J. Yang, J. Leconte, E. T. Wolf, T. Merlis, D. D. B. Koll, F. Forget, and D. S. Abbot. Simulations of Water Vapor and Clouds on Rapidly Rotating and Tidally Locked Planets: A 3D Model Intercomparison. Astrophysical Journal, 875:46, 2019. [ bib | DOI | PDF version | ADS link ]

Robustly modeling the inner edge of the habitable zone is essential for determining the most promising potentially habitable exoplanets for atmospheric characterization. Global climate models (GCMs) have become the standard tool for calculating this boundary, but divergent results have emerged among the various GCMs. In this study, we perform an intercomparison of standard GCMs used in the field on a rapidly rotating planet receiving a G-star spectral energy distribution and on a tidally locked planet receiving an M-star spectral energy distribution. Experiments both with and without clouds are examined. We find relatively small difference (within 8 K) in global-mean surface temperature simulation among the models in the G-star case with clouds. In contrast, the global-mean surface temperature simulation in the M-star case is highly divergent (2030 K). Moreover, even differences in the simulated surface temperature when clouds are turned off are significant. These differences are caused by differences in cloud simulation and/or radiative transfer, as well as complex interactions between atmospheric dynamics and these two processes. For example we find that an increase in atmospheric absorption of shortwave radiation can lead to higher relative humidity at high altitudes globally and, therefore, a significant decrease in planetary radiation emitted to space. This study emphasizes the importance of basing conclusions about planetary climate on simulations from a variety of GCMs and motivates the eventual comparison of GCM results with terrestrial exoplanet observations to improve their performance.

D. Turrini, Y. Miguel, T. Zingales, A. Piccialli, R. Helled, A. Vazan, F. Oliva, G. Sindoni, O. Panić, J. Leconte, M. Min, S. Pirani, F. Selsis, V. Coudé du Foresto, A. Mura, and P. Wolkenberg. The contribution of the ARIEL space mission to the study of planetary formation. Experimental Astronomy, 46:45-65, 2018. [ bib | DOI | arXiv | PDF version | ADS link ]

The study of extrasolar planets and of the Solar System provides complementary pieces of the mosaic represented by the process of planetary formation. Exoplanets are essential to fully grasp the huge diversity of outcomes that planetary formation and the subsequent evolution of the planetary systems can produce. The orbital and basic physical data we currently possess for the bulk of the exoplanetary population, however, do not provide enough information to break the intrinsic degeneracy of their histories, as different evolutionary tracks can result in the same final configurations. The lessons learned from the Solar System indicate us that the solution to this problem lies in the information contained in the composition of planets. The goal of the Atmospheric Remote-Sensing Infrared Exoplanet Large-survey (ARIEL), one of the three candidates as ESA M4 space mission, is to observe a large and diversified population of transiting planets around a range of host star types to collect information on their atmospheric composition. ARIEL will focus on warm and hot planets to take advantage of their well-mixed atmospheres, which should show minimal condensation and sequestration of high-Z materials and thus reveal their bulk composition across all main cosmochemical elements. In this work we will review the most outstanding open questions concerning the way planets form and the mechanisms that contribute to create habitable environments that the compositional information gathered by ARIEL will allow to tackle.

M. Turbet, E. Bolmont, J. Leconte, F. Forget, F. Selsis, G. Tobie, A. Caldas, J. Naar, and M. Gillon. Modeling climate diversity, tidal dynamics and the fate of volatiles on TRAPPIST-1 planets. Astronomy Astrophysics, 612:A86, 2018. [ bib | DOI | arXiv | PDF version | ADS link ]

