Temporal evolution of the high-energy irradiation and water content of TRAPPIST-1 exoplanets
V. Bourrier, J. de Wit, E. Bolmont, V. Stamenkovic, P. J. Wheatley, A. J. Burgasser, L. Delrez, B. -O. Demory, D. Ehrenreich, M. Gillon, E. Jehin, J. Leconte, S… 展开作者
V. Bourrier, J. de Wit, E. Bolmont, V. Stamenkovic, P. J. Wheatley, A. J. Burgasser, L. Delrez, B. -O. Demory, D. Ehrenreich, M. Gillon, E. Jehin, J. Leconte, S. M. Lederer, N. Lewis, A. H. M. J. Triaud, V. Van Grootel
备注：
19 pages, 14 figures, accepted for publication in AJ
The ultracool dwarf star TRAPPIST-1 hosts seven Earth-size transiting planets, some of which could harbour liquid water on their surfaces. UV observations are essential to measure their high-energy irradiation, and to search for photodissociated water escaping from their putative atmospheres. Our new observations of TRAPPIST-1 Ly-$α$ line during the transit of TRAPPIST-1c show an evolution of the star emission over three months, preventing us from assessing the presence of an extended hydrogen exosphere. Based on the current knowledge of the stellar irradiation, we investigated the likely history of water loss in the system. Planets b to d might still be in a runaway phase, and planets within the orbit of TRAPPIST-1g could have lost more than 20 Earth oceans after 8 Gyr of hydrodynamic escape. However, TRAPPIST-1e to h might have lost less than 3 Earth oceans if hydrodynamic escape stopped once they entered the habitable zone. We caution that these estimates remain limited by the large uncertainty on the planet masses. They likely represent upper limits on the actual water loss because our assumptions maximize the XUV-driven escape, while photodissociation in the upper atmospheres should be the limiting process. Late-stage outgassing could also have contributed significant amounts of water for the outer, more massive planets after they entered the habitable zone. While our results suggest that the outer planets are the best candidates to search for water with the JWST, they also highlight the need for theoretical studies and complementary observations in all wavelength domains to determine the nature of the TRAPPIST-1 planets, and their potential habitability.
We present an XMM-Newton X-ray observation of TRAPPIST-1, which is an ultracool dwarf star recently discovered to host three transiting and temperate Earth-sized planets. We find the star is a relatively strong and variable coronal X-ray source with an X-ray luminosity similar to that of the quiet Sun, despite its much lower bolometric luminosity. We find L_x/L_bol=2-4x10^-4, with the total XUV emission in the range L_xuv/L_bol=6-9x10^-4, and XUV irradiation of the planets that is many times stronger than experienced by the present-day Earth. Using a simple energy-limited model we show that th…
The James Webb Space Telescope (JWST) will offer the first opportunity to characterize terrestrial exoplanets with sufficient precision to identify high mean molecular weight atmospheres, and TRAPPIST-1's seven known transiting Earth-sized planets are particularly favorable targets. To assist community preparations for JWST, we use simulations of plausible post-ocean-loss and habitable environments for the TRAPPIST-1 exoplanets, and test simulations of all bright object time series spectroscopy modes and all MIRI photometry filters to determine optimal observing strategies for atmospheric dete…
The TRAPPIST-1 system offers the opportunity to characterize terrestrial, potentially habitable planets orbiting a nearby ultracool dwarf star. We performed a four-orbit reconnaissance with the Space Telescope Imaging Spectrograph onboard the Hubble Space Telescope to study the stellar emission at Lyman-$α$, to assess the presence of hydrogen exospheres around the two inner planets, and to determine their UV irradiation. We detect the Lyman-$α$ line of TRAPPIST-1, making it the coldest exoplanet host star for which this line has been measured. We reconstruct the intrinsic line profile, showing…
Three Earth-sized exoplanets were recently discovered close to the habitable zone of the nearby ultracool dwarf star TRAPPIST-1. The nature of these planets has yet to be determined, since their masses remain unmeasured and no observational constraint is available for the planetary population surrounding ultracool dwarfs, of which the TRAPPIST-1 planets are the first transiting example. Theoretical predictions span the entire atmospheric range from depleted to extended hydrogen-dominated atmospheres. Here, we report a space-based measurement of the combined transmission spectrum of the two inn…
