flux. the goal of our work is understanding how the observability of escaping helium evolves. we couple our one-dimensional hydrodynamic non-isothermal model of atmospheric escape with a ray-tracing technique to achieve this. we consider the evolution of the stellar radiation and the planet's gravitational potential. | the evolution of atmospheric escape of highly irradiated gassy exoplanets |
stellar extreme ultraviolet (euv, 100-1000 å) radiation can influence factors for habitability of planets in k dwarf systems, including driving atmospheric escape and water loss from close-in exoplanets. this wavelength range is currently unobservable due to interstellar contamination and a lack of operational instruments, and so modeling euv spectra becomes important. we investigated k stars because their atmospheric planetary o 2 and ch 4 spectral features and high planet-star contrast ratio make these systems favorable targets for atmospheric characterization. we modeled the euv spectra of k stars using the phoenix atmospheric code, utilizing hubble space telescope (hst) far and near ultraviolet (fuv, nuv) data for semi-empirical guidance. we categorized each spectral subtype according to temperature and surface gravity and computed the photospheric contribution to each uv flux. then upper atmosphere models, including chromosphere and transition regions, were computed, added, and compared to hst and galaxy evolution explorer (galex) data and adjusted for flux accuracy, including increasing the number of computed non-local thermodynamic equilibrium (non-lte) species. strong lines in the fuv and nuv of the synthetic emission spectra that did not match hst data were masked for a more accurate integrated flux calculation. we generated grids of spectra based on the characteristics of k0-k9 stars, which revealed that individualized grids for each spectral subtype will need to be computed, rather than computing one generalized grid applicable for all k dwarf stars. this suite of models will be added to the upcoming pegasus (phoenix euv grid and stellar uv spectra) website which will be accessible to the community and searchable using galex data. these synthetic spectra involving the euv can be utilized for studies of star-planet interactions and other simulations involving planet habitability, and will be influential in the identification of potentially habitable planets in k dwarf systems. | the great escape! extreme-uv spectra grids for k-type stars |
although exomoons, natural satellites beyond our solar system, are still undetectable in direct searches with state-of-the-art instruments, their existence has been hypothesized to explain various inconsistencies in exoplanetary spectra. exogenic sources of sodium and potassium have been considered at multiple exoplanets, where abundances exceed the exoplanet's source rates, and hydrostatic exoplanet atmospheres are limited in their ability to explain increased line broadening seen in na & k spectra, where an orbiting body naturally provides broadening with variable ±∼10-20 km/s.a semi-analytic atmospheric escape and evolution model dishoom approximates the minimum mass flux needed for an exomoon to provide volcanic material for the absorption of star light. we develop a 3-d test-particle monte carlo simulation module called serpens (simulating the evolution of ring particles emergent from natural satellites) to be coupled to dishoom. serpens is designed to be highly adaptive, open-source, and easy to use. we simulate the neutral outgassing and evolution of a satellite at multiple candidate exoplanet-exomoon systems including hd189733 b ii, hd209458 b i, wasp-49 a b i, hat-p-1 b i, and wasp-96 b i, in order to provide a number density n[cm-3] and line-of-sight column density n[cm-2] map of the particle environment in a non-hydrostatic medium, characteristic of a volcanic exosphere akin to jupiter's na exosphere fueled by io. the neutral species maps are then fed into a non-hydrostatic radiative transfer model, prometheus, which computes an exospheric spectrum that can be directly compared to ongoing ground and space-based spectra of candidate exomoon systems. we model masses ranging from earth, io, and enceladus to emulate long-term effects of mass loss and present the respective particle distributions. photoionization is set as the prime constraint for the lifetime of atoms and molecules.in contrast to previous works, our code serpens focuses on exomoons and their imprint as a neutraland plasma torus. serpens is designed to eject particles via sputtering and thermal evaporation atregular time intervals allowing us to simulate an evolving cloud/torus. multiple species including na, kand so2, as well as their chemical networks, are supported.our results demonstrate how exomoons similar to io, referred to as exo-ios, can affect line-of-sight column densities depending on the phase of the exomoon at the time of observation. this means that it is possible to model time-variable spectra by taking into account the phase of the exomoon. | exo-io simulations of toroidal exospheres |
ariel [1] is the m4 mission of the esa's cosmic vision program 2015-2025, whose aim is to characterize by lowresolution transit spectroscopy the atmospheres of over one thousand warm and hot exoplanets orbiting nearby stars. the operational orbit of the spacecraft is baselined as a large amplitude halo orbit around the sun-earth l2 lagrangian point, as it offers the possibility of long uninterrupted observations in a fairly stable radiative and thermo-mechanical environment. a direct escape injection with a single passage through the earth radiation belts and no eclipses is foreseen. the space environment around earth and l2 presents significant design challenges to all spacecraft, including the effects of interactions with sun radiation and charged particles owning to the surrounding plasma environment, potentially leading to dielectrics charging and unwanted electrostatic discharge (esd) phenomena endangering the payload operations and its data integrity. here, we present some preliminary simulations and analyses about the ariel payload dielectrics and semiconductors charging along the transfer orbit from launch to l2 included. | preliminary surface charging analysis of ariel payload dielectrics in early transfer orbit and l2-relevant space environment |
intense, high-energy stellar irradiation is believed to control the atmospheric chemistry and mass loss of short-period planets. these effects are most dramatic in the early stages of planetary evolution, when young stars produce higher levels of x-ray and ultraviolet (uv) radiation, and experience extreme stellar activity, which manifests itself in great starspot surface coverage and more short-term high-energy bursts of radiation called stellar flares. observing how close-in young planets react and change due to these environments is key to understanding the population of mature exoplanets. with the discovery of a dozen young (<100 myr) transiting exoplanets, now is the opportune moment to start observing their atmospheres with the hopes of illuminating the processes that affect planetary evolution. in this talk, i will present three benchmark studies of young stellar activity and planetary atmospheres as observed in the uv, optical, and near-infrared wavelengths. i will discuss the evolution of flare rates and energies of young stars as observed in the uv and near-infrared. we found that flare rates and energies decrease for stars older than 50 myr for stars with teff ≥ 4000k, while colder stars show no evolution in their flare-frequency distributions. i will demonstrate how these statistics can be incorporated into exoplanet atmospheric removal calculations and the implications of such additions. additionally, i will present uv and optical observations probing for evidence of atmospheric escape on two young transiting super-neptune-sized planets: au mic b (23 myr) and v1298 tau c (30-40 myr). while each study presented highlights the challenges associated with studying young planetary systems, i am hopeful continued monitoring and future jwst observations will help answer questions about how young exoplanet atmospheres evolve. | a multi-wavelength investigation of young stellar and planetary systems |
the k2-33 b is a planet of 5 earth radii orbiting the young m-type host star, recently emerged from the interplanetary disc. the extreme youth of the system (<20 myr) gives the unprecedented opportunity to study the earliest phases of planetary evolution, at a stage when the planet is exposed to an extremely high level of high-energy radiation emitted by the host star. since the planetary mass remains unknown (the estimated upper limit is 5.4 jupiter mass), we perform a series of 1d hydrodynamic simulations of the planet's upper atmosphere considering a range of most probable possible planetary masses in the range from super-earth to the twice of neptune. to account for internal heating as a result of contraction, we set the temperature range from the black body temperature of the planet of 850 k to 1300 k. as the result, we obtain temperature profiles mostly controlled by the planet's mass, while the equilibrium temperature has a secondary effect. for planetary masses below 7-10 earth mass, the atmosphere is exposed to extremely high escape rates, driven by the planet's weak gravity and high thermal energy, which increase with decreasing mass and/or increasing temperature. for higher masses, the escape is instead driven by the absorption of the high-energy stellar radiation. a rough comparison of the timescales for complete atmospheric escape and age of the system indicates that the planet is more massive than 10 earth masses. | young planets under extreme uv irradiation: upper atmosphere modelling of the young exoplanet k2-33b |
high-energy stellar irradiation can photoevaporate planetary atmospheres, which can be observed in spectroscopic transits of hydrogen lines. for the close-in giant exoplanet hd189733b, multiple observations in the ly-alpha line have shown that atmospheric evaporation is variable, going from undetected to enhanced evaporation in a 1.5-yr interval. coincidentally or not, when hd189733b was observed to be evaporating, a stellar flare had just occurred 8 h prior to the observation. this led to the question of whether this temporal variation in evaporation occurred due to the flare, an unseen associated cme, or even the simultaneous effect of both. in this work, we investigate the impact of flares (radiation), winds, and cmes (particles) on the atmosphere of hd189733b using three-dimensional radiation hydrodynamic simulations that self-consistently include stellar photon heating. we find that the flare alone cannot explain the observed high blueshifted velocities seen in the ly-alpha. the cme, however, leads to an increase in the velocity of escaping atmospheres, enhancing the blueshifted transit depth. | the impact of cmes and flares on the atmospherere of close-in exoplanets: comparison with observations |
atmospheric escape from exoplanets is a topic of great interest for the exoplanet community since atmospheric retention is an important component of surface habitability. while atmospheric escape has been detected from large exoplanets, it remains difficult to measure for smaller (rocky) planets. indeed, for rocky planets orbiting active stars it is thought that it may be difficult for atmospheres to be retained at all. in the absence of detailed observations, one option is to leverage observations and models for planets in our own solar system. here we consider atmospheric escape from mars - if it orbited an m dwarf star similar to barnard's star. our analysis considers five escape processes: hydrodynamic escape, thermal escape, photochemical escape, ion escape, and sputtering. to estimate the escape rate via each process from our hypothetical "exomars", we employ models for escape that have either been validated using observations or verified against other models. we provide escape rate estimates for important species in the martian upper atmosphere: o, o2, h, and co2, and use them to estimate the lifetime of the martian atmosphere. | atmospheric lifetime from a hypothetical mars-sized planet orbiting barnard's star |
we have studied the input of the exothermic photochemistry into the formation of the non-thermal escape flux in the transition h 2 − h region of the extended upper atmosphere of the hot exoplanet - the sub-neptune π men c. the formation rate and the energy spectrum of hydrogen atoms formed with an excess of kinetic energy due to the exothermic photochemistry forced by the stellar xuv radiation were calculated using a numerical kinetic monte carlo model of a hot planetary corona. the escape flux was estimated to be equal to 2.5×1012 cm−2s−1 for the mean level of stellar activity in the xuv radiation flux. this results in the mean estimate of the atmospheric loss rate due to the exothermic photochemistry equal to 6.7×108 g s−1. the calculated estimate is close to the observational estimates of the possible atmospheric loss rate for the exoplanet π men c in the range less than 1.0×109 gs−1. | study of the non-thermal atmospheric loss for exoplanet π men c |