TRAPPIST-1 planets are invaluable for the study of comparative planetary science outside our solar system and possibly habitability. Both transit timing variations (TTV) of the planets and the compact, resonant architecture of the system suggest that TRAPPIST-1 planets could be endowed with various volatiles today. First, we derived from N-body simulations possible planetary evolution scenarios, and show that all the planets are likely in synchronous rotation. We then used a versatile 3D global climate model (GCM) to explore the possible climates of cool planets around cool stars, with a focus on the TRAPPIST-1 system. We investigated the conditions required for cool planets to prevent possible volatile species to be lost permanently by surface condensation, irreversible burying or photochemical destruction. We also explored the resilience of the same volatiles (when in condensed phase) to a runaway greenhouse process. We find that background atmospheres made of N₂, CO, or O₂ are rather resistant to atmospheric collapse. However, even if TRAPPIST-1 planets were able to sustain a thick background atmosphere by surviving early X/EUV radiation and stellar wind atmospheric erosion, it is difficult for them to accumulate significant greenhouse gases like CO₂, CH₄, or NH₃. CO₂ can easily condense on the permanent nightside, forming CO₂ ice glaciers that would flow toward the substellar region. A complete CO₂ ice surface cover is theoretically possible on TRAPPIST-1g and h only, but CO₂ ices should be gravitationally unstable and get buried beneath the water ice shell in geologically short timescales. Given TRAPPIST-1 planets large EUV irradiation (at least 10³ × Titan's flux), CH₄ and NH₃ are photodissociated rapidly and are thus hard to accumulate in the atmosphere. Photochemical hazes could then sedimentate and form a surface layer of tholins that would progressively thicken over the age of the TRAPPIST-1 system. Regarding habitability, we confirm that few bars of CO₂ would suffice to warm the surface of TRAPPIST-1f and g above the melting point of water. We also show that TRAPPIST-1e is a remarkable candidate for surface habitability. If the planet is today synchronous and abundant in water, then it should very likely sustain surface liquid water at least in the substellar region, whatever the atmosphere considered.

J. de Wit, H. R. Wakeford, N. K. Lewis, L. Delrez, M. Gillon, F. Selsis, J. Leconte, B.-O. Demory, E. Bolmont, V. Bourrier, A. J. Burgasser, S. Grimm, E. Jehin, S. M. Lederer, J. E. Owen, V. Stamenković, and A. H. M. J. Triaud. Atmospheric reconnaissance of the habitable-zone Earth-sized planets orbiting TRAPPIST-1. Nature Astronomy, 2:214-219, 2018. [ bib | DOI | arXiv | PDF version | ADS link ]

Seven temperate Earth-sized exoplanets readily amenable for atmospheric studies transit the nearby ultracool dwarf star TRAPPIST-1 (refs ^1,2). Their atmospheric regime is unknown and could range from extended primordial hydrogen-dominated to depleted atmospheres^3-6. Hydrogen in particular is a powerful greenhouse gas that may prevent the habitability of inner planets while enabling the habitability of outer ones^6-8. An atmosphere largely dominated by hydrogen, if cloud-free, should yield prominent spectroscopic signatures in the near-infrared detectable during transits. Observations of the innermost planets have ruled out such signatures⁹. However, the outermost planets are more likely to have sustained such a Neptune-like atmosphere^{10, 11}. Here, we report observations for the four planets within or near the system's habitable zone, the circumstellar region where liquid water could exist on a planetary surface^12-14. These planets do not exhibit prominent spectroscopic signatures at near-infrared wavelengths either, which rules out cloud-free hydrogen-dominated atmospheres for TRAPPIST-1 d, e and f, with significance of 8σ, 6σ and 4σ, respectively. Such an atmosphere is instead not excluded for planet g. As high-altitude clouds and hazes are not expected in hydrogen-dominated atmospheres around planets with such insolation^{15, 16}, these observations further support their terrestrial and potentially habitable nature.

M. Turbet, F. Forget, J. Leconte, B. Charnay, and G. Tobie. CO₂ condensation is a serious limit to the deglaciation of Earth-like planets. Earth and Planetary Science Letters, 476:11-21, 2017. [ bib | DOI | arXiv | PDF version | ADS link ]

