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Context. With seven planets, the TRAPPIST-1 system has among the largest number of exoplanets discovered in a single system so far. The system is of astrobiological interest, because three of its planets orbit in the habitable zone of the ultracool M dwarf. Aims. We aim to determine interior structures for each planet and estimate the temperatures of their rock mantles due to a balance between tidal heating and convective heat transport to assess their habitability. We also aim to determine the precision in mass and radius necessary to determine the planets' compositions. Methods. Assuming the planets are composed of uniform-density noncompressible materials (iron, rock, H2O), we determine possible compositional models and interior structures for each planet. We also construct a tidal heat generation model using a single uniform viscosity and rigidity based on each planet's composition. Results. The compositions for planets b, c, d, and e remain uncertain given the error bars on mass and radius. With the exception of TRAPPIST-1c, all have densities low enough to indicate the presence of significant H2O. Planets b and c experience enough heating from planetary tides to maintain magma oceans in their rock mantles; planet c may have surface eruptions of silicate magma, potentially detectable with next-generation instrumentation. Tidal heat fluxes on planets d, e, and f are twenty times higher than Earth's mean heat flow. Conclusions. Planets d and e are the most likely to be habitable. Planet d avoids the runaway greenhouse state if its albedo is &0.3. Determining the planet's masses within ~0.1-0.5 Earth masses would confirm or rule out the presence of H2O and/or iron. Understanding the geodynamics of ice-rich planets f, g, and h requires more sophisticated modeling that can self-consistently balance heat production and transport in both rock and ice layers. [ABSTRACT FROM AUTHOR]

We study the evolution of protoplanetary discs that would have been precursors of a Trappist-1-like system under the action of accretion and external photoevaporation in different radiation environments. Dust grains swiftly grow above the critical size below which they are entrained in the photoevaporative wind, so although gas is continually depleted, dust is resilient to photoevaporation after only a short time. This means that the ratio of the mass in solids (dust plus planetary) to the mass in gas rises steadily over time. Dust is still stripped early on, and the initial disc mass required to produce the observed 4M⊕ of Trappist-1 planets is high. For example, assuming a Fatuzzo & Adams distribution of UV fields, typical initial disc masses have to be >30 per cent the stellar (which are still Toomre Q stable) for the majority of similar mass M dwarfs to be viable hosts of the Trappist-1 planets. Even in the case of the lowest UV environments observed, there is a strong loss of dust due to photoevaporation at early times from the weakly bound outer regions of the disc. This minimum level of dust loss is a factor of 2 higher than that which would be lost by accretion on to the star during 10Myr of evolution. Consequently, even in these least irradiated environments, discs that are viable Trappist-1 precursors need to be initially massive (>10 per cent of the stellar mass). [ABSTRACT FROM AUTHOR]

The recently detected TRAPPIST-1 planetary system, with its seven planets transiting a nearby ultracool dwarf star, offers the first opportunity to perform comparative exoplanetology of temperate Earth-sized worlds. To further advance our understanding of these planets' compositions, energy budgets, and dynamics, we are carrying out an intensive photometric monitoring campaign of their transits with the Spitzer Space Telescope. In this context, we present 60 new transits of the TRAPPIST-1 planets observed with Spitzer/Infrared Array Camera (IRAC) in 2017 February and March. We combine these observations with previously published Spitzer transit photometry and perform a global analysis of the resulting extensive data set. This analysis refines the transit parameters and provides revised values for the planets' physical parameters, notably their radii, using updated properties for the star. As part of our study, we also measure precise transit timings that will be used in a companion paper to refine the planets' masses and compositions using the transit timing variations method. TRAPPIST-1 shows a very low level of low-frequency variability in the IRAC 4.5-μmband, with a photometric RMS of only 0.11 per cent at a 123-s cadence. We do not detect any evidence of a (quasi-)periodic signal related to stellar rotation.We also analyse the transit light curves individually, to search for possible variations in the transit parameters of each planet due to stellar variability, and find that the Spitzer transits of the planets are mostly immune to the effects of stellar variations. These results are encouraging for forthcoming transmission spectroscopy observations of the TRAPPIST-1 planets with the James Webb Space Telescope. [ABSTRACT FROM AUTHOR]