Context. The TRAPPIST-1 system hosts seven Earth-sized, temperate exoplanets orbiting an ultra-cool dwarf star. As such, it represents a remarkable setting to study the formation and evolution of terrestrial planets that formed in the same protoplanetary disk. While the sizes of the TRAPPIST-1 planets are all known to better than 5% precision, their densities have significant uncertainties (between 28% and 95%) because of poor constraints on the planet's masses. Aims.The goal of this paper is to improve our knowledge of the TRAPPIST-1 planetary masses and densities using transit-timing variati…
We have obtained the highest resolution images available of TRAPPIST-1 using the Gemini-South telescope and our speckle imaging camera. Observing at 692 and 883 nm, we reached the diffraction limit of the telescope providing a best resolution of 27 mas or, at the distance of TRAPPIST-1, a spatial resolution of 0.32 AU. Our imaging of the star extends from 0.32 to 14.5 AU. We show that to a high confidence level, we can exclude all possible stellar and brown dwarf companions, indicating that TRAPPIST-1 is a single star.
We explore the occurrence and detectability of planet-planet occultations (PPOs) in exoplanet systems. These are events during which a planet occults the disk of another planet in the same system, imparting a small photometric signal as its thermal or reflected light is blocked. We focus on the planets in TRAPPIST-1, whose orbital planes we show are aligned to within 0.3 degrees at 90% confidence. We present a photodynamical model for predicting and computing PPOs in TRAPPIST-1 and other systems for various assumptions of the planets' atmospheric states. When marginalizing over the uncertainti…
Context. New estimates of the masses and radii of the seven planets orbiting the ultracool M-dwarf TRAPPIST-1 star permit improved modelling of their compositions, heating by tidal dissipation, and removal of tidal heat by solid-state convection. Aims. Here, we compute the heat flux due to insolation and tidal heating for the inner four planets. Methods. We apply a Maxwell viscoelastic rheology to compute the tidal response of the planets using the volume-weighted average of the viscosities and rigidities of the metal, rock, high-pressure ice and liquid water/ice I layers. Results. We show tha…
The era of atmospheric characterization of terrestrial exoplanets is just around the corner. Modeling prior to observations is crucial in order to predict the observational challenges and to prepare for the data interpretation. This paper presents the report of the TRAPPIST Habitable Atmosphere Intercomparison (THAI) workshop (14-16 September 2020). A review of the climate models and parameterizations of the atmospheric processes on terrestrial exoplanets, model advancements and limitations, as well as direction for future model development was discussed. We hope that this report will be used …
Exoplanets residing close to their stars can experience evolution of both their physical structures and their orbits due to the influence of their host stars. In this work, we present a coupled analysis of dynamical tidal dissipation and atmospheric mass loss for exoplanets in XUV irradiated environments. As our primary application, we use this model to study the TRAPPIST-1 system, and place constraints on the interior structure and orbital evolution of the planets. We start by reporting on a UV continuum flux measurement (centered around $\sim1900$ Angstroms) for the star TRAPPIST-1, based on…
The recent discovery of seven potentially habitable Earth-size planets around the ultra-cool star TRAPPIST-1 has further fueled the hunt for extraterrestrial life. Current methods focus on closely monitoring the host star to look for biomarkers in the transmission signature of exoplanet's atmosphere. However, the outcome of these methods remain uncertain and difficult to disentangle with abiotic alternatives. Recent exoplanet direct imaging observations by THIRSTY, an ultra-high contrast coronagraph located in La Trappe (France), lead us to propose a universal and unambiguous habitability crit…