It is widely believed that the carbonate-silicate cycle is the main agent, through volcanism, to trigger deglaciations by CO₂ greenhouse warming on Earth and on Earth-like planets when they get in a frozen state. Here we use a 3D Global Climate Model to simulate the ability of planets initially completely frozen to escape from glaciation episodes by accumulating enough gaseous CO₂. The model includes CO₂ condensation and sublimation processes and the water cycle. We find that planets with Earth-like characteristics (size, mass, obliquity, rotation rate, etc.) orbiting a Sun-like star may never be able to escape from a glaciation era, if their orbital distance is greater than ~1.27 Astronomical Units (Flux 847 Wm^-2 or 62% of the Solar constant), because CO₂ would condense at the poles - here the cold traps - forming permanent CO₂ ice caps. This limits the amount of CO₂ in the atmosphere and thus its greenhouse effect. Furthermore, our results indicate that for (1) high rotation rates (P_rot 24 h), (2) low obliquity (obliquity 23.5deg), (3) low background gas partial pressures (1 bar), and (4) high water ice albedo (H₂O albedo 0.6), this critical limit could occur at a significantly lower equivalent distance (or higher insolation). For each possible configuration, we show that the amount of CO₂ that can be trapped in the polar caps depends on the efficiency of CO₂ ice to flow laterally as well as its gravitational stability relative to subsurface water ice. We find that a frozen Earth-like planet located at 1.30 AU of a Sun-like star could store as much as 1.5, 4.5 and 15 bars of dry ice at the poles, for internal heat fluxes of 100, 30 and 10 mW m^-2, respectively. But these amounts are in fact lower limits. For planets with a significant water ice cover, we show that CO₂ ice deposits should be gravitationally unstable. They get buried beneath the water ice cover in geologically short timescales of ~10⁴ yrs, mainly controlled by the viscosity of water ice. CO₂ would be permanently sequestered underneath the water ice cover, in the form of CO₂ liquids, CO₂ clathrate hydrates and/or dissolved in subglacial water reservoirs (if any). This would considerably increase the amount of CO₂ trapped and further reduce the probability of deglaciation.

V. Bourrier, J. de Wit, E. Bolmont, V. Stamenković, 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, and V. Van Grootel. Temporal Evolution of the High-energy Irradiation and Water Content of TRAPPIST-1 Exoplanets. , 154:121, 2017. [ bib | DOI | arXiv | PDF version | ADS link ]

The ultracool dwarf star TRAPPIST-1 hosts seven Earth-size transiting planets, some of which could harbor liquid water on their surfaces. Ultraviolet observations are essential to measuring their high-energy irradiation and searching for photodissociated water escaping from their putative atmospheres. Our new observations of the TRAPPIST-1 Lyα line during the transit of TRAPPIST-1c show an evolution of the star 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 three Earth oceans if hydrodynamic escape stopped once they entered the habitable zone (HZ). 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 X-rays to ultraviolet-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 HZ. 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.

M. Gillon, A. H. M. J. Triaud, B.-O. Demory, E. Jehin, E. Agol, K. M. Deck, S. M. Lederer, J. de Wit, A. Burdanov, J. G. Ingalls, E. Bolmont, J. Leconte, S. N. Raymond, F. Selsis, M. Turbet, K. Barkaoui, A. Burgasser, M. R. Burleigh, S. J. Carey, A. Chaushev, C. M. Copperwheat, L. Delrez, C. S. Fernandes, D. L. Holdsworth, E. J. Kotze, V. Van Grootel, Y. Almleaky, Z. Benkhaldoun, P. Magain, and D. Queloz. Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1. Nature, 542:456-460, 2017. [ bib | DOI | arXiv | PDF version | ADS link ]

One aim of modern astronomy is to detect temperate, Earth-like exoplanets that are well suited for atmospheric characterization. Recently, three Earth-sized planets were detected that transit (that is, pass in front of) a star with a mass just eight per cent that of the Sun, located 12 parsecs away. The transiting configuration of these planets, combined with the Jupiter-like size of their host starnamed TRAPPIST-1makes possible in-depth studies of their atmospheric properties with present-day and future astronomical facilities. Here we report the results of a photometric monitoring campaign of that star from the ground and space. Our observations reveal that at least seven planets with sizes and masses similar to those of Earth revolve around TRAPPIST-1. The six inner planets form a near-resonant chain, such that their orbital periods (1.51, 2.42, 4.04, 6.06, 9.1 and 12.35 days) are near-ratios of small integers. This architecture suggests that the planets formed farther from the star and migrated inwards. Moreover, the seven planets have equilibrium temperatures low enough to make possible the presence of liquid water on their surfaces.