The presence of an atmosphere over sufficiently long timescales is widely perceived as one of the most prominent criteria associated with planetary surface habitability. We address the crucial question of whether the seven Earth-sized planets transiting the recently discovered ultracool dwarf star TRAPPIST-1 are capable of retaining their atmospheres. To this effect, we carry out numerical simulations to characterize the stellar wind of TRAPPIST-1 and the atmospheric ion escape rates for all of the seven planets. We also estimate the escape rates analytically and demonstrate that they are in good agreement with the numerical results. We conclude that the outer planets of the TRAPPIST-1 system are capable of retaining their atmospheres over billion-year timescales. The consequences arising from our results are also explored in the context of abiogenesis, biodiversity, and searches for future exoplanets. In light of the many unknowns and assumptions involved, we recommend that these conclusions must be interpreted with due caution. [ABSTRACT FROM AUTHOR]

With the discovery of rocky planets in the temperate habitable zone (HZ) of the closeby cool star TRAPPIST-1, the question of whether such planets could harbour life arises. Habitable planets around red dwarf stars can orbit in radiation environments that can be life-sterilizing. Ultraviolet (UV) flares from these stars are more frequent and intense than solar flares. Additionally, their temperate HZs are closer to the star. Here we present UV surface environment models for TRAPPIST-1's HZ planets and explore the implications for life. TRAPPIST-1 has high X-ray/extreme-ultraviolet activity, placing planetary atmospheres at risk from erosion. If a dense Earth-like atmosphere with a protective ozone layer existed on planets in the HZ of TRAPPIST-1, UV surface environments would be similar to the present-day Earth. However, an eroded or an anoxic atmosphere would allow more UV to reach the surface, making surface environments hostile even to highly UV tolerant terrestrial extremophiles. If future observations detect ozone in the atmospheres of any of the planets in the HZ of TRAPPIST-1, these would be interesting targets for the search for surface life. We anticipate our assay to be a starting point for in-depth exploration of stellar and atmospheric observations of the TRAPPIST-1 planets to constrain their UV surface habitability. [ABSTRACT FROM AUTHOR]

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 with the Earth-to-Mars case. As a consequence, we argue that the probability of abiogenesis is enhanced on the TRAPPIST-1 planets compared with the solar system. By adopting models from theoretical ecology, we show that the number of species transferred and the number of lifebearing planets are also likely to be higher because of the increased rates of immigration. We propose observational metrics for evaluating whether life was initiated by panspermia on multiple planets in the TRAPPIST-1 system. These results are also applicable to habitable exoplanets and exomoons in other planetary systems. [ABSTRACT FROM AUTHOR]

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 LX/Lbol = 2–4 × 10-4, with the total XUV emission in the range LXUV/Lbol = 6–9 × 10-4, 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 the relatively close-in Earth-sized planets, which span the classical habitable zone of the star, are subjected to sufficient X-ray and EUV irradiation to significantly alter their primary and any secondary atmospheres. Understanding whether this high-energy irradiation makes the planets more or less habitable is a complex question, but our measured fluxes will be an important input to the necessary models of atmospheric evolution. [ABSTRACT FROM AUTHOR]

The Rossiter–McLaughlin effect is the principal method of determining the sky-projected spin–orbit angle (β) of transiting planets. Taking the example of the recently discovered TRAPPIST-1 system, we explore how ultracool dwarfs facilitate the measurement of the spin– orbit angle for Earth-sized planets by creating an effect that can be an order of magnitude more ample than the Doppler reflex motion caused by the planet if the star is undergoing rapid rotation. In TRAPPIST-1’s case we expect the semi-amplitudes to be 40–50 ms−1 for the known transiting planets. Accounting for stellar jitter expected for ultracool dwarfs and instrumental noise, and assuming radial velocity precisions both demonstrated and anticipated for upcoming near-infrared spectrographs, we quantify the observational effort required to measure the planets’ masses and spin–orbit angles.We conclude that if the planetary system is well-aligned then β can be measured to a precision of ≲10° if the spectrograph is stable at the level of 2 ms−1.We also investigate the measure of β, the mutual inclination, when multiple transiting planets are present in the system. Lastly, we note that the rapid rotation rate of many late M-dwarfs will amplify the Rossiter–McLaughlin signal to the point where variations in the chromatic Rossiter–McLaughlin effect from atmospheric absorbers should be detectable. [ABSTRACT FROM AUTHOR]