The closest stars that harbor potentially habitable planets are cool M-stars. Upcoming ground- and space-based telescopes will be able to search the atmosphere of such planets for a range of chemicals. To facilitate this search and to inform upcoming observations, we model the high-resolution reflection spectra of two of the closest potentially habitable exoplanets for a range of terrestrial atmospheres and surface pressures for active and inactive phases of their host stars for both oxic and anoxic conditions: Proxima b, the closest potentially habitable exoplanet, and Trappist-1e, one of 3 E…
Small exoplanets of nearby M dwarf stars present the possibility to find and characterize habitable worlds within the next decade. TRAPPIST-1, an ultracool M dwarf star, was recently found to have seven Earth-sized planets of predominantly rocky composition. The planets e, f, and g can have a liquid water ocean on their surface given appropriate atmospheres of N2 and CO2. Particularly, climate models have shown that the planets e and f can sustain a global liquid water ocean, for >=0.2 bar CO2 plus 1 bar N2, or >=2 bars CO2, respectively. These atmospheres are irradiated by ultraviolet emissio…
Rocky exoplanets orbiting in the habitable zone (HZ) of nearby M dwarfs provide unique opportunities for characterizing their atmospheres and searching for biosignature gases. TRAPPIST-1e, a temperate Earth-sized exoplanet in the HZ of a nearby M dwarf, is arguably the most favorable target for ground- and space-based atmospheric characterization by the extremely large telescopes (ELTs) and the James Webb Space Telescope (JWST). To inform future observations in reflected and emitted lights using these upcoming telescopes, we simulate the high-resolution reflection and emission spectra for TRAP…
The atmospheres of rocky exoplanets are close to being characterized by astronomical observations, in part due to the commissioning of the James Webb Space Telescope. These observations compel us to understand exoplanetary atmospheres, in the voyage to find habitable planets. With this aim, we investigate the effect that CO$_2$ partial pressure (pCO$_2$) has on exoplanets' climate variability, by analyzing results from ExoCAM model simulations of the tidally locked TRAPPIST-1e exoplanet, an Earth-like aqua-planet and Earth itself. First, we relate the differences between the planets to their e…
Modeling Volcanic Plume Heights Across Exoplanet Atmospheres: Insights from TRAPPIST-1
NASA Goddard Space Flight Center（美国国家航空航天局戈达德太空飞行中心）；Integrated Space Science and Technology Institute, Department of Physics, American University（美国大学物理系综合空间科学与技术研究所）
The TRAPPIST-1 system is home to at least seven terrestrial planets and is a target of interest for future James Webb Space Telescope (JWST) observations. Additionally, these planets will be of interest to future missions making observations in the ultraviolet (UV). Although several of these planets are located in the traditional habitable zone, where liquid water could exist on the surface, TRAPPIST-1h is interesting to explore as a potentially habitable ocean world analog. In this study, we evaluate the observability of a Titan-like atmosphere on TRAPPIST-1h. The ability of the JWST or a fut…
The star TRAPPIST-1 hosts a system of seven transiting, terrestrial exoplanets apparently in a resonant chain, at least some of which are in or near the Habitable Zone. Many have examined the roles of tides in this system, as tidal dissipation of the orbital energy of the planets may be relevant to both the rotational and orbital dynamics of the planets, as well as their habitability. Generally, tides are calculated as being due to the tides raised on the planets by the star, and tides raised on the star by the planets. I write this research note to point out a tidal effect that may be at leas…
One of the most significant goals of modern science is establishing whether life exists around other suns. The most direct path towards its achievement is the detection and atmospheric characterization of terrestrial exoplanets with potentially habitable surface conditions. The nearest ultracool dwarfs (UCDs), i.e. very-low-mass stars and brown dwarfs with effective temperatures lower than 2700 K, represent a unique opportunity to reach this goal within the next decade. The potential of the transit method for detecting potentially habitable Earth-sized planets around these objects is drastical…