E. Bolmont, F. Selsis, J. E. Owen, I. Ribas, S. N. Raymond, J. Leconte, and M. Gillon. Water loss from terrestrial planets orbiting ultracool dwarfs: implications for the planets of TRAPPIST-1. , 464:3728-3741, 2017. [ bib | DOI | arXiv | PDF version | ADS link ]

Ultracool dwarfs (UCD; T_eff 3000 K) cool to settle on the main sequence after 1 Gyr. For brown dwarfs, this cooling never stops. Their habitable zones (HZ) thus sweeps inward at least during the first Gyr of their lives. Assuming they possess water, planets found in the HZ of UCDs have experienced a runaway greenhouse phase too hot for liquid water prior to enter the HZ. It has been proposed that such planets are desiccated by this hot early phase and enter the HZ as dry worlds. Here, we model the water loss during this pre-HZ hot phase taking into account recent upper limits on the XUV emission of UCDs and using 1D radiation-hydrodynamic simulations. We address the whole range of UCDs but also focus on the planets recently found around the 0.08 M_&sun; dwarf TRAPPIST-1. Despite assumptions maximizing the FUV photolysis of water and the XUV-driven escape of hydrogen, we find that planets can retain significant amount of water in the HZ of UCDs, with a sweet spot in the 0.04-0.06 M_&sun; range. We also studied the TRAPPIST-1 system using observed constraints on the XUV flux. We find that TRAPPIST-1b and c may have lost as much as 15 Earth oceans and planet d - which might be inside the HZ - may have lost less than 1 Earth ocean. Depending on their initial water contents, they could have enough water to remain habitable. TRAPPIST-1 planets are key targets for atmospheric characterization and could provide strong constraints on the water erosion around UCDs.

M. Turbet, J. Leconte, F. Selsis, E. Bolmont, F. Forget, I. Ribas, S. N. Raymond, and G. Anglada-Escudé. The habitability of Proxima Centauri b. II. Possible climates and observability. Astronomy Astrophysics, 596:A112, 2016. [ bib | DOI | arXiv | PDF version | ADS link ]

Radial velocity monitoring has found the signature of a Msini = 1.3M_⊕ planet located within the habitable zone (HZ) of Proxima Centauri. Despite a hotter past and an active host star, the planet Proxima b could have retained enough volatiles to sustain surface habitability. Here we use a 3D Global Climate Model (GCM) to simulate the atmosphere and water cycle of Proxima b for its two likely rotation modes (1:1 and 3:2 spin-orbit resonances), while varying the unconstrained surface water inventory and atmospheric greenhouse effect. Any low-obliquity, low-eccentricity planet within the HZ of its star should be in one of the climate regimes discussed here. We find that a broad range of atmospheric compositions allow surface liquid water. On a tidally locked planet with sufficient surface water inventory, liquid water is always present, at least in the substellar region. With a non-synchronous rotation, this requires a minimum greenhouse warming ( 10 mbar of CO₂ and 1 bar of N₂). If the planet is dryer, 0.5 bar or 1.5 bars of CO₂ (for asynchronous or synchronous rotation, respectively) suffice to prevent the trapping of any arbitrary, small water inventory into polar or nightside ice caps. We produce reflection and emission spectra and phase curves for the simulated climates. We find that atmospheric characterization will be possible via direct imaging with forthcoming large telescopes. The angular separation of 7λ/D at 1 μm (with the E-ELT) and a contrast of 10^-7 will enable high-resolution spectroscopy and the search for molecular signatures, including H₂O, O₂, and CO₂. The observation of thermal phase curves can be attempted with the James Webb Space Telescope, thanks to a contrast of 2 × 10^-5 at 10 μm. Proxima b will also be an exceptional target for future IR interferometers. Within a decade it will be possible to image Proxima b and possibly determine whether the surface of this exoplanet is habitable.

I. Ribas, E. Bolmont, F. Selsis, A. Reiners, J. Leconte, S. N. Raymond, S. G. Engle, E. F. Guinan, J. Morin, M. Turbet, F. Forget, and G. Anglada-Escudé. The habitability of Proxima Centauri b. I. Irradiation, rotation and volatile inventory from formation to the present. Astronomy Astrophysics, 596:A111, 2016. [ bib | DOI | arXiv | PDF version | ADS link ]