The model and initial data: Mixing of planetesimals in the TRAPPIST-1 exoplanetary system is studied at the late gas-free stage of formation of almost formed planets. The previous formation of embryos of planets could include their migration from greater distances and pebble accretion when gas presented in the protoplanetary disk. The TRAPPIST-1 system consists of a star with a mass equal to 0.0898 of the mass of the Sun and 7 planets. The motion of planetesimals under the gravitational influence of the star and seven planets (from b to h) was calculated with the use of the symplectic code from [1]. In different variants, the step ts of integration equaled to 0.1 day or 0.01 day. Planetesimals that collided with planets or the star or reached 50 AU from the star were excluded from integration. In each variant of the calculations, a disk of planetesimals was located near the orbit of one of the planets and is marked by the same letter as the planet. The initial values of semi-major axes of orbits of 250 planetesimals varied from amin to amax, their initial eccentricities were equal to eo=0.02 or eo=0.15, and the initial inclinations equaled to eo/2 rad. The orbital elements and masses of the planets and the values of amin and amax are presented in Table 1. Table 1. Semi-major axes a (in AU), eccentricities e, and masses m (in Earth masses mE) of exoplanets in the TRAPPIST-1 system, and the values of amin and amax for the considered disks near orbits of planets b, c, d, e, f, g, h. T0.02 and T0.15 are the times of evolution of disks at eo equaled to 0.02 or 0.15, respectively. f0.02 and f0.15 are the fractions of planetesimals collided with the 'host' planet during evolution at eo=0.02 and eo=0.15, respectively. The left and right values in the colums are for integrations with the step ts of integration equaled to 0.1 day or 0.01 day, respectively. Results of calculations: The results of calculations showed that several planets in the TRAPPIST-1 system accumulated planetesimals initially located at the same distance. The below conclusions are the same for calculations with the step ts of integration equaled to 0.1 day and 0.01 day. There were no collisions of planetesimals with the host star. Not more than 3.2% of planetesimals were ejected into hyperbolic orbits. Often there was no ejection for disks b - d. More than a half of planetesimals from disks b - g collided with planets in less than 1000 yr, and for disks b - d even in 250 yr. Times of evolution of disks b - h varied from 12 Kyr to more than 60 Myr (see Table 1). In variants with large time of evolution of a disk, most of the time could be due to the last planetesimal, which often was ejected into a hyperbolic orbit. For disk h at eo=0.02 and ts=0.1d, the last planetesimal moved for tens of millions of years at a distance R=0.09 AU from the star (at the resonance 4/7 with planet h). The fraction of planetesimals collided with the 'host' planet was 0.36-0.8 at eo=0.02 and 0.22-0.75 at eo=0.15 (see Table 1). The fraction of collisions of planetesimals with the 'host' planet was usually smaller for disks located farther from the star. In each calculation variant, there was at least one planet for which the number of collided planetesimals was greater than 25% of the number of collisions of planetesimals with the 'host' planet. For planetesimals initially located at R>0.7 AU for eo=0.15, the ratio of planetesimals collided with planets h and g equaled to 1.7. For disk h, this ratio was 0.6-0.8 at eo=0.15 and 1.1-1.5 at eo=0.02. Planetesimals could collide with all planets for disks d - h and at least with planets b - e for disks b - c. Therefore, outer layers of neighbouring planets in the TRAPPIST-1 system can include similar material, if there was a lot of planetesimals near their orbits at the late stages of the accumulation of the planets. For comparison, it was concluded in [2-3] that each terrestrial planet incorporated planetesimals from the feeding zone of all terrestrial planets. The amounts of material from different parts of the zone from 0.7 to 1.5 AU from the Sun, which entered into almost formed the Earth and Venus, differed, probably, by no more than 2 or 3 times [3]. Acknowledgements: The studies of fractions of planetesimals collided with TRAPPIST-1 planets were supported by the Ministry of Science and Higher Education of the Russian Federation under the grant 075-15-2020-780. Computer simulations of formation of the TRAPPIST-1 system were partly supported by the Vernadsky Institute. [ABSTRACT FROM AUTHOR]