with jwst up and running, astronomers are getting a first look at the quirks of individual exoplanets. features never before examined are coming into view: for instance, a recent study has revealed that while hat-p-18b may not have much methane, it does have a tiny tail.jwst shows off, finds a corginow more than a year past its launch, jwst is finally doing what it was designed to do: collecting photons and wowing astronomers with the precision of its data. one of the earliest flexes of its scientific power occurred last summer, when it trained its attention on the transit of a jupiter-sized, saturn-mass exoplanet named hat-p-18b.while the team, led by guangwei fu (johns hopkins university), found several molecules in the upper atmosphere of the planet using the near infrared imager and slitless spectrograph (niriss) instrument, what they didnt find was more surprising.a subsample of the data, orange, and the best-fit model, blue, showing the helium absorption signature. the y-axis is in units of transit depth, meaning enhanced absorption appears as a positive bump. [fu et al. 2022]the first of these surprises was a helium absorption signature, but not surrounding the planet: instead, their results indicate that hat-p-18b is dragging along a faint tail of escaping helium. similar features have been spotted trailing behind other planets, but this one was so subtle that it was previously missed by ground based observatories. in other words, hat-p-18b is the corgi of the exoplanets: it has a tail, but its not a dominant structure.but what about methane?the second surprise concerned a molecule not displaced from the planet, but possibly missing entirely. one of the primary motivations for targeting hat-p-18b specifically is its position in a uniquely helpful corner of parameter space for modelers working on a methane mystery.hot planets with surface temperatures over 1000k are not expected to have any methane in their atmospheres, since thermodynamics at these extreme conditions prefer other species. however, simple models suggest that any worlds cooler than this should show signs of absorption caused by methane molecules in the upper atmosphere intercepting photons with a specific wavelength.strangely, however, this prediction has not panned out in previous studies. searches of several planets that should have held methane turned up none. this tension called for a closer look: were the assumptions baked into the models wrong, or was there something strange about the first worlds surveyed? with an equilibrium temperature of 800k, hat-p-18b was the perfect target to help move the needle one way or another.the niriss data, black, and several possible model atmospheres to explain it, colored on top. the green and red models were produced assuming equilibrium chemistry. the x-axis denotes wavelength, and the ticks range linearly from 0.5 to 2.5 microns. [fu et al. 2022]fu and collaborators made no conclusive methane detection, further deepening the model mismatch puzzle. models which assume the atmosphere is in chemical equilibrium struggled to reproduce the combination of no-methane, yes-water seen in the data, which suggested that some other mechanism(s) were involved to remove the expected gas. even more striking, other models which made no assumption about an equilibrium also did not confidently prefer including methane in the final fit over leaving it out entirely.in all, jwst revealed hat-p-18b to be a strange world, one which subverts our expectations of atmospheric chemistry but charms with a helium tail. well have to wait for jwst observations of other planets before we know just how weird either of those traits truly is.citationwater and an escaping helium tail detected in the hazy and methane-depleted atmosphere of hat-p-18b from jwst niriss/soss, guangwei fu et al 2022 apjl 940 l35. doi:10.3847/2041-8213/ac9977the post the corgi of exoplanets: methane mystery on hat-p-18b appeared first on aas nova. | the corgi of exoplanets: methane mystery on hat-p-18b |
the terrestrial planets of the solar system are rocky worlds that did not accrete envelopes of hydrogen and helium, but instead possess thin secondary atmospheres, or no atmosphere at all. until recently, most exoplanet atmospheric studies have centered around hot jupiters, for which high planet-to-star radius ratios and short orbital periods allowed for observable transmission spectra. now we have the opportunity to probe the atmosphere of a small, rocky exoplanet. gj1132b has a radius of 1.2 earth radii and a mass of 1.6 earth masses, and orbits an m-dwarf 12 parsecs away. determining the composition of gj1132b's atmosphere is essential to understanding the nature of atmospheric evolution on terrestrial planets. we observed five transits of gj1132b using the magellan clay telescope with the ldss3c multi-object spectrograph. we compare the transit depth of gj1132b in wavelength bins ranging from 0.65 -- 1.04 microns to infer whether or not gj1132b has maintained its primordial hydrogen-dominated atmosphere. should we find evidence of a hydrogen-dominated atmosphere, this would imply that a terrestrial planet is able to accrete and retain a low mean-molecular weight atmosphere from the planetary nebula. coupled with recent uv spectra of the host star, our results can clarify the process of atmospheric escape on terrestrial worlds, with implications for formation histories of m-dwarf planets and the potential for habitability in these systems. if instead gj1132b possesses a low mean-molecular weight atmosphere, we look to future observations with jwst and the ground-based extremely large telescopes to characterize its atmosphere.this material is based upon work supported by the national science foundation graduate research fellowship program. this work was made possible by a grant from the john templeton foundation. | exploring an earth-sized neighbor: ground-based transmission spectroscopy of gj1132b, a rocky planet transiting a small nearby m-dwarf |
we present the colorado ultraviolet transit experiment (cute), a 6u cubesat mission that will perform exoplanet transmission spectroscopy to study the physics of atmospheric escape and possibly detect the presence of magnetic fields on exoplanets. | cute: a small nuv satellite mission to study exoplanet atmospheres |
atmospheres of the early earth with the young sun or close-in exoplanets around k and m dwarfs are subject to large fluxes of solar/stellar radiation from the hosts stars. the solar/stellar radiative flux in the x-ray and extreme ultraviolet ray (xuv) are the key elements for heating the upper atmosphere such as the ionosphere, thermosphere, and mesosphere. first, xuv is absorbed by neutral molecules in the upper atmosphere, excites the molecular quantum states, and consequently heats the upper atmosphere. second, xuv ionizes neutral molecules and enhances the ionospheric conductivity increasing ion and electron densities together with the increased upper atmospheric temperature. this also leads to the upper atmospheric heating through joule heating with the ionospheric currents. here, we present the comparison of xuv and joule heating in the upper atmospheric heating and hydrodynamic escape rates. in this study, we use a newly developed 3-d upper atmospheric dynamics model, (exo)planetary ionosphere and thermosphere tool for research (planet-ittr) to calculate xuv heating, ionospheric conductivity, and joule heating. we simulate the xuv and joule heating rates and hydrodynamic escape rates in early earth (earth-sized exoplanets around active stars) at different distances from the sun (the host stars). in this presentation, we will also discuss the habitability of the earth-sized exoplanet around active stars in terms of distance from the host stars. | xuv and joule heating driven atmospheric escape from the early earth and earth-sized exoplanets around active stars |
the absence of global dipole magnetic fields at terrestrial planets is widely perceived as a major impediment in protecting planetary atmospheres from being eroded by stellar winds. in this work, we re-examine this idea by focusing on the rate of atmospheric ion escape from mars for nominal solar wind parameters and an extreme ``carrington-type'' space weather event. we carry out extensive numerical simulations using a sophisticated multi-fluid magnetohydrodynamics (mhd) model, thereby demonstrating that the escape rate is a non-monotonic function of the martian dipole magnetic field strength, and that it varies by more than an order of magnitude in certain instances. our work illustrates, for the very first time using the state-of-the-art global simulations, that the lack of dipole fields is not necessarily a disadvantage from the viewpoint of atmospheric losses. we are led to the conclusion that the ion escape rates from mars may have actually been higher when it possessed stronger magnetic moment in the past. we conclude our analysis by briefly exploring the ensuing implications for martian and exoplanetary habitability. | global dipole magnetic field: boon or bane for the martian atmospheric retention? |
luvoir is powerful and flexible observatory designed to revolutionize our view of the universe. in addition to searching for signs of life on habitable worlds, luvoir will be capable of detecting and characterizing hundreds of non-habitable exoplanets orbiting nearby stars dramatically advancing the field of "comparative exoplanetology". operating at l2, with a large aperture of 8-15 m and a sophisticated instrument suite, luvoir will allow for fantastic characterization of planets across parameter space both with direct imaging and transmission spectroscopy. at fuv wavelengths the luvoir ultravoilet multi object spectrograph (lumos) will obtain high signal to noisetransmission spectra at high spectral resolution, allowing us to detect transiting planetary exospheres and constrain the physics of atmospheric escape. meanwhile, high definition imager (hdi) instrument we can obtain high signal to noise, medium resolution spectra from the nuv to the nir, allowing us to constrain the properties of clouds, map absorption from alkali metals, and measure abundances for a wide range of molecules including h2o, co2, and o2. finally, direct imaging and spectroscopy with the eclips coronograph will enable a systematic investigation of system architectures and the diversity of exoplanet atmospheres at wide orbits. | studying the diversity of exoplanets with luvoir |
stellar flares are considered an impediment to habitability, especially in the case of close-in exoplanets around m-dwarfs since these stars are highly active. in recent times, there has been a growing awareness that coronal mass ejections (cmes) - sometimes termed as stellar storms or superstorms, depending on the flare energy - associated with stellar flares pose severe threats to planetary habitability. interplanetary cmes (or icmes), corresponding to fast-moving magnetic clouds, act to impact the planets with significantly high dynamic pressure. semi-analytical models imply that planets around active stars ( 1 flare/day) may experience escape rates that are 1-3 orders of magnitude higher than those arising from erosion by stellar winds alone. understanding atmospheric escape is very important from the standpoint of habitability since atmospheric evolution influences the climate and the fluxes of ionizing radiation reaching the surface, among other factors. here we carry out sophisticated 3d multi-species mhd simulations to assess how the atmospheric escape rates of the trappist-1 planets evolve during the passage of an icme, where the icme is initialized and modeled according to the flare observations. | how flares regulate atmospheric losses from the trappist-1 planets |
in this investigation, we present 3-d simulations of the upper thermosphere and ionosphere of the hot jupiter hd 209458 b (nicknamed osiris). we examine the 3-d neutral and ion dynamics under the influence of jovian-type magnetic fields and compare these with a non-magnetic case. our initial results suggest that the jovian-type magnetic field significantly reduces the ability of the ions to escape at low-to-mid latitudes, and therefore limits the adiabatic cooling associated with this escape. thus, the magnetized case exhibits strong escape from the high-latitude polar regions, but significantly reduced escape over the lower latitudes. this configuration results in a significantly warmer global thermosphere in the magnetized case with an energy balance that is driven primarily by stellar euv/xuv heating and radiative cooling, which contrasts with the typical non-magnetized case of stellar heating balanced by the adiabatic cooling associated with bulk atmospheric escape. | simulating the three-dimensional upper atmospheres of close-in exoplanets with strong magnetic fields |
i will address two questions: (1) do tess/kepler super-earths have atmospheres? (2) for sub-neptunes, how does magma-atmosphere exchange set the composition of the atmosphere? both involve coupling between magma oceans and h2-rich atmospheres. (1) after stellar xuv flux declines, super-earth volcanism continues for gyr. late volcanism might rejuvenate atmospheres, but only if the solid mantle has enough volatiles. volatiles that have high-molecular-mass (high-μ) are delivered in the first 108 yr. on this timescale, for short-period exoplanets, early-accreted nebular atmospheres are ablated. thus, rocky exoplanet volatiles might be entrained by escaping h2. the extent of high-μ loss depends on magma ocean crystallization timescales. if this is slow, then because the atmosphere and magma are equilibrated, all of the high-μ volatiles can escape. however, early crystallization favors later existence of an atmosphere. the (small) fraction of volatiles which are trapped within the crystals are shielded during the era of high xuv. crystallization timescale depends on stellar luminosity, so this model suggests a period threshold for atmosphere absence on super-earths. (2) sub-neptunes are mostly magma by mass, and are mostly atmosphere by volume. i explore the effect of the magma on the atmosphere. for the fe-mg-si-o-h system, the fate of h (and the sub-neptune atmosphere's composition) is set by magma oxidation state. in turn this oxidation state is set by the details of planet assembly. depending on magma redox state, a nebular-derived atmosphere can masquerade as an outgassed atmosphere and vice versa. fortunately, planet radius and atmosphere scale height can be used to tell these scenarios apart. h2o dissolved in the magma is a major h sink for sub-neptunes: the total volatile budget can be mostly dissolved h2o even when μatm 2. because of these sinks, the amount of h2 needed to explain the radius of sub-neptunes is much greater than usually assumed. these results point the way to future modeling work, combining magma-ocean chemistry with fractionating escape during the transition from primary to secondary atmospheres. | the transition from primary to secondary atmospheres |