We present a simple model for estimating the probability of interplanetary panspermia in the recently discovered system of seven planets orbiting the ultracool dwarf star TRAPPIST-1, and find that panspermia is potentially orders of magnitude more likely to occur in the TRAPPIST-1 system compared to the Earth-to-Mars case. As a consequence, we argue that the probability of abiogenesis is enhanced on the TRAPPIST-1 planets compared to the Solar system. By adopting models from theoretical ecology, we show that the number of species transferred and the number of life-bearing planets is also likel…
Using the Andrade-derived Sundberg-Cooper rheology, we apply several improvements to the secular tidal evolution of TRAPPIST-1e and the early history of Pluto-Charon under the simplifying assumption of homogeneous bodies. By including higher-order eccentricity terms (up to and including $e^{20}$), we find divergences from the traditionally used $e^{2}$ truncation starting around $e=0.1$. Order-of-magnitude differences begin to occur for $e>0.6$. Critically, higher-order eccentricity terms activate additional spin-orbit resonances. Worlds experiencing non-synchronous rotation can fall into and …
The Trappist-1 system contains seven roughly Earth-sized planets locked in a multi-resonant orbital configuration, which has enabled precise measurements of the planets' masses and constrained their compositions. Here we use the system's fragile orbital structure to place robust upper limits on the planets' bombardment histories. We use N-body simulations to show how perturbations from additional objects can break the multi-resonant configuration by either triggering dynamical instability or simply removing the planets from resonance. The planets cannot have interacted with more than ${\sim 5\…
The high energy radiation environment around M dwarf stars strongly impacts the characteristics of close-in exoplanet atmospheres, but these wavelengths are difficult to observe due to geocoronal and interstellar contamination. On account of these observational restrictions, a stellar atmosphere model may be used to compute the stellar extreme ultraviolet (EUV; 100 - 912 Å) spectrum. We present a case study of the ultra-cool M8 dwarf star, TRAPPIST-1, which hosts seven transiting short-period terrestrial sized planets whose atmospheres will be probed by the James Webb Space Telescope. We const…
Recently, transmission spectroscopy in the atmospheres of the TRAPPIST-1 planets revealed flat and featureless absorption spectra, which rule out cloud-free hydrogen-dominated atmospheres. Earth-sized planets orbiting TRAPPIST-1 likely have either a clear or a cloudy/hazy hydrogen-poor atmosphere. In this paper, we investigate whether a proposed formation scenario is consistent with expected atmospheric compositions of the TRAPPIST-1 planets. We examine the amount of a hydrogen-rich gas that TRAPPIST-1-like planets accreted from the ambient disk until disk dispersal. Since TRAPPIST-1 planets a…
Degenerate interpretations of O$_3$ spectral features in exoplanet atmosphere observations due to stellar UV uncertainties: a 3D case study with TRAPPIST-1e
Many stars of different spectral types with planets in the habitable zone are known to emit flares. Until now, studies that address the long-term impact of stellar flares and associated Coronal Mass Ejections (CMEs) assumed that the planet's interior remains unaffected by interplanetary CMEs, only considering the effect of plasma/UV interactions on the atmosphere of planets. Here, we show that the magnetic flux carried by flare-associated CMEs results in planetary interior heating by Ohmic dissipation and leads to a variety of interior--exterior interactions. We construct a physical model to s…
Upcoming telescopes such as the James Webb Space Telescope (JWST), or the Extremely Large Telescope (ELTs), may soon be able to characterize, through transmission, emission or reflection spectroscopy, the atmospheres of rocky exoplanets orbiting nearby M dwarfs. One of the most promising candidates is the late M dwarf system TRAPPIST-1 which has seven known transiting planets for which Transit Timing Variation (TTV) measurements suggest that they are terrestrial in nature, with a possible enrichment in volatiles. Among these seven planets, TRAPPIST-1e seems to be the most promising candidate t…