We model the long-term X-ray and ultraviolet (XUV) luminosity of TRAPPIST-1 to constrain the evolving high-energy radiation environment experienced by its planetary system. Using a Markov Chain Monte Carlo (MCMC) method, we derive probabilistic constraints for TRAPPIST-1's stellar and XUV evolution that account for observational uncertainties, degeneracies between model parameters, and empirical data of low-mass stars. We constrain TRAPPIST-1's mass to m⋆ = 0.089 ± 0.001 M⊙ and find that its early XUV luminosity likely saturated at . From the posterior distribution, we infer that there is a ∼40% chance that TRAPPIST-1 is still in the saturated phase today, suggesting that TRAPPIST-1 has maintained high activity and LXUV/Lbol ≈ 10−3 for several gigayears. TRAPPIST-1's planetary system therefore likely experienced a persistent and extreme XUV flux environment, potentially driving significant atmospheric erosion and volatile loss. The inner planets likely received XUV fluxes ∼103–104 times that of the modern Earth during TRAPPIST-1's ∼1 Gyr long pre-main-sequence phase. Deriving these constraints via MCMC is computationally nontrivial, so scaling our methods to constrain the XUV evolution of a larger number of M dwarfs that harbor terrestrial exoplanets would incur significant computational expenses. We demonstrate that approxposterior, an open source Python machine learning package for approximate Bayesian inference using Gaussian processes, accurately and efficiently replicates our analysis using 980 times less computational time and 1330 times fewer simulations than MCMC sampling using emcee. We find that approxposterior derives constraints with mean errors on the best-fit values and 1σ uncertainties of 0.61% and 5.5%, respectively, relative to emcee. [ABSTRACT FROM AUTHOR]

TRAPPIST-1 is an ultracool dwarf hosting a system consisting of seven planets. While orbital properties, radii, and masses of the planets are nowadays well constrained, one of the fascinating open issues is the possibility that an environment hospitable to life could develop on some of these planets. Here, we use a simple formulation of an energy balance model that includes vegetation coverage to investigate the possibility of life affecting the climate of the planets in the TRAPPIST-1 system. Results confirm that planet TRAPPIST-1e has the best chance to be a habitable world and indicate that vegetation coverage significantly affects the resulting temperatures and habitability properties. The influence of vegetation has been evaluated in different scenarios characterized by different vegetation types, land–sea distributions and levels of greenhouse effect. While changes in vegetation type produce small changes, about 0.1%, in the habitable surface fraction, different land–sea distributions, by also affecting the vegetation growth, produce different temperature distributions. Finally, at latitudes where vegetation grows, the lowering of local albedo still represents a relevant contribution in settling the planetary temperature profiles even when levels of greenhouse effect higher than the Earth-like case are considered. [ABSTRACT FROM AUTHOR]

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 hydrogen-rich gas that TRAPPIST-1-like planets accreted from the ambient disk until disk dispersal. Since TRAPPIST-1 planets are trapped into a resonant chain, we simulate disk gas accretion onto a migrating TRAPPIST-1-like planet. We find that the amount of accreted hydrogen-rich gas is as small as 10−2 wt% and 0.1 wt% for TRAPPIST-1 b and 1 c, 10−2 wt% for 1 d, 1 wt% for 1 e, a few wt% for 1 f and 1 g and 1 wt% for 1 h, respectively. We also calculate the long-term thermal evolution of TRAPPIST-1-like planets after disk dissipation and estimate the mass loss of their hydrogen-rich atmospheres driven by stellar X-ray and UV irradiation. We find that all the accreted hydrogen-rich atmospheres can be lost via hydrodynamic escape. Therefore, we conclude that TRAPPIST-1 planets should have no primordial hydrogen-rich gases but secondary atmospheres such as a Venus-like one and water vapor, if they currently retain atmospheres. [ABSTRACT FROM AUTHOR]

Small exoplanets of nearby M-dwarf stars present the possibility of finding and characterizing 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 could have a liquid water ocean on their surface given appropriate atmospheres of N2 and CO2. In particular, 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 bar CO2, respectively. These atmospheres are irradiated by ultraviolet emission from the star’s moderately active chromosphere, and the consequence of this irradiation is unknown. Here we show that chemical reactions driven by the irradiation produce and maintain more than 0.2 bar O2 and 0.05 bar CO if the CO2 is ≥0.1 bar. The abundance of O2 and CO can rise to more than 1 bar under certain boundary conditions. Because of this O2–CO runaway, habitable environments on the TRAPPIST-1 planets entail an O2- and CO-rich atmosphere with coexisting O3. The only process that would prevent runaway is direct recombination of O2 and CO in the ocean, a reaction that is facilitated biologically. Our results indicate that O2, O3, and CO should be considered together with CO2 as the primary molecules in the search for atmospheric signatures from temperate and rocky planets of TRAPPIST-1 and other M-dwarf stars. [ABSTRACT FROM AUTHOR]