so far, atmospheric evaporation has been detected in exoplanets with direct and indirect methods. from these observations, it was found that close-in exoplanets show very high escape rates that are on the order of 1e9 -- 1e10 g/s and even higher. there is a number of reasons of why these rates are so high, including the high levels of activity from the host star and the very extreme environment within which close-in exoplanets are embedded. in this talk, i will present an outside-the-solar-system perspective on planetary atmospheric evaporation and will discuss current debate which questions the effectiveness of planetary magnetic fields in reducing evaporation in exoplanets. | an exoplanetary view on atmospheric evaporation |
the helium line triplet at 1083 nm has recently been established as a powerful new diagnostic of upper atmospheres of exoplanets. these observations opened a new wavelength window into the extended atmospheres of exoplanets, which can ultimately improve our understanding of the physical processes that drive atmospheric escape and mass loss. additional information about planetary atmospheres can be obtained from the radiation polarization at 1083 nm. linear and circular polarization signals in the helium 1083 nm line are created in the presence of a magnetic field due to the atomic level polarization induced by anisotropic stellar radiation and the combined action of the zeeman and hanle effects. polarization signals in the helium 1083 nm line resulting from exoplanet magnetic fields with a wide range of field strengths, including those observed in the solar system planets, could be detectable with current and future high-resolution spectropolarimeters operating in the near-infrared wavelength range. | a new method for detecting magnetic fields in exoplanets |
mars has always represented an important target from the standpoint of planetary science, especially on account of its long-term climate evolution. one of the most striking differences between ancient and current mars is that the former had a thicker atmosphere compared to the present-day value, thereby making noachian mars potentially more conducive to hosting life. this discrepancy immediately raises the question of how and when the majority of the martian atmosphere was lost, as well as the channels through which it occurred. there are compelling observational and theoretical reasons to believe that the majority of atmospheric escape must have occurred early in the planet's geological history, when the extreme ultraviolet (euv) flux and the solar wind from the sun were much stronger than today. our understanding of present-day martian atmospheric escape has improved greatly thanks to observations undertaken by, e.g., the maven in conjunction with detailed theoretical modeling. in this study, we adopted the one-way coupled framework, which has been employed to study the ion and photochemical losses at the current epoch. we adopted the 3-d mars thermosphere from the mars global ionosphere thermosphere model (m-gitm) and the hot atomic oxygen density from the mars exosphere monte carlo model adaptive mesh particle simulator (amps) as the input for the 3-d bats-rus mars multi-fluid mhd (mf-mhd) model. the mars amps hot oxygen corona and the associated photochemical loss rate were calculated based on the thermospheric/ionospheric background from m-gitm. our simulations indicate that the total photochemical and ion atmospheric losses over the span 0-4 ga are approximately equal to each other, and their sum amounts to 0.1 bar being lost over this duration. if we assume that the oxygen lost through a combination of ion and photochemical escape mechanisms was originally derived from surface water, we find 3.8 × 10^17 kg of water has been lost from mars between 0 and 4 ga; this mass corresponds to a global surface depth of 2.6 m. this study offers fresh insights concerning the long-term climate evolution and habitability of the increasing number of exoplanets discovered yearly due to atmospheric losses. | modeling martian atmospheric losses over time: implications for (exo)planetary climate evolution and habitability |
transmission spectroscopy is one of our most powerful tools for characterizing exoplanet atmospheres and hanks to the recent launch of nasa's tess mission we will soon have a large sample of planets around bright stars, ideally suited to this technique. with it's unique combination of uv to nir wavelength coverage and incredibly high s/n, luvoir would build upon the powerful legacy of hubble and revolutionize our ability to characterize the atmospheres of a wide range of transiting exoplanets. at uv wavelengths we would be able detect plantary exospheres and constrain the physics of atmospheric escape; in the optical we can constrain the properties of clouds and search absorption for alkali metals; and in the nir we can detect molecules like h2o, co2, and o2. | transmission spectroscopy of exoplanets with luvoir |
high euv irradiation near the parent star significantly heats and ionizes hydrogen-rich exoplanet upper atmospheres. the resulting weakly-bound gas, and frozen-in magnetic fields, can escape the planet in a gas pressure-driven outflow, aided by stellar tidal forces. this upper atmosphere of atomic hydrogen has been detected by the hst during transit in the lyman α line for a handful of planets. for one planet in particular, gj436b, the blue-shifted absorption during transit may imply that lyman alpha radiation forces have accelerated hydrogen atoms away from the star to large velocities. the distribution of hydrogen, at densities that give rise to an observable transit, may also extend well behind the planet in this system, perhaps over a large fraction of the orbit. two theoretical problems are raised by invoking radiation forces to explain gj436b. first, collisions of accelerated hydrogen atoms with neighboring protons may limit the speed to which the gas can be accelerated, by increasing the inertia as the hydrogen atom drags the protons with it. second, the lyman alpha line may be optically thick, so that acceleration by photons near line center may only occur in a thin skin on the outside of the planetary gas. hence an accurate wavelength-dependent treatment of lyman alpha radiation forces coupled to gas motion is required. we propose to implement a monte-carlo calculation of radiation transport with the zeus magnetohydrodynamics code in order to study exoplanet upper atmospheres and outflows, and their observable consequences in both transit observations as well as proposed scattered light observations at other orbital phases. heating by euv radiation and atomic line cooling will be used to self-consistently determine the gas temperature and lyman alpha source function within the planetary gas. scattering including partial frequency redistribution, true absorption, and collisional de-excitation will be included in the monte-carlo transport model. a prescription for joint diffusion in frequency and space will be used to speed up calculations in the optically thick regions. the radiation field will be determined at each time step and the radiation forces will act to accelerate the gas. previously, only the forces due to the stellar lyman α have been simulated. we will also include for the first time the radiation force from lyman alpha generated inside the planetary gas through recombination and electron impact excitation. the resulting radiation field is comparable in size to the stellar lyman alpha, and may give rise to a line-driven wind from the planet. models will be computed separately for a planetary wind centered on the planet, as well as streams of gas orbiting the star, treating the planet as a point source of gas. in addition to including the lyman alpha radiative force on the gas, the outgoing intensity will be tabulated over the grid boundary, allowing detailed three-dimensional calculations of transmission spectra, including absorption and emission by the gas. once code development is completed, models will be computed for known systems with lyman alpha observations. the resulting models will aid in interpreting previous hst observations of lyman alpha transmission spectra, and will make predictions for promising planet-star conditions for future observations. | exoplanetary mhd outflows driven by euv heating, lyman alpha radiation forces and stellar tides |
we study the atmospheric escape from m-dwarf exoplanets (including proxima b and seven earth-sized planets in the trappist-1 system) and its implications for long-term climate evolution and habitability by using the state-of-the-art numerical tools. | atmospheric escape from m-dwarf exoplanets and implications for long-term climate evolution and habitability |
we present a uniform analysis of the atmospheric escape rate of neptune-like planets with known radius and mass. we find that nearly 15% of neptunes exhibit extremely high mass-loss rates, that defy evolution and composition models of their atmospheres. we conclude that at least one of the established techniques to constrain the size or mass of exoplanets is consistently producing biased values for neptune planets. | mass-loss rate constraints on the observed distribution of exoplanets |
planetary oxidation is critical to the evolution of terrestrial-type atmospheres and consequently to astrobiology and the search for biosignatures. the atmospheres and surfaces of all rocky planets oxidize to some degree due to gravitational differentiation: hydrogen is lost to space and iron moves to the core, leaving more electronegative elements, particularly oxygen, in their wake. under some conditions, this process may continue until abiotic o2-dominated atmospheres form. to understand this important phenomenon in detail, we have performed the first fully coupled atmosphere-interior modeling of the oxidation of earth-like planets around different star types. we include atmospheric photochemistry, diffusion and escape, line-by-line climate calculations and interior thermodynamics and chemistry, which allows self-consistent calculation of a planet's atmospheric and mantle redox state as a function of time. we find that planets that receive a low stellar flux and/or have high h2o, n2 and mantle fe inventories are least likely to build up abiotic o2 atmospheres. in particular, the transiting exoplanets lhs1140b and trappist-1f and g have low probability of abiotic o2 buildup via water loss, making them important future targets for ground-based biosignature searches. furthermore, our predictions for hotter planets such as gj1132b provide a way to test our model via observations in future, without the confounding influence of a possible biosphere on the results. | planetary oxidation and abiotic o2 atmospheres |
the work presents the multi-fluid numerical modeling to interpret the observed absorption in mg resonance lines during the transit of wasp-12b and to quantify the crucial mechanisms responsible for the exoplanetary upper atmosphere mass loss. the model simulates the expansion of upper atmosphere due to stellar xuv radiation and includes the hydrogen chemistry and effects of stellar wind. the two case-scenarios of the planetary material escape and interaction with the stellar wind, namely the 'blown by the wind' (without the inclusion of tidal force) and 'captured by the star' (with the tidal force) have been modeled for different stellar xuv radiation fluxes and different stellar wind parameters. the simulation results demonstrate that in the 'blown by the wind' regime when a slow sw condition and no tidal force is considered in the model, we observe a very strong absorption of mg. however, in the 'captured by the star' regime, we found that the change in the xuv radiation fluxes does not make a significant change in the absorption profile of mg line. however, the sw condition has a major impact on the mg absorption line. in the case of 'captured by the star' regime, the calculated absorption shows a good agreement with the observed value of absorption in mg line which is 3-4%. | multi-fluid modeling of upper atmosphere mass loss and absorption line for wasp-12b |
exoplanetary environments are made of an intricate mixture of plasmas, radiation, energetic particles, winds, and magnetic fields; these all play crucial roles for the structure and evolution of planetary atmospheres and the formation and possibly protection of planetary habitable environments. interactions between planetary atmospheric particles and solar-wind ions result in various non-thermal loss mechanisms that are relevant for atmospheric erosion; energetic neutral atoms from charge exchange interactions can even deposit their energy in upper atmospheres and contribute to their heating. we present results from simulations and discuss the effects of magnetospheric obstacles, the resulting atmospheric loss rates and neutral hydrogen clouds detectable through ly a absorption. we also present estimates for secondary x-ray production as a result of charge exchange interactions. combined modeling of expanding hydrogen clouds resulting from such interactions are now also used to estimate magnetic moments of exoplanets. we emphasize that the interplay between all these mechanisms, also including radiation-driven thermal escape of atmospheres, changes with stellar evolution; for a full understanding of the state of an observed exoplanetary atmosphere, the long-term evolution of the host star, in particular its rotation and magnetic activity, needs to be studied. in this respect, radio astronomy plays a central role as it sensitively probes these environments and their constituents in time, such as magnetospheres, high-energy particles, stellar magnetic fields and winds, and therefore contributes to our understanding of the emergence of habitable planetary environments. | exoplanetary habitability: radiation, particles, plasmas, and magnetic fields |