To identify promising exoplanets for atmospheric characterization and to make the best use of observational data, a thorough understanding of their atmospheres is needed. 3D general circulation models (GCMs) are one of the most comprehensive tools available for this task and will be used to interpret observations of temperate rocky exoplanets. Due to parameterization choices made in GCMs, they can produce different results, even for the same planet. Employing four widely-used exoplanetary GCMs -- ExoCAM, LMD-G, ROCKE-3D and the UM -- we continue the TRAPPIST-1 Habitable Atmosphere Intercompari…
The TRAPPIST-1 system provides an exquisite laboratory for advancing our understanding exoplanetary atmospheres, compositions, dynamics and architectures. A remarkable aspect of TRAPPIST-1 is that it represents the longest known resonance chain, where all seven planets share near mean motion resonances with their neighbors. Prior to the measurement of 1h's period, Luger et al. (2017) showed that six possible and highly precise periods for 1h were expected, assuming it also participated in the resonant chain. We show here that combining this argument with a Titius-Bode law fit of the inner six …
Limits on forming coreless terrestrial worlds in the TRAPPIST-1 system
SKLab-DeepMinE, MOEKLab-OBCE, School of Earth and Space Sciences, Peking University（北京大学地球与空间科学学院）；Research Institute of Extraterrestrial Material at PKU (RIEMPKU)（北京大学地外物质研究所）；Institute for Particle Physics and Astrophysics, ETH Zürich（苏黎世联邦理工学院粒子物理与天体物理研究所）
The recent discovery of the planetary system hosted by the ultracool dwarf star TRAPPIST-1 could open new perspectives into the investigation of planetary climates of Earth-sized exoplanets, their atmospheres and their possible habitability. In this paper, we use a simple climate-vegetation energy-balance model to study the climate of the seven TRAPPIST-1 planets and the climate dependence on the global albedo, on the fraction of vegetation that could cover their surfaces and on the different greenhouse conditions. The model allows us to investigate whether liquid water could be maintained on …
The TRAPPIST-1 system provides an exquisite laboratory for understanding exoplanetary atmospheres and interiors. Their mutual gravitational interactions leads to transit timing variations, from which Grimm et al. (2018) recently measured the planetary masses with precisions ranging from 5% to 12%. Using these masses and the <5% radius measurements on each planet, we apply the method described in Suissa et al. (2018) to infer the minimum and maximum CRF (core radius fraction) of each planet. Further, we modify the maximum limit to account for the fact that a light volatile envelope is excluded …
The TRAPPIST-1 planetary system provides an unprecedented opportunity to study terrestrial exoplanet evolution with the James Webb Space Telescope (JWST) and ground-based observatories. Since M dwarf planets likely experience extreme volatile loss, the TRAPPIST-1 planets may have highly-evolved, possibly uninhabitable atmospheres. We used a versatile, 1D terrestrial-planet climate model with line-by-line radiative transfer and mixing length convection (VPL Climate) coupled to a terrestrial photochemistry model to simulate environmental states for the TRAPPIST-1 planets. We present equilibrium …
The nearby (d = 12 pc) M8 dwarf star TRAPPIST-1 (2MASS J23062928-0502285) hosts a compact system of at least seven exoplanets with sizes similar to Earth. Given its importance for testing planet formation and evolution theories, and for assessing the prospects for habitability among Earth-size exoplanets orbiting the most common type of star in the Galaxy, we present a comprehensive assessment of the age of this system. We collate empirical age constraints based on the color-absolute magnitude diagram, average density, lithium absorption, surface gravity features, metallicity, kinematics, rota…