The TRAPPIST-1 system, consisting of an ultracool host star having seven known Earth-sized planets, will be a prime target for atmospheric characterization with the James Webb Space Telescope (JWST). However, the detectability of atmospheric molecular species may be severely impacted by the presence of clouds and/or hazes. In this work, we perform 3D general circulation model (GCM) simulations with the LMD-G model supplemented by 1D photochemistry simulations at the terminator with the Atmos model to simulate several possible atmospheres for TRAPPIST-1e, 1f, and 1g: (1) modern Earth, (2) Archean Earth, and (3) CO2-rich atmospheres. The JWST synthetic transit spectra were computed using the GSFC Planetary Spectrum Generator. We find that the TRAPPIST-1e, 1f, and 1g atmospheres, with clouds and/or hazes, could be detected using JWST's NIRSpec Prism from the CO2 absorption line at 4.3 μm in less than 15 transits at 3σ or less than 35 transits at 5σ. However, our analysis suggests that other gases would require hundreds (or thousands) of transits to be detectable. We also find that H2O, mostly confined in the lower atmosphere, is very challenging to detect for these planets or similar systems if the planets' atmospheres are not in a moist greenhouse state. This result demonstrates that the use of GCMs, self-consistently taking into account the effect of clouds and subsaturation, is crucial to evaluate the detectability of atmospheric molecules of interest, as well as for interpreting future detections in a more global (and thus robust and relevant) approach. [ABSTRACT FROM AUTHOR]

We present the distance-calibrated spectral energy distribution (SED) of TRAPPIST-1 using a new medium-resolution (R ∼ 6000) near-infrared (NIR) Folded-port InfraRed Echellette (FIRE) spectrum and its Gaia parallax. We report an updated bolometric luminosity (Lbol) of −3.216 ± 0.016, along with semiempirical fundamental parameters: effective temperature Teff = 2628 ± 42 K, mass = 90 ± 8 MJup, radius = 1.16 ± 0.03 RJup, and log g = 5.21 ± 0.06 dex. Its kinematics point toward an older age, while spectral indices indicate youth; therefore, we compare the overall SED and NIR bands of TRAPPIST-1 to field-age, low-gravity, and low-metallicity dwarfs of similar Teff and Lbol. We find field dwarfs of similar Teff and Lbol best fit the overall and band-by-band features of TRAPPIST-1. Additionally, we present new Allers & Liu spectral indices for the SpeX SXD and FIRE spectra of TRAPPIST-1, both classifying it as intermediate gravity. Examining Teff, Lbol, and absolute JHKW1W2 magnitudes versus optical spectral type places TRAPPIST-1 in an ambiguous location containing both field and intermediate-gravity sources. Kinematics place TRAPPIST-1 within a subpopulation of intermediate-gravity sources lacking bona fide membership in a moving group with higher tangential and UVW velocities. We conclude that TRAPPIST-1 is a field-age source with subtle spectral features reminiscent of a low surface gravity object. To resolve the cause of TRAPPIST-1's intermediate-gravity indicators we speculate on two avenues that might be correlated to inflate the radius: (1) magnetic activity or (2) tidal interactions from planets. We find the M8 dwarf LHS 132 is an excellent match to TRAPPIST-1's spectral peculiarities along with the M9 β dwarf 2MASS J10220489+0200477, the L1 β 2MASS J10224821+5825453, and the L0 β 2MASS J23224684−3133231, which have distinct kinematics, making all four intriguing targets for future exoplanet studies. [ABSTRACT FROM AUTHOR]

TRAPPIST-1 is an ultracool dwarf (UCD) with a system of seven terrestrial planets, at least three of which orbit in the habitable zone. The radio emission of such low-mass stars is poorly understood; few UCDs have been detected at radio frequencies at all, and the likelihood of detection is only loosely correlated with stellar properties. Relative to other low-mass stars, UCDs with slow rotation such as TRAPPIST-1 tend to be radio dim, whereas rapidly rotating UCDs tend to have strong radio emission—although this is not always the case. We present radio observations of TRAPPIST-1 using the Atacama Large Millimeter/submillimeter Array at 97.5 GHz and the Very Large Array at 44 GHz. TRAPPIST-1 was not detected at either frequency, and we place 3σ upper flux limits of 10.6 and 16.2 μJy, respectively. We use our results to constrain the magnetic properties and possible outgoing high-energy particle radiation from the star. The presence of radio emission from UCDs is indicative of a stellar environment that could pose a threat to life on surrounding planets. Gyrosynchrotron emission, discernible at frequencies between 20 and 100 GHz, is one of the only processes that can be used to infer the presence of high-energy particles released during magnetic reconnection events. Since M dwarfs are frequent hosts of terrestrial planets, characterizing their stellar emission is a crucial part of assessing habitability. Exposure to outgoing high-energy particle radiation—traceable by radio flux—can erode planetary atmospheres. While our results do not imply that the TRAPPIST-1 planets are suitable for life, we find no evidence that they are overtly unsuitable due to proton fluxes. [ABSTRACT FROM AUTHOR]