we will present the results of magma ocean models incorporating ferrous iron disproportionation [1] in the silicate melt [2], combined with ferrous/ferric iron fractionation during crystallization of the magma ocean. the magma ocean model is coupled to a steam atmosphere model [3], which controls the rate of cooling. escape of hydrogen from the atmosphere leads to production of free oxygen which can either escape, build up in the atmosphere, or oxidize the magma ocean. ferrous iron disproportionation is likely to be more important on larger planets, whereas atmospheric oxidation will likely be more important on smaller planets or those receiving more stellar radiation. ferric iron partitioning is likely to result in an increase in the oxidation state of all magma oceans throughout their thermal evolution, although the magnitude of this effect will depend on equilibrium vs. fractional crystallization and the phases crystallizing from the magma ocean. we will discuss the relative importance of these three effects for venus, earth, mars and make generalizations for rocky exoplanets. [1] frost, dj et al (2004) nature, 428, 409 [2] zhang, hl et al (2017) gca, 204, 83 [3] schaefer, l et al (2016) apj, 829, 63 | combined redox evolution of magma oceans due to disproportionation, ferric iron partitioning, and atmospheric oxidation |
the uppermost layer of the atmosphere, the exosphere,is not well-known in its global structure since the densities are very low compared to instrument detection capabilities. because of rare collisions and high knudsen numbers, the motion of light species (h,h2, ...)in the corona is essentially determined by the external forces : the gravitation from the planet, the radiation pressure, as well the stellar gravity. in this work, we calculate rigorously and analytically,based on the hamiltonian mechanics and liouville theorem, the impact of the radiation pressure and gravitation from the planet on the structure of the exosphere. this approach was partially used by bishop and chamberlain (1989) but only in the 2d case : we extend it to the 3d case. assuming a collisionless exosphere and a constant radiation pressure near the planet, we determine the density profiles for ballistic particles (the main contribution for densities in the lower exosphere) for light species as a function of the angle with respect to the sun direction. we also obtain an analytical formula for the escape flux at the subsolar point, which can be compared with the jeans' escape flux. finally, we study the effect of the radiation pressure on the zero velocity curves, position of the roche lobe and hill's region for the well-known three-body problem especially for hot jupiters and discuss about the validity of our model. the goal is to bring some constraints on modelling of exoplanet atmospheres. | influence of the radiation pressure on the planetary exospheres: density profiles, escape flux and atmospheric stability |
short-period sub-neptunes with substantial volatile envelopes are among the most common type of known exoplanets. however, these planets are typically on highly-irradiated orbits where they are vulnerable to atmospheric photoevaporation. in particular, recent studies of the kepler planet population have suggested a dearth of sub-neptunes on orbits receiving more than 650x the broadband irradiation of the earth (lundkvist et al. 2016). physically, we expect this "photoevaporation desert" to depend on the lifetime integrated x-ray and extreme ultraviolet flux, which is the main driver of atmospheric escape for these planets. in this work, we compute planet occurrence as a function of integrated x-ray flux for the latest samples of confirmed kepler and k2 planets. our objective is to constrain the desert in a parameter space that ties directly to the photoevaporation process. we find a sharp drop-off in planet occurrence for integrated x-ray flux greater than 4 x 10^21 erg/cm^2, and we also investigate how this drop-off varies with stellar spectral type. | investigation of the sub-neptune photoevaporation desert for m-dwarfs to sun-like stars |
we aim at investigating the possible internal structures of the exoplanet gj 1132 b whose physical properties have been recently refined from multi-wavelength observations of a series of transits. because this planet potentially harbors an atmosphere, we also discuss the influence of hydrodynamic escape on its internal structure. | the atmosphere and internal structure of gj 1132 b |
ongoing and future space missions aim to identify potentially habitable planets in our solar system and beyond. planetary habitability is determined not only by a planet's current stellar insolation and atmospheric properties, but also by the evolutionary history of its climate. it has been suggested that icy planets and moons become habitable after their initial ice shield melts as their host stars brighten. here we show from global climate model simulations that a habitable state is not achieved in the climatic evolution of those icy planets and moons that possess an inactive carbonate-silicate cycle and low concentrations of greenhouse gases. examples for such planetary bodies are the icy moons europa and enceladus, and certain icy exoplanets orbiting g and f stars. we find that the stellar fluxes that are required to overcome a planet's initial snowball state are so large that they lead to significant water loss and preclude a habitable planet. specifically, they exceed the moist greenhouse limit, at which water vapour accumulates at high altitudes where it can readily escape, or the runaway greenhouse limit, at which the strength of the greenhouse increases until the oceans boil away. we suggest that some icy planetary bodies may transition directly to a moist or runaway greenhouse without passing through a habitable earth-like state. | abrupt climate transition of icy worlds from snowball to moist or runaway greenhouse |
exoplanets orbiting at close distances from their stars could lose some fraction of their atmospheres because of the extreme stellar irradiation. hot rocky planets might have lost all of their atmospheres through this process, having evolved from larger progenitors with gas envelopes, possibly neptune-mass exoplanets. the signature of this mass loss could be observed with hst/stis at lyman-alpha; however, there were no convincing detections of this effect for planets less massive than gas giants. new hst observations of the neptune-mass exoplanet gj 436b reveal a spectacular atmospheric escape. although not strong enough to deplete the atmosphere of the planet in the lifetime of the parent star, it creates a huge exospheric cloud of atomic hydrogen eclipsing over half the stellar disk at lyman-alpha. the exospheric cloud transit has an early ingress and numerical simulations predict it could last for almost half the 2.6-day revolution period of the planet, because of an extended comet-like tail. however, the current data do not stretch over this duration, and we propose here to use hst/stis to cover the full transit. this will reveal the exact shape of gj 436b exospheric cloud and allow us to discriminate between the two possible mechanism scuplting the cloud. the results could be applied to all moderately-irradiated exoplanets, in particular the planets around m dwarfs that will be detected by the tess mission. | full hst coverage of a comet-like exoplanet in transit |
young planetary systems offer the opportunity to study the early stages of planetary and atmospheric evolution. exoplanets in young stellar clusters, which have known ages, provide a valuable benchmark for models of these processes. k2-25b is a neptune-sized exoplanet recently found to transit a mid m dwarf member of the hyades open cluster (625 myr from isochrone fitting). atmospheric escape is thought to play an important role in sculpting the final compositions of low-mass planets like k2-25b. at 3.7 earth-radii, k2-25b may represent the puffy precursor to the close-in super-earths and mini-neptunes found around older mid m dwarfs. this star's youth, proximity and high levels of activity offer a unique opportunity to study atmospheric evaporation. we propose to observe two transits of k2-25b at lyman alpha wavelengths in order to determine the size of the planetary exosphere. we have simulated the expected lyman alpha spectrum of this star using representative a stellar profile and interstellar absorption, and considered the intrinsic stellar variability. we estimate that we will obain a s/n of 14.7 in the integrated flux in the blue wing of the lyman alpha line, where the transit signal is expected. a 50% lyman alpha transit depth was recently observed for gl 436b, which is a neptune-sized planet in orbit around an old, inactive m dwarf. our observations will result in a 10.6 sigma detection of the planetary exosphere if the transit is the same depth as for gl 436b. | the evaporating exosphere of a young planet |
with stis/uv observations we detected magnesium atoms at high altitude in the atmosphere of the hot-jupiter hd209458b, probing lower regions in the atmosphere than previously done with lyman-alpha observations (vidal-madjar et al. 2013). with the present program, we will search for magnesium and other heavy species in escaping atmospheres of 2 giant planets orbiting hot a and f-type stars: wasp-94ab and wasp-33b.the observations will provide unprecedented information on the physical conditions (velocity, temperature, and density) in the upper atmosphere of these two hot-jupiters. targets have been selected for the expected high significance level of the atmospheric detections (>10 sigma). these exoplanets present favorable configuration for upper atmosphere observations because of the combination of high escape rates and large spatial extensions of the magnesium clouds surrounding them. the atmospheric signatures of the magnesium and other metals are therefore expected to be easily detectable. moreover, the two selected exoplanets have highly different equilibrium temperatures, below and above the mgsio3 condensation temperature. consequently, because the metals observed in the escaping flow originate from deeper in the atmosphere where haze can condensate, the observations will constrain the physical processes taking place in the clouds that cannot be observed directly. | metals from deep atmosphere to exosphere in hot-jupiters |
observations of exoplanets during the transit of their host star allow probing the structure and composition of their atmosphere. the intense stellar energy input into exoplanets orbiting close to their star can lead to a dramatic expansion of their upper atmosphere, and the 'evaporation' of large amounts of gas into space. uv observations of hot jupiters revealed the extended exospheres formed by this escaping gas, and showed that these planets are too massive to lose a substantial fraction of their atmosphere. lower-mass planets are expected to be much more sensitive to evaporation, which has long been thought to play a role in forming the desert of hot neptunes (a deficit of neptune-size exoplanets on very short orbits). i will present the discovery of a giant hydrogen exosphere around gj3470b, a warm neptune located at the border of the desert. this is the first uv result of the panchromatic comparative exoplanet treasury (pancet) survey, a hubble program targeting 20 exoplanets across the entire spectrum. our numerical simulations of the resolved exospheric transit show that gj3470b is subjected to mass losses comparable to that of hot jupiters, making it the most extreme case of evaporation observed to date. gj3470b could already have lost up to 40% of its mass over its 2 gyr lifetime, bringing direct observational confirmation that evaporation shaped the population of close-in exoplanets. i will compare gj3470b with other known evaporating planets and discuss the reasons for its dramatic escape. our results strengthen the interest of observing the upper atmosphere of exoplanets to determine their properties and understand how they depend on their past evolution. this is particularly important for super-earth and earth-size planets, whose lower atmosphere could be hidden by clouds. the development of new tracers of atmospheric escape at optical/infrared wavelengths opens thrilling perspectives for the characterization of exoplanets via their upper atmosphere. | the most extreme case of atmospheric escape detected on the warm neptune gj 3470b with hst |
we present new quasi-global 3-d radiative-hydrodynamic simulations of close-in exoplanets undergoing atmospheric escape. by tracking the ionization state of outflows driven by ionization heating, we produce self-consistent synthetic observations. the resulting synthetic lyman-α observations find several distinct large-scale features of atmospheric escape, which we classify into three regimes dependent upon the properties of the interplanetary medium (e.g., stellar wind, ionizing flux, orbital separation). several of these new features produce substantial obscuration of the star many hours outside of transit. we therefore demonstrate that long-baseline transit observations in lyman-α and other non-optical lines are needed to constrain mass loss mechanisms. we compare to the several observations of known systems undergoing atmospheric escape, and discuss which aspects of the theory are still missing. by incorporating recent developments in transit observations of escaping exospheres, such as ground-based observations in h-α and helium 10830, we expand our models and probe the core of hydrodynamic escape missed in lyman-α. we conclude by discussing how our results inform our understanding of the evaporation valley evident in super-earth demographics. | atmospheric escape: new windows, longer baselines and demographic influences |
exoplanets in short-period orbits provide a unique opportunity to observe phenomena critical to the development and evolution of our own solar system, including atmospheric escape, interaction with the host star, and the potential to study exoplanetary magnetism. i will present the colorado ultraviolet transit experiment (cute), a 6u cubesat mission that aims at studying the physics of atmospheric escape and possibly detect the presence of magnetic fields on exoplanets. cute will be launched in mid-2020 and its flexible observing plan allows for coordinated uv-optical-infrared observations of particularly interesting bright targets with a number of current and future facilities. | the cute small satellite mission |