Multiple planet systems provide an ideal laboratory for probing exoplanet composition, formation history and potential habitability. For the TRAPPIST-1 planets, the planetary radii are well established from transits (Gillon et al., 2016, Gillon et al., 2017), with reasonable mass estimates coming from transit timing variations (Gillon et al., 2017, Wang et al., 2017) and dynamical modeling (Quarles et al., 2017). The low bulk densities of the TRAPPIST-1 planets demand significant volatile content. Here we show using mass-radius-composition models, that TRAPPIST-1f and g likely contain substant…
Variations in the reflective properties of the bulk material that comprises the surface of land-dominated planets will affect the planetary energy balance by interacting differently with incident radiation from the host star. Furthermore, low-mass cool stars, such as nearby M8V dwarf TRAPPIST-1, emit a significant fraction of their flux in longer wavelengths relative to the Sun in regions where terrestrial materials may exhibit additional variability in albedo. Using the Community Earth System Model (CESM) we investigate the effect of the composition of the land surface and its albedo on plane…
The TRAPPIST-1 planetary system is an excellent candidate for study of the evolution and habitability of M-dwarf planets. Transmission spectroscopy observations performed with the Hubble Space Telescope (HST) suggest the innermost five planets do not possess clear hydrogen atmospheres. Here we reassess these conclusions with recently updated mass constraints and expand the analysis to include limits on metallicity, cloud top pressure, and the strength of haze scattering. We connect recent laboratory results of particle size and production rate for exoplanet hazes to a one-dimensional atmospher…
TRAPPIST-1 planets are invaluable for the study of comparative planetary science outside our Solar System and possibly habitability. First, we derive from N-body simulations possible planetary evolution scenarios, and show that each of the planets are likely to be in synchronous rotation. We then use a 3-D Global Climate Model to explore the possible climates of cool planets of the TRAPPIST-1 system. In particular, we look at the conditions required for cool planets to prevent possible volatile species to be lost by permanent condensation, irreversible burying or photochemical destruction. We …
The seven approximately Earth-sized transiting planets in the \object{TRAPPIST-1} system provide a unique opportunity to explore habitable zone and non-habitable zone small planets within the same system. Its habitable zone exoplanets -- due to their favorable transit depths -- are also worlds for which atmospheric transmission spectroscopy is within reach with the Hubble Space Telescope (HST) and with the James Webb Space Telescope (JWST). We present here an independent reduction and analysis of two \textit{HST} Wide Field Camera 3 (WFC3) near-infrared transit spectroscopy datasets for six pl…
TRAPPIST: a robotic telescope dedicated to the study of planetary systems
我们在这里介绍一个新的机器人望远镜称为TRAPPIST（TRAnsiting Planets and PlanetesImals Small Telescope）。TRAPPIST配备了高质量的CCD相机，安装在0.6米的轻型光学管上，于2010年4月安装在ESO La Silla天文台（智利），现在正在开始其科学计划。TRAPPIST的科学目标是通过两种方法研究行星系：探测和研究系外行星以及研究彗星。我们在这里描述的项目目标，硬件，我们提出了一些在调试阶段获得的第一批成果。
Although high energetic radiation from flares is a potential threat to exoplanet atmospheres and may lead to surface sterilization, it might also provide the extra energy for low-mass stars needed to trigger and sustain prebiotic chemistry. We investigate two flares on TRAPPIST-1, an ultra-cool dwarf star that hosts seven exoplanets of which three lie within its habitable zone. The flares are detected in all four passbands of the MuSCAT2 allowing a determination of their temperatures and bolometric energies. We analyzed the light curves of the MuSCAT1 and MuSCAT2 instruments obtained between 2…
The precise characterization of terrestrial atmospheres with the James Webb Space Telescope (JWST) is one of the utmost goals of exoplanet astronomy in the next decade. With JWST's impending launch, it is crucial we are well prepared to understand the subtleties of terrestrial atmospheres - particularly ones we may have not needed to consider before due to instrumentation limitations. In this work we show that patchy ice cloud variability is present in the upper atmospheres of M-dwarf terrestrial planets, particularly along the limbs. Here we test whether these variable clouds will introduce u…