The TRAPPIST-1 system is sufficiently closely packed that tides raised by one planet on another are significant. We investigate whether this source of tidal heating is comparable to eccentricity tides raised by the star. Assuming a homogeneous body with a Maxwell rheology, we find that energy dissipation from stellar tides always dominates over that from planet–planet tides across a range of viscosities. TRAPPIST-1 g may experience the greatest proportion of planet–planet tidal heating, where it can account for between 2% and 20% of the total amount of tidal heating, for high-viscosity (1021 Pa s) and low-viscosity (1014 Pa s) regimes, respectively. If planet–planet tidal heating is to exceed that from stellar eccentricity tides, orbital eccentricities must be no more than e = 10−3–10−4 for most of the TRAPPIST-1 planets. [ABSTRACT FROM AUTHOR]

Small exoplanets of nearby red dwarf stars present the possibility to find and characterizehabitable worlds within the next decade. TRAPPIST-1, an ultracool red dwarf star, wasrecently found to have seven Earth-sized planets of predominantly rocky composition. Theplanets e, f, and g can have a liquid water ocean on their surface given appropriateatmospheres of N2 and CO2. Particularly, climate models have shown that the planets eand f can sustain a global liquid water ocean, for ≥0.2 bar CO2 plus 1 bar N2,and ≥2 bars CO2, respectively. These atmospheres are irradiated by ultravioletemission from the star’s moderately active chromosphere. Using an atmosphericphotochemistry model, we investigate how the irradiation drives chemical reactions in theatmospheres of TRAPPIST-1 e and f, where we assume habitable compositionspredicted from the climate models and include the effects of lightning and oxidation ofthe crust. Our models show that chemical reactions driven by the irradiation inthe atmosphere produce and maintain more than 1 bar of O2 and 0.1 bar of CO ifthe CO2 is ≥0.1 bar. Because of this O2 runaway, the habitable environments onthe TRAPPIST-1 planets entail an O2-dominated atmosphere, with co-existingCO, CO2, and N2. The sole process that would prevent the O2 runaway is a directrecombination of O2 and CO in the ocean, a reaction not operating on Earth butmight be facilitated biologically. Our results indicate that O2 and CO should beconsidered together with CO2 as the primary molecules in the search for atmosphericsignatures from potentially habitable planets of TRAPPIST-1 and other red dwarf stars. [ABSTRACT FROM AUTHOR]

Being the first, and currently the only, multiple planet hosting dwarf star that is sufficiently cool to form condensate clouds in its atmosphere, Trappist-1 provides a unique opportunity to test the efficiency of image polarimetry as a tool to detect and characterize exoplanets around L- and late M-dwarfs and exomoons around directly imaged self-luminous giant exoplanets. Although scattering of light by atmospheric dust particles should produce a significant amount of linear polarization in the far optical and near infrared, the disk-averaged net detectable polarization of the star must be zero owing to spherical symmetry. However, the transit of its planets would give rise to significant asymmetry and produce phase-dependent polarization with the peak polarization occurring at the inner contact points of planetary transit ingress and egress epochs. Adopting the known stellar and planetary physical parameters and employing a self-consistent cloudy atmosphere model of the M8 dwarf star, the transit polarization profiles and the expected amount of polarization of Trappist-1 during the transit phase of each individual planet, as well as that during the simultaneous transit of two planets, are presented in this paper. It is emphasized that the amount of polarization expected is within the detection limit of a few existing facilities. If polarization is detected confirming the prediction, time-resolved image polarimetry will emerge as a potential tool to detect and characterize small planets around cloudy ultra-cool dwarfs. [ABSTRACT FROM AUTHOR]

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, rotation, and magnetic activity; and conclude that TRAPPIST-1 is a transitional thin/thick disk star with an age of 7.6 ± 2.2 Gyr. The star’s color–magnitude position indicates that it is slightly metal-rich, which is consistent with the previously reported near-infrared spectroscopic metallicity; and it has a radius ) that is larger by 8%–14% than the predictions of solar-metallicity evolutionary models. We discuss some implications of the old age of this system with regard to the stability and habitability of its planets. [ABSTRACT FROM AUTHOR]