the most widely-studied mechanism of mass loss from irradiated exoplanets is photoevaporation via xuv ionization. however, lower-energy fuv dissociation of hydrogen molecules can also theoretically drive atmospheric evaporation on low-mass planets because the dissociation energy of hydrogen is an order of magnitude greater than the escape energy per proton from the gravity well of an earth-sized planet. this implies that a significant fraction of a star's blackbody flux can contribute to photoevaporation, potentially to a greater degree than ionizing radiation. for temperate planets such as the early earth, impact erosion is expected to dominate over photoevaporation in most formation models, but for highly irradiated planets such as those near the "evaporation valley" observed in kepler planets, or for pebble accretion formation models, they could plausibly be sculpted primarily by photodissociation. i present results of a survey of various mass loss processes and their relative contributions to mass loss from an early earth-like planet. in particular, we find that photodissociation could strip an atmosphere up to 0.5% of the mass of the planet even at solar levels of irradiation. i then apply this prescription for mass loss to models of highly irradiated super-earths and mini-neptunes and discuss the implications of these results for rocky planet formation and the interpretation of the evaporation valley. | photodissociation-driven mass loss from young and highly-irradiated exoplanets |
one of the great strengths of the proposed luvoir space observatory is that in addition to searching for potentially habitable terrestrial planets, it will find and characterize hundreds of other types of planets as well. this large population of planets will offer unparalleled opportunities for comparative planetary science over a wide range of masses, radii, and orbital separations. direct imaging and spectroscopy with eclips, an ultra-high contrast coronagraph, will enable a systematic investigation of system architectures and the diversity of exoplanet atmospheres. an investment of 424 hours with luvoir-a would obtain optical spectra of 30 known planets for example. for the more massive of these planets we will have previous mass measurements from gaia or radial velocity surveys, creating a powerful data set for testing theories of planet formation, atmospheric evolution, photochemistry, and cloud processes. for newly discovered planets, astrometric observations with hdi, the telescope's high resolution imager, or ground-based radial velocity observations will constrain planet masses. luvoir will also offer unparalled transit science, particularly at uv wavelengths, enabling studies of photochemistry and atmospheric escape. in our poster presentation we will illustrate a few examples of luvoir capabilities with both the a and b architectures to explore comparative planetary science, illustrating the diversity of studies possible within a relatively modest application of observatory time. | the capabilities of luvoir for studies of exoplanet diversity |
transmission spectroscopy is one of our most powerful tools for characterizing exoplanet atmospheres and thanks to the recent launch of nasa's tess mission we will soon have a large sample of planets around bright stars, ideally suited to this technique. with it's unique combination of uv to nir wavelength coverage and incredibly high s/n, luvoir would build upon the powerful legacy of hubble and revolutionize our ability to characterize the atmospheres of a wide range of transiting exoplanets. at fuv wavelengths the luvoir ultravoilet multi object spectrograph (lumos) will obtain high s/n transmission spectra and high spectral resolution, allowing us to detect transiting planetary exospheres and constrain the physics of atmospheric escape. meanwhile, using the uvis channel on the high definition imager (hdi) instrument we can obtain high s/n spectra at r≈500 in the optical and nuv, which allow us to constrain the properties of clouds and search for absorption from alkali metals. finally, with the nir channel in hdi we will be able to detect molecular absorption and measure abundances for a wide range of species including h2o, co2, and o2. | transmission spectroscopy of exoplanets with luvoir |
the atmospheres of highly irradiated exoplanets are observed to undergo hydrodynamic escape, resulting in planetary mass loss. however, stellar winds can shape and even prevent atmospheric escape, affecting observable signatures of escape such as lyman-alpha and h-alpha line profiles. in this work, we simulate atmospheric escape of close-in exoplanets and investigate whether they are affected by stellar winds. we show that, although younger hot-jupiters experience higher levels of atmospheric escape, owing to a favourable combination of higher irradiation levels and weaker planetary gravity, stellar winds are also stronger at this young age, which act as to reduce/inhibit escape rates of young exoplanets. | how stellar winds can affect atmospheric escape in close-in exoplanets |
we use maven data and models to assess how crustal magnetic fields modify the escape rates of atmospheric ions. this has applicability to a big picture question for mars, planetary, and exoplanetary science: do magnetic fields influence habitability? | do magnetic fields matter? |
with a simple transmission code, we compute high-resolution transit spectra of the sodium/potassium doublet with arbitrary number density profiles as input (in particular non-hydrostatic ones). while hydrostatic atmospheres absorb mostly in an optically thick regime, we find that non-hydrostatic atmospheres (such as escaping atmospheres or exogenic sources) have significant absorption in an optically thin regime. this leads to a distinct shape of non-hydrostatic transit spectra: while such atmospheres are able to absorb as much flux as hydrostatic ones in the line cores, non-hydrostatic atmospheres produce only negligible absorption in the line wings. they furthermore have a d2/d1 line ratio which is close to two (compared to hydrostatic atmospheres where this ratio is close to one). we compare our findings to data from the hot jupiter wasp-49b. we find that a non-hydrostatic atmosphere (namely an exogenic source) fits the observation significantly better than hydrostatic models. we conclude that current high-resolution observations of the sodium-/potassium doublet might contain signatures of escaping atmspheres or exogenic sources, and that the canonically used hydrostatic number density profile and transit spectra computations based thereon are no longer valid for such non-hydrostatic scenarios. | nonhydrostatic density profiles of sodium & potassium at close-in gas giant exoplanets |
host stars create environments that shape the atmospheric structure and composition of their exoplanets, the effects of which we observe today. the stellar wind and high energy radiation present within astrospheres can erode planetary atmospheres, leading to the detections of atmospheric evaporation for short-period planets gl 436b and gj 3470b. we have obtained 2 transit observations of the neptune-sized exoplanet k2-25b in lyman α with hst/stis. the young age of the k2-25 system and k2-25b's size and similarities with gl 436b suggest the possibility of a large amount of atmospheric escape. studying this planet will further our understanding of the evolution of small planets by probing how planetary properties, like size and youth, impact atmospheric escape. by characterizing the total intrinsic lyman α and x-ray flux from k2-25, we will present constraints on the euv environment the exoplanet resides in and the mass loss it is experiencing. | the evaporating atmosphere of the young exoplanet k2-25b |
most planets in the solar system have or previously had a global magnetic field, yet not much is known about magnetic fields in exoplanets. information about the presence of a magnetic field and its strength could give us valuable insights into the interior structure and thermal evolution of an exoplanet. furthermore, a global magnetic field on an exoplanet could have important consequences for the extent, composition, and evolution of its atmosphere, by controlling atmospheric escape and its interaction with the stellar wind. in this talk, i will present a new method for detecting magnetic fields in the atmospheres of close-in exoplanets, based on spectropolarimetric transit observations at the wavelength of the helium line at 1083 nm. strong absorption signatures (transit depths on the order of a few percent) in the 1083 nm line have recently been observed for several close-in exoplanets. most of the work so far has been focused on measuring and interpreting the effects of extended or escaping planetary atmospheres on the radiation intensity at 1083 nm; however, a wealth of information can be stored in radiation polarization as well. i will describe how linear and circular polarization signals in the helium 1083 nm line arise in the presence of an external magnetic field due to atomic level polarization induced by anisotropic stellar radiation, and the combined action of the zeeman and hanle effects. this phenomenon has been well established in solar physics as a means to probe the magnetic field properties of the solar chromosphere and corona, and i will demonstrate how the diagnostic power of this method can be extended to the field of exoplanets. assuming exoplanetary magnetic fields with strengths comparable to the magnetic fields observed in the solar system planets, polarization signals in the helium 1083 nm line should be detectable with modern high-resolution spectropolarimeters operating at these wavelengths. | detecting magnetic fields in exoplanets with spectropolarimetry in the helium line at 1083 nm |
we present 3d mhd simulations of the wind-wind interactions that occur between a solar-type star and a short period hot jupiter exoplanet. a planetary outflow results from atmospheric escape induced by the host stars incident radiation. circumstellar and circumplanetary material which accretes onto the stellar surface in a form of coronal rain, we characterise this interaction for a representative hot jupiter hosting system and predict the accretion point, size and extent. the nature of this accretion is variable in both location and rate, with the final accretion point occurring at 133 degrees west and 53 degrees east of the subplanetary point. the size of the accretion spot itself has been found to vary with a period of 67 ks (approximately 1/5 of the orbital period). the results are highly dependent on the magnetic fields of both the star and the planet and on the atmospheric conditions of the hot jupiter. we characterise this behaviour as star-planet-wind interaction (spwi). | it's raining hot jupiters: 3d mhd simulations of planetary atmospheric escape |
observational breakthroughs in the exoplanet field of the last decade motivated the development of numerous theoretical models describing atmospheres and mass loss, which is believed to be one of the main drivers of planetary evolution. we aim to outline for which types of close-in planets in the neptune-mass range the accurate treatment of photoionisation effects is most relevant concerning atmospheric escape and the parameters relevant for interpreting observations. we developed the chain (cloudy e hydro ancora insieme) model combining 1d hydrodynamic upper atmosphere model with the non-lte photoionisation and radiative transfer code cloudy accounting for photochemistry, detailed atomic level populations, and chemical reactions for all elements up to zinc. we apply chain to model the upper atmospheres of a range of neptune-like planets with masses between 1 and 50 m$_{\oplus}$, varying also the orbital parameters. for the majority of warm and hot neptunes, we find slower and denser outflows, with lower ion fractions, compared to the predictions of the hydrodynamic model alone. furthermore, we find significantly different temperature profiles between chain and the hydrodynamic model alone, though the peak values are similar for similar atmospheric compositions. the mass-loss rates predicted by chain are higher for hot, strongly irradiated planets and lower for more moderate planets. all differences between the two models are strongly correlated with the amount of high-energy irradiation. finally, we find that the hydrodynamic effects impact significantly ionisation and heating. the impact of the precise photoionisation treatment provided by cloudy strongly depends on the system parameters. this suggests that some of the simplifications typically employed in hydrodynamic modelling might lead to systematic errors when studying planetary atmospheres, even at a population-wide level. | precise photoionisation treatment and hydrodynamic effects in atmospheric modelling of warm and hot neptunes |
a large fraction of known exoplanets orbit their host stars at very short periods. due to such close proximity to the parent star, their atmospheres are exposed to high levels of stellar radiation and stellar wind, as well as strong tidal interactions, which can lead to atmospheric escape operating in a regime much more efficient than in the solar system planets. efficient atmospheric mass loss has the power to radically transform planets by changing their atmospheric structure or by removing the atmosphere altogether. even though the first evidence of ongoing atmospheric escape in exoplanets was acquired for close-in gas giants, exoplanet demographics suggest that this process likely plays a much greater evolutionary role in planets the size of neptune and smaller. it is believed to be responsible for shaping the so-called 'hot neptune desert', the observed paucity of intermediate-sized planets at close orbital separations. ltt 9779b is a rare example of a planet residing well within this desert, in the part of the parameter space where atmospheric escape driven by stellar high-energy radiation can be further assisted by roche lobe overflow, resulting in extremely high mass-loss rates. the goal of our proposal is to observe the ongoing atmospheric escape for this ultra-hot neptune through transmission spectroscopy at nuv wavelengths with stis. the nuv wavelength range gives us the best opportunity to observe the upper atmosphere in multiple lines of different species, which is necessary in order to break modeling degeneracies and constrain the atmospheric mass-loss rate. | heavy metals escaping from a hot neptune |
intuition for the various regimes of atmospheric escape driven by stellar interactions will be given through order of magnitude sketches. consideration will be given to both planetary parameters and the stellar environment in which the planet resides. thereafter, detailed models will be presented focusing on the most dramatic regime, hydrodynamic escape. a demonstration of our publicly available 1-d hydrodynamic escape code will be given to estimate mass-loss rates for various known systems. the simplicity and speed of the code makes it ideal for survey follow-up candidate selection. next, we present quasi-global 3-d radiative-hydrodynamic simulations of exoplanets undergoing hydrodynamic escape. by tracking the ionization state of outflows driven by ionization heating, we are able to produce self-consistent synthetic observations. the resulting synthetic lyman-alpha observations find several distinct large-scale features of atmospheric escape, which we classify into three regimes dependent upon the properties of the interplanetary medium (e.g., stellar wind, ionizing flux, orbital separation). of note, several of these new features produce substantial obscuration of the star many hours outside of transit. we therefore demonstrate that long-baseline transit observations in lyman-alpha and other non-optical lines are needed to constrain mass-loss mechanisms. we compare to the several observations of known systems undergoing atmospheric escape, and discuss which aspects of the theory are still missing. by incorporating recent developments in transit observations of escaping exospheres, such as ground-based observations of the balmer series, we expand our models and probe the core of hydrodynamic escape missed in lyman-alpha due to the interstellar medium. we conclude by discussing how our results inform our understanding of the evaporation valley evident in super-earth demographics. | atmospheric escape for close-in exoplanets |
both rocky super-earths and volatile-rich sub-neptunes have been found simultaneously in multi-planetary systems, suggesting that these systems are appropriate to study different composition and formation pathways within the same environment. to estimate their composition, we present an interior structure model that includes self-consistently an atmosphere in radiative-convective equilibrium. we use our interior-atmosphere model within a bayesian adaptive markov chain monte carlo (mcmc) for the detailed analysis of individual planets, allowing to estimate the uncertainties of the compositional parameters, core mass fraction and water mass fraction, given the error bars of the observed mass and radius. in this talk, i will present the development of this compositional model for super-earths and sub-neptunes, as well as its application to different planetary systems. i will focus in particular on the homogeneous analysis of a sample of multi-planetary systems hosting 5 or more exoplanets. their composition gives us clues about their possible formation site in the protoplanetary disk and their formation mechanisms, including atmospheric escape. i will also discuss the uncertainties and degeneracies interior models face when estimating the volatile content of low-mass planets, and how jwst and future telescopes can help narrow down their possible compositions. | water content trends in low-mass multiplanetary systems |
ultra-hot jupiter is a new class of exoplanets emerging in the recent years. their extremely hot temperatures cause thermal dissociation of molecules and even ionisation of atoms. we have detected an extended hot hydrogen atmosphere around kelt-9b — the hottest exoplanet discovered so far. the detection was achieved by measuring the atomic hydrogen absorption during transit with the balmer hα line using the carmenes spectrograph. the obtained hα transmission spectrum has a strong extra absorption of 1.15% at the line centre. the observation implies that the effective radius at the line centre is ∼ 1.64 times the size of the planetary radius, indicating that the planet has a largely extended hydrogen envelope close to the size of the roche lobe and is probably undergoing dramatic atmosphere escape. | high resolution transmission spectrum of ultra-hot jupiters |
in this white paper we focus on the science case for exoplanet observations at near-uv/blue optical wavelengths. scattering information encoded in at these wavelengths can distinguish the cause of muted spectral features. these wavelengths also capture atmospheric escape processes, and help characterize mass loss from exoplanet atmospheres. | understanding exoplanet atmospheres with uv observations i: nuv and blue/optical |
of the three terrestrial worlds that have significant atmospheres (venus, earth, and mars), only earth also possesses a global dynamo magnetic field. this magnetic field is often thought to have shielded the planet from the impinging solar wind, preventing the atmosphere from being stripped away to space. the atmospheres of mars and venus, by contrast, are thought to have escaped to space or been dessicated (respectively) due at least in part to their planet's lack of global magnetic field. the assumption that global scale magnetic fields are a necessary requirement for surface habitability is widely used both in the planetary and exoplanetary communities, but this assumption has been called into question in recent years based both on theoretical arguments and on observations returned by spacecraft. here we summarize the arguments "for" and "against" the importance of magnetic fields for planetary habitability, and review the observations that teach us about the role of magnetic fields. we then identify several ongoing efforts and likely fruitful avenues for determining whether a dynamo field is necessary for life to be possible at a planet's surface. | do habitable worlds require magnetic fields? |
in this white paper we focus on the science case for exoplanet observations in the far-uv (fuv); an accompanying white paper led by jessie christiansen will focus on the science case in the nuv and blue/optical. | understanding exoplanet atmospheres with uv observations ii: the far uv and atmospheric escape |
atmospheric escape is the prevailing evolutionary theory that explains current exoplanet demographics - pertaining to the most common exoplanet types, short-period sub-neptunes and super-earths, in particular. two planets have been detected around the 23 myr pre-main sequence m dwarf au mic; their known age makes them good probes for early stages of exoplanet evolution. au mic b is the 4.2 earth radius inner planet orbiting with a period of 8 days. the planet's youth, high levels of x-ray and uv radiation, and proximity to its bright host indicate this planet is likely experiencing atmospheric mass loss. previous stis uv observations of this planet show a highly variable lyman-alpha transit, going from no detected planetary outflow to detected. even more strange, the detection occurs before the white-light transit of the planet, meaning the neutral hydrogen outflow is moving ahead of the planet. this could be explained by 1) variable photoionization of the outflow, or 2) turbulent interactions with the stellar wind environment that cause a "burping" of the outflow. we propose for two additional transits of this planet to test this hypothesis. observing the extreme behavior of this young planet will provide critical constraints on atmospheric escape models. | a young burping planet: characterizing the variable atmospheric escape of the exciting au mic b |
atmospheres of exoplanets are our main observing window into the physical and chemical properties of these remote worlds. spectacular progress has already been made on the atmospheric characterisation of exoplanets, highlighting a huge diversity compared to our solar system planets. in particular, hot exoplanets found very close to their stars have been found to lose a substantial fraction of their atmospheres, because of the tremendous stellar irradiation they receive. it is surmised that such planets could be entirely stripped from their atmospheres, raising questions as to the possibility that gaseous planets in milder irradiation regimes could give birth to smaller planets with residual, earth-like atmospheres. the study of atmospheric escape is thus essential to our understanding of exoplanet evolution, and the origins and stability of habitable worlds. upper atmospheres (thermosphere + exosphere) give rise to spectacular spectroscopic signatures in the uv and more recently in the ir. the metastable helium lines (10832.1, 10833.2 and 10833.3 \aa) represent recent tracers of escape in a wavelength range where the spectra of the star and of the planetary atmosphere are more easily observed than in the uv. observations of exospheres in the uv only yield part of the information about the escaping gas which need to be completed by probing the thermosphere in the ir. it is thus essential to simulate self-consistently thermosphere and exosphere with the most relevant codes, to interpret accurately observations of these layers. the joint interpretation can be done using sophisticated simulations with the model of upper atmosphere that we developed to the description of both gaseous and ultra-hot rocky planets, evaporation of exoplanets (eve). this model is a 3d model simulating the different atmospheric layers of exoplanets: the lower layers, described in a fluid regime, are modelled with a high-resolution cartesian grid containing millions of cells. the upper atmospheric layers are modelled in a collisionless regime, with tens of millions of particles of gas. the main purpose of the eve code is to calculate the absorption of stellar light across the simulated atmosphere. the better coupling of thermospheric and exospheric structure allow us to re-interpret and in greater details available uv (hst) and ir (carmenes, giano, spirou, keck/nirspec) observations of upper atmospheres. we also aim to interpret and predict signatures of escape in near-infrared jwst spectra of small exoplanets, and in near-infrared and optical spectra respectively obtained by the gtos of the nirps and espresso spectrographs. | toward the characterization of upper atmospheres with a 3d coupled thermosphere-exosphere model |
spectroscopy of transiting exoplanets has revealed a wealth of information about their atmospheric compositions and thermal structures. in particular, studies of highly irradiated exoplanets at temperatures much higher than those found in our solar system have provided detailed information on planetary chemistry and physics because of the high level of precision which can be obtained from such observations. here we use a variety of techniques to study the atmospheres of highly irradiated transiting exoplanets and address three large, open questions in exoplanet atmosphere spectroscopy. first, we use secondary eclipse and phase curve observations to investigate the thermal structures and heat redistribution of ultra-hot jupiters, the hottest known exoplanets. we demonstrate how these planets form an unique class of objects influenced by high-temperature chemical effects such as molecular dissociation and h- opacity. second, we use observations of helium in the upper atmosphere of the exo-neptune hat-p-11b to probe atmospheric escape processes. third, we develop tools to interpret jwst observations of highly irradiated exoplanets, including a data analysis pipeline to perform eclipse mapping of hot jupiters and a method to infer albedos of and detect atmospheres on hot, terrestrial planets. finally, we discuss remaining open questions in the field of highly irradiated exoplanets and opportunities to advance our understanding of these unique bodies in the coming years. | revealing the atmospheres of highly irradiated exoplanets: from ultra-hot jupiters to rocky worlds |
we present a simple analytic description of atmospheric mass loss by aerial bursts and demonstrate that mass loss from aerial bursts becomes significant when the maximum impactor size that leads to an aerial burst rather than a ground explosion, ro, is larger than the minimum impactor size needed to achieve atmospheric loss, rmin. for vertical trajectories, which give the most stringent limit, this condition is approximately satisfied when ρo/ρi ≳ 0.4v, which implies atmospheric densities need to be comparable to impactor densities for impactor velocities that are a few times the escape velocity of the planet. the range of impactor radii resulting in aerial burst-induced mass loss, ro-rmin, increases with the ratio of the atmosphere to the impactor density and with the trajectory angle of the impactor. the range of impactor radii that result in aerial burst-induced mass loss and the atmospheric mass lost is larger in adiabatic atmospheres than in isothermal atmospheres of equivalent total mass, scale height, and atmospheric surface density. our results imply that aerial bursts are not expected to significantly contribute to the atmospheric mass loss history of earth, but are expected to play an important role for planets and exoplanets similar to neptune with significant atmospheres. for neptune-like atmospheres, the atmospheric mass ejected per impactor mass by aerial bursts is comparable to that lost by ground explosions, which implies that, for impactors following a dohnanyi size distribution, overall loss by aerial bursts is expected to exceed that by ground explosions by a factor of (rground/raerial)0.5. | atmosphere loss by aerial bursts |
ultra-hot jupiters (uhjs) provide a natural laboratory of rapid atmospheric escape that can be used to test models of this process. uhjs have dayside temperatures in excess of 2200 k, which means that metals and heavy elements are not likely to condense on the dayside. metals like fe and mg can often escape their atmospheres and are detectable in the near-uv. for example, the ultra-hot jupiter wasp-121b is among the known transiting planets with the highest potential for atmospheric escape. this is confirmed by transmission spectra at different wavelengths, including in the nuv, that show evidence of the escape of hydrogen and metals from the planet's atmosphere. a one-size-fits-all approach, however, does not apply to mass loss rates from uhjs. many uhjs are relatively stable to mass loss by atmospheric escape, often because of their relatively high masses. we use detailed models of uhj atmospheres, spanning from the lower atmosphere to the thermosphere, to demonstrate that the mass loss rate should depend much more strongly on planet mass than expected based on the standard energy-limited approach. we then discuss the nuv and other observations in the context of the results to further constrain the energy balance in uhj atmospheres. in particular, we focus on nuv observations of wasp-121b and wasp-189b. while the nuv spectrum of wasp-121b was observed with the hubble space telescope (hst), the nuv spectrum of wasp-189b was observed by the colorado ultraviolet transit experiment (cute) and it constitutes the first exoplanet transit spectrum observed with a small satellite. | atmospheric escape and near-uv signatures of ultra-hot jupiters |
as the exoplanet wasp-107b orbits its host star, its atmosphere escapes to form a comet-like tail. helium atoms detected in the escaping gases give astronomers a powerful tool for investigating exoplanetary atmospheres. | helium discovered in the tail of an exoplanet |
exoplanet radii show a bimodal distribution, with two peaks corresponding to smaller planets (likely rocky) and larger intermediate-size planets, respectively. we apply interior structure model, growth model, as well as atmospheric escape model, and conduct monte carlo simulations, to demonstrate that many intermediate-size planets are water worlds. this result has profound implications on planet formation theory and origins of life. (https://arxiv.org/abs/1906.04253) | iceland, iceline, iceplanets |
this work shows results from magma ocean outgassing and hydrodynamic escape processes on the mass and longevity of secondary atmospheres on rocky exoplanets. | the transition from primary to secondary atmospheres on rocky exoplanets |
the atmospheres of close-in, low-mass exoplanets are extremely vulnerable to the effects of stellar uv to x-ray radiation. photoevaporation can significantly ablate planetary atmospheres or even strip them entirely, potentially rendering a planet uninhabitable. existing hydrodynamical studies of this important atmospheric mass loss mechanism have mainly considered hydrogen/helium dominated atmospheres. currently, the effect of higher mean compositions on photoevaporative mass loss has only been the subject of a limited number of studies. in the era of more advanced exoplanet atmospheric observations, it is more important than ever to determine what, if any, atmosphere these planets may have been able to retain. here, we present preliminary results of hydrodynamic simulations, showing a water-rich atmosphere of a low-mass planet undergoing photoevaporation. we show accurate modelling of higher mean molecular weight atmospheres can result in different escape rates than those predicted by current theoretical models. | photoevaporation of water dominated exoplanet atmospheres |
the observed exoplanet population unveiled by recent detection programs is billions of years old, distinctly separated in time from the planet formation process that only lasted ~10-100 myr. i will argue that photo evaporation driven atmospheric escape has been one of the key evolutionary drivers shaping the exoplanet population we observed today. by understanding how these planet evolve in time, i will show we can place some intriguing constraints on how they formed. | the evaporation driven evolution of close-in exoplanets |
exoplanet research has become a major focus due to advancements like the transit method, which allows us to observe the features of exoplanet atmospheres. shared features between exoplanetary atmospheres and their host stars limit confidence on any atmospheric interpretation. our ability to characterize the variability of these shared stellar features is critical in accurately characterizing planetary atmospheres. the helium i 1083 nm line is one such shared feature and is an ideal absorption line to study when probing the upper atmosphere of certain exoplanets for atmospheric escape. by investigating the variability of the he i 1083nm absorption line in the sun we can begin to understand how the feature behaves in other sunlike stars. i analyzed publicly available solis/iss spectra of the sun as a star to document how the he i 1083nm line strength changed as a function of time. using the sherpa model-fitting python package, i was able to calculate the equivalent widths for nearly 3,000 observations between 2007 and 2017. tracking these fluctuations through both low and high stellar activity, which can be approximated by the s-index, can reveal more precise constraints on how we expect the line to vary during specific points in a star's cycle. this analysis will allow us to better disentangle the stellar component of the he i 1083nm signal from exoplanetary atmospheres. this work was supported by the nsf-reu solar physics program at sao, grant number ags-1560313. | characterizing variability for solar he i 1083 nm |
the photoionization-driven evaporation of planetary atmospheres appears to be a fundamental process for exoplanets on short period orbits. given its importance, it is vital that photoevaporation be explored as a fully 3-d multi-physics process facilitating detailed comparison with existing observations. using astrobear, an amr multiphysics code, we model the transfer of ionizing photons into the atmosphere of a hot jupiter and track the subsequent launch of the wind and its large-scale evolution subject to tidal and non-inertial forces. we run simulations for planets of 0.263 and 0.07 jupiter masses and stellar fluxes of 2x1013 and 2x1014 photons/cm2/s. these simulations reveal new, potentially observable planetary wind flow patterns, including the development of an extended neutral tail lagging behind the planet in its orbit. in addition, the role of radiation pressure in shaping exoplanet photoevaporation remains a topic of contention. radiation pressure from the exoplanet's host star has been proposed as a mechanism to drive the escaping atmosphere into a "cometary" tail and explain the high velocities observed in systems where mass loss is occurring. using simulations of a planet modeled after hd 209458b and subject to both ionizing and lyman-α radiation, we demonstrate that, for the lyman-α flux expected for hd 209458, radiation pressure is unlikely to significantly affect photoevaporative winds or to explain the high velocities at which wind material is observed. charge exchange between the stellar and planetary winds has also been suggested as a method for creating the observed lyman-α absorption signature. we present results of new simulations that explore the effect of charge exchange on our synthetic observations. finally, we present simulations of the effect of stellar and planetary magnetic fields on our self-consistently launched wind. | exoplanet photoevaporation and mass loss: why don't theory and observation match? |
the extreme ultraviolet (euv) emission from stars is the dominant source of heating to the upper atmospheres of planets and drives atmospheric escape, but this wavelength regime is unobserved for most exoplanet host stars. the differential emission measure technique is a method to infer the euv emission of stars using fuv emission lines and x-ray fluxes as inputs. this method can function in the limits of low precision data with appropriately propagated uncertainties and can be effectively applied to a large ensemble of stars. we will use archival hubble stis/cos data of 115 gkm dwarfs, supplemented by archival x-ray data, to infer these stars' euv spectra using the differential emission measure technique. the selected sample spans a wide range of stellar masses and ages encompassing a parameter space that includes the vast majority of exoplanet hosts. the euv spectral library of this sample will be published in an easy-to-use format for the broader exoplanet community to use as inputs for models of planetary atmosphere formation and evolution and to study the present-day euv irradiation of known exoplanets. to make statistically and scientifically rigorous claims about the relationship between euv emission of planet hosts and the properties of their planetary systems we need population-wide euv estimates with well-characterized uncertainties. this proposal makes use of the hubble archive to provide homogeneously determined euv spectra, enabling comparative studies of exoplanets formation, evolution, and habitability. | a library of extreme ultraviolet spectra for 115 main-sequence stars to enable comparative exoplanet studies |
the recent observations on wasp-39 b by jwst have revealed hints of high metallicity within the atmosphere compared to its host star. there are various theories on how these high metallic atmospheres emerge. in this study, we closely investigate the impact of extreme escape in the form of hydrodynamic escape to see its impact on atmospheric metallicity and spectral features such as ch4, co2 and so2. we perform a grid simulation, with an adapted version of mesa that includes hydrodynamic escape to fully evolve planets with similar masses and radii to the currently observed wasp-39 b estimates. by making use of (photo)chemical kinetics and radiative transfer codes, we evaluate the transmission spectra at various time intervals throughout the simulation. our results indicate that the massive size of wasp-39 b limits the metal enhancement to a maximum of ~1.23× the initial metallicity. when incorporating metal drag, this enhancement factor is repressed to an even greater degree, resulting in an enrichment of at most ~0.4%. as a consequence, when assuming an initial solar metallicity, metal-enriched spectral features like so2 are still missing after ~9 gyr into the simulation. this paper, thus, demonstrates that hydrodynamic escape cannot be the primary process behind the high metallicity observed in the atmosphere of wasp-39 b, suggesting instead that a metal-enhanced atmosphere was established during its formation. | metallicity and spectral evolution of wasp 39b: the limited role of hydrodynamic escape |
kepler's observations show most of the exoplanets are super-earths. the formation of a super-earth is generally related to the atmospheric mass loss that is crucial in the planetary structure and evolution. the shock driven by the giant impact will heat the planet, resulting in the atmosphere escape. we focus on whether self-gravity changes the efficiency of mass loss. without self-gravity, if the impactor mass is comparable to the envelope mass, there is a significant mass-loss. the radiative-convective boundary will shift inward by self-gravity. as the temperature and envelope mass increase, the situation becomes more prominent, resulting in a heavier envelope. therefore, the impactor mass will increase to motivate the significant mass loss, as the self-gravity is included. with the increase of envelope mass, the self-gravity is particularly important. | effects of self-gravity on mass-loss of the post-impact super-earths |
characterizing rocky exoplanet atmospheres is a key goal of exoplanet science, but interpreting such observations will require understanding the stellar ultraviolet (uv) irradiation incident on the planet from its host star. stellar uv mediates atmospheric escape, photochemistry, and planetary habitability, and observations of rocky exoplanets can only be understood in the context of the uv spectral energy distribution (sed) of their host stars. particularly important are seds from observationally favorable but poorly understood low-mass m-dwarf stars, which are the only plausible targets for rocky planet atmospheric characterization for the next 1-2 decades. in this work, we explore the utility of astrosat ultraviolet imaging telescope (uvit) for the characterization of the uv seds of low-mass stars. we present observations of the nearby m0 star hip 23309 in the far-uv (fuv) and near-uv (nuv) gratings of uvit. our fuv spectra are consistent with contemporaneous hubble space telescope (hst) data and our nuv spectra are stable between orbits, suggesting uvit is a viable tool for the characterization of the seds of low-mass stars. we apply our measured spectra to simulations of photochemistry and habitability for a hypothetical rocky planet orbiting hip 23309 and elucidate the utility and limitations of uvit in deriving uv seds of m-dwarf exoplanet hosts. our work validates uvit as a tool to complement hst in the characterization of exoplanet host stars and carries implications for its successor missions like insist. | uv spectral characterization of low-mass stars with astrosat uvit for exoplanet applications: the case study of hip 23309 |
x-ray observations of star-planet systems are important to grow our understanding of exoplanets; these observation allow for studies of photoevaporation of the exoplanetary atmosphere, and in some cases even estimations of the size of the outer planetary atmosphere. the german-russian erosita instrument onboard the srg (spectrum roentgen gamma) mission is performing the first all-sky x-ray survey since the 1990s, and provides x-ray fluxes and spectra of exoplanet host stars over a much larger volume than was accessible before. using new erosita data as well as archival data from xmm-newton, chandra and rosat we estimate mass loss rates of exoplanets under an energy-limited escape scenario, and identify several exoplanets with strong x-ray irradiation and expected mass-loss that are amenable to follow-up observations at other wavelengths. we model sample spectra using a toy model of an exoplanetary atmosphere to predict what exoplanet transit observations with future x-ray missions such as athena will look like, and estimate the observable x-ray transmission spectrum for a typical hot jupiter-type exoplanet. | identifying interesting planetary systems for future x-ray observations |
the atmosphere of a terrestrial planet that is replenished with secondary gases should have accumulated hydrogen-rich gas from its protoplanetary disk. although a giant impact blows off a large fraction of the primordial atmosphere of a terrestrial planet in the late formation stage, the remaining atmosphere can become water-rich via chemical reactions between hydrogen and vaporized core material. we find that a water-rich postimpact atmosphere forms when a basaltic or ci chondrite core is assumed. in contrast, little postimpact water is generated for an enstatite chondrite core. we investigate the x-ray- and uv-driven mass loss from an earth-mass planet with an impact-induced multicomponent h2-he-h2o atmosphere for gyr. we show that water is left in the atmosphere of an earth-mass planet when the low flux of escaping hydrogen cannot drag water upward via collisions. for a water-dominated atmosphere to form, the atmospheric mass fraction of an earth-mass planet with an oxidizing core after a giant impact must be less than a few times 0.1%. we also find that earth-mass planets with water-dominated atmospheres can exist at semimajor axes ranging from a few times 0.1 au to a few au around a sun-like star, depending on the mass-loss efficiency. such planets are important targets for atmospheric characterization in the era of jwst. our results indicate that efficient mixing between hydrogen and rocky components during giant impacts can play a role in the production of water in an earth-mass planet. | evolution of a water-rich atmosphere formed by a giant impact on an earth-sized planet |
the absorption signals of metastable he in hd 209458b and several other exoplanets can be explained via an escaping atmosphere model with a subsolar he/h ratio. the low abundance of helium can be a result of planet formation if there is a small amount of helium in their primordial atmosphere. however, another possibility is that the low he/h ratio is caused by the process of mass fractionation of helium in the atmosphere. in order to investigate the effect of fractionation in the hydrogen-helium atmosphere, we developed a self-consistent multi-fluid 1d hydrodynamic model based on the well-known open-source mhd code pluto. our simulations show that a lower he/h ratio can be produced spontaneously in the multi-fluid model. we further modeled the transmission spectra of he 10830 lines for hd 209458b in a broad parameter space. the transmission spectrum of the observation can be fitted in the condition of 1.80 times the x-ray and extreme-ultraviolet flux of the quiet sun. meanwhile, the ratio of the escaping flux of helium to hydrogen, f he/f h, is 0.039. our results indicate that the mass fractionation of helium to hydrogen can naturally interpret the low he/h ratio required by the observation. thus, in the escaping atmosphere of hd 209458b, decreasing the abundance of helium in the atmosphere is not needed even if its he abundance is similar to that of the sun. the simulation presented in this work hints that in the escaping atmosphere, mass fractionation can also occur on other exoplanets, which needs to be explored further. *revised manuscript on 2022 june 7 | the mass fractionation of helium in the escaping atmosphere of hd 209458b |
spectral observations in the ly-α line have shown that atmospheric escape is variable and for the exoplanet hd189733b, the atmospheric evaporation goes from undetected to enhanced evaporation in a 1.5 years interval. to understand the temporal variation in the atmospheric escape, we investigate the effect of flares, winds, and cmes on the atmosphere of hot jupiter hd189733b using 3d self-consistent radiation hydrodynamic simulations. we consider four cases: first, the quiescent phase including stellar wind; secondly, a flare; thirdly, a cme; and fourthly, a flare followed by a cme. we find that the flare alone increases the atmospheric escape rate by only 25%, while the cme leads to a factor of 4 increments, in comparison to the quiescent case. we also find that the flare alone cannot explain the observed high blue-shifted velocities seen in the ly-α. the cme, however, leads to an increase in the velocity of escaping atmospheres, enhancing the blue-shifted transit depth. | effect of stellar flares and coronal mass ejections on the atmospheric escape from hot jupiters |
the lyα emission line is the brightest uv emission line in m and k dwarf spectra and serves as an important tool for studies of stellar chromospheres, the interstellar medium, and exoplanet atmospheres. however, lyα observations have proven difficult due to the strong absorption by the interstellar medium, necessitating a reconstruction of the intrinsic stellar line from the observed spectrum. we have performed new lyα reconstructions on the muscles treasury survey stars, incorporating improved parameterizations for the intrinsic line wings and line core. we present an analysis of how the updated lyα fluxes could impact photochemical and atmospheric escape studies and flux-flux scaling relations with other chromospheric emission lines such as ca ii h and k. we find the overall intrinsic lyα flux of our star sample decreases by as little as 10% to as much as ~5× fainter compared to previous findings. the exception to this flux decrease is the m dwarf gj 581, whose lyα flux increased by 4%. these results will likely have a limited impact on the aforementioned studies that rely on lyα fluxes. | new and improved lyα reconstructions for m and k dwarfs |
in this presentation, we make use of sophisticated 3d numerical simulations to assess the extent of atmospheric ion (e.g., o+, o2+ and co2+) losses from venus over time. a sophisticated upper atmospheric model (venus thermospheric general circulation model - vtgcm) is coupled with a magnetohydrodynamic (mhd) code, bats-r-us, to incorporate the effects of varying solar radiation and wind conditions over the past four billion years. we demonstrate that the atmospheric ion escape rates were significantly higher (by orders of magnitude) in the past at ∼4 ga compared to the present-day values owing to the stronger solar wind and higher ultraviolet fluxes from the young sun. we found that the atmospheric ion loss is likely to become much more important at ancient times. we briefly discuss the ensuing implications of high atmospheric ion escape rates in the context of ancient venus, and exoplanets with similar atmospheric compositions around young solar-type stars and m-dwarfs. | modeling venusian atmospheric losses over time |
multi-planet systems offer insight into a host star's influence on its planets' atmospheres. k2-3 is an m0 star (effective temperature of 3880 k) orbited by three small transiting planets, k2-3b, c, and d (2.0, 1.6, and 1.4 earth radii, respectively). we combine stellar spectral information with self-consistently derived stellar and planetary properties to investigate the k2-3 system. we measure the uv flux of k2-3 with hst/cos and place an upper limit on its x-ray flux with xmm-newton. we use empirical scaling techniques and a differential emission measure to fill in gaps in the high energy spectrum. we constrain the k2-3 system properties by simultaneously fitting transit photometry, radial velocity, and spectral energy density information. the radii of all three planets are constrained to 2% and the masses of the inner two planets, k2-3b and c, to 10%, while the outermost planet, k2-3d, is not detected in the rv measurements. k2-3b and c are enhanced in volatiles and consistent with sub-neptune-like compositions, while k2-3d is likely terrestrial due to its small radius. the compositions of sub-neptune planets are highly degenerate, and indeed we find that the radii and masses of k2-3b and c allow for solar composition envelopes making up 0.30% and 0.05% of their masses, respectively, as well as water steam atmospheres making up for 50% and 13% of their masses. however, the solar composition scenarios are unlikely given the high energy output of k2-3; the timescale for atmospheric escape (1-2 gyr) is short compared to the estimated age of the system (3-12 gyr). additionally, assuming that k2-3d is terrestrial, we find that the architecture of the k2-3 planets is inconsistent with photoevaporation as the sole mechanism for sculpting their atmospheres. rather we must invoke more stochastic processes such as gas-depleted planet formation or giant impacts to account for the observed system architecture. finally, we present model transmission spectra that include photochemical reactions and disequilibrium chemistry for a range of atmospheric cases: for k2-3 b and c we model the aforementioned solar composition and steam atmosphere cases, while for k2-3d we model early venus, modern venus, and modern earth cases. discriminating between these atmospheric cases will require spectral information about the atmospheres of the k2-3 planets, something that is achievable with current observatories. this work serves as a template for future investigations into the atmospheres of small exoplanets in multi-planet systems in the context of their host stars. | a visit to the k2-3 system: models of three planetary atmospheres spanning the radius valley informed by a high energy stellar spectrum and updated system properties |
dust clouds are ubiquitous in the atmospheres of hot jupiters and affect their observable properties. the alignment of dust grains in the clouds and resulting dust polarization is a promising method to study magnetic fields of exoplanets. moreover, the grain size distribution plays an important role in physical and chemical processes in the atmospheres, which is rather uncertain in atmospheres. in this paper, we first study grain alignment of dust grains in the atmospheres of hot jupiters by radiative torques (rats). we find that silicate grains can be aligned by rats with the magnetic fields (b-rat) due to strong magnetic fields of hot jupiters, but carbonaceous grains of diamagnetic material tend to be aligned with the radiation direction (k-rat). at a low altitude of $r<2r_{\rm p}$ with $r_{\rm p}$ being the planet radius, only large grains can be aligned, but tiny grains of $a\sim 0.01\mu$m can be aligned at a high altitude of $r>3r_{\rm p}$. we then study rotational disruption of dust grains by the radiative torque disruption (ratd) mechanism. we find that large grains can be disrupted by ratd into smaller sizes. grains of high tensile strength are disrupted at an altitude of $r>3r_{\rm p}$, but weak grains can be disrupted at a lower altitude. we suggest that the disruption of large grains into smaller ones can facilitate dust clouds to escape to high altitudes due to lower gravity and may explain the presence of high-altitude clouds in hot jupiter as well as super-puff atmospheres. | grain alignment and rotational disruption by radiative torques in exoplanet atmospheres |
we present two novel additions to the semianalytic solution of lyα radiative transfer in spherical geometry: (1) implementation of the correct boundary condition for a steady source, and (2) solution of the time-dependent problem for an impulsive source. for the steady-state problem, the solution can be represented as a sum of two terms: a previously known analytic solution of the equation with mean intensity j = 0 at the surface, and a novel, semianalytic solution which enforces the correct boundary condition of zero-ingoing intensity at the surface. this solution is compared to that of the monte carlo method, which is valid at arbitrary optical depth. it is shown that the size of the correction is of order unity when the spectral peaks approach the doppler core and decreases slowly with line center optical depth, specifically as ${(a{\tau }_{0})}^{-1/3}$ , which may explain discrepancies seen in previous studies. for the impulsive problem, the time, spatial, and frequency dependence of the solution are expressed using an eigenfunction expansion in order to characterize the escape time distribution and emergent spectra of photons. it is shown that the lowest-order eigenfrequency agrees well with the decay rate found in the monte carlo escape time distribution at sufficiently large line center optical depths. the characterization of the escape time distribution highlights the potential for a monte carlo acceleration method, which would sample photon escape properties from distributions rather than calculating every photon scattering, thereby reducing computational demand. | a novel solution for resonant scattering using self-consistent boundary conditions |
one of jwst's four pillars of science points to finding the building blocks of life elsewhere in the universe. planets orbiting m-dwarf stars represent our best (and only) opportunity to measure the spectrum of a potentially-habitable planet in the next decade. the quest towards habitability begins with a simple question: does this planet have an atmosphere? whether or not terrestrial m-dwarf planets can retain their atmospheres is a hotly debated topic and only a large observational campaign acquiring exoplanet transmission spectra can provide unequivocal evidence of atmospheres. understanding which m-dwarf planets have atmospheres will focus future theoretical efforts and could provide the first evidence of a "cosmic shoreline", a universal division between planets with and without substantial atmospheres. even the population of planets with tenuous atmospheres will inform us about atmospheric escape processes. in this study,we will obtain transmission spectra of nine terrestrial planets orbiting the nearest m dwarfs using instrument modes that are sensitive to co2 at 4.3 microns and ch4 at 3.3 microns, the strongest such features in jwst's wavelength range. upon successful completion of this campaign, we will know which transiting m-dwarf planets within 15 parsecs have atmospheres and, of those that do, the fundamental diversity in their basic atmospheric compositions. we will know how the presence of an atmosphere correlates with planet irradiation and escape velocity, and how the evolutionary history of m dwarfs shapes the atmospheres of the planets that orbit them. ultimately, this study will generate new sparks of life in m-dwarf planet research. | tell me how i?m supposed to breathe with no air: measuring the prevalence and diversity of m-dwarf planet atmospheres |
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