Proxima b is a planet with a minimum mass of 1.3M_⊕ orbiting within the habitable zone (HZ) of Proxima Centauri, a very low-mass, active star and the Sun's closest neighbor. Here we investigate a number of factors related to the potential habitability of Proxima b and its ability to maintain liquid water on its surface. We set the stage by estimating the current high-energy irradiance of the planet and show that the planet currently receives 30 times more extreme-UV radiation than Earth and 250 times more X-rays. We compute the time evolution of the star's spectrum, which is essential for modeling the flux received over Proxima b's lifetime. We also show that Proxima b's obliquity is likely null and its spin is either synchronous or in a 3:2 spin-orbit resonance, depending on the planet's eccentricity and level of triaxiality. Next we consider the evolution of Proxima b's water inventory. We use our spectral energy distribution to compute the hydrogen loss from the planet with an improved energy-limited escape formalism. Despite the high level of stellar activity we find that Proxima b is likely to have lost less than an Earth ocean's worth of hydrogen (EO_H) before it reached the HZ 100-200 Myr after its formation. The largest uncertainty in our work is the initial water budget, which is not constrained by planet formation models. We conclude that Proxima b is a viable candidate habitable planet.

J. Yang, J. Leconte, E. T. Wolf, C. Goldblatt, N. Feldl, T. Merlis, Y. Wang, D. D. B. Koll, F. Ding, F. Forget, and D. S. Abbot. Differences in Water Vapor Radiative Transfer among 1D Models Can Significantly Affect the Inner Edge of the Habitable Zone. Astrophysical Journal, 826:222, 2016. [ bib | DOI | arXiv | PDF version | ADS link ]

An accurate estimate of the inner edge of the habitable zone is critical for determining which exoplanets are potentially habitable and for designing future telescopes to observe them. Here, we explore differences in estimating the inner edge among seven one-dimensional radiative transfer models: two line-by-line codes (SMART and LBLRTM) as well as five band codes (CAM3, CAM4_Wolf, LMDG, SBDART, and AM2) that are currently being used in global climate models. We compare radiative fluxes and spectra in clear-sky conditions around G and M stars, with fixed moist adiabatic profiles for surface temperatures from 250 to 360 K. We find that divergences among the models arise mainly from large uncertainties in water vapor absorption in the window region (10 μm) and in the region between 0.2 and 1.5 μm. Differences in outgoing longwave radiation increase with surface temperature and reach 10-20 W m^-2 differences in shortwave reach up to 60 W m^-2, especially at the surface and in the troposphere, and are larger for an M-dwarf spectrum than a solar spectrum. Differences between the two line-by-line models are significant, although smaller than among the band models. Our results imply that the uncertainty in estimating the insolation threshold of the inner edge (the runaway greenhouse limit) due only to clear-sky radiative transfer is 10% of modern Earths solar constant (I.e., 34 W m^-2 in global mean) among band models and 3% between the two line-by-line models. These comparisons show that future work is needed that focuses on improving water vapor absorption coefficients in both shortwave and longwave, as well as on increasing the resolution of stellar spectra in broadband models.

E. Bolmont, A.-S. Libert, J. Leconte, and F. Selsis. Habitability of planets on eccentric orbits: Limits of the mean flux approximation. Astronomy Astrophysics, 591:A106, 2016. [ bib | DOI | arXiv | PDF version | ADS link ]

Unlike the Earth, which has a small orbital eccentricity, some exoplanets discovered in the insolation habitable zone (HZ) have high orbital eccentricities (e.g., up to an eccentricity of ˜0.97 for HD 20782 b). This raises the question of whether these planets have surface conditions favorable to liquid water. In order to assess the habitability of an eccentric planet, the mean flux approximation is often used. It states that a planet on an eccentric orbit is called habitable if it receives on average a flux compatible with the presence of surface liquid water. However, because the planets experience important insolation variations over one orbit and even spend some time outside the HZ for high eccentricities, the question of their habitability might not be as straightforward. We performed a set of simulations using the global climate model LMDZ to explore the limits of the mean flux approximation when varying the luminosity of the host star and the eccentricity of the planet. We computed the climate of tidally locked ocean covered planets with orbital eccentricity from 0 to 0.9 receiving a mean flux equal to Earth's. These planets are found around stars of luminosity ranging from 1 L_&sun; to 10^-4L_&sun;. We use a definition of habitability based on the presence of surface liquid water, and find that most of the planets considered can sustain surface liquid water on the dayside with an ice cap on the nightside. However, for high eccentricity and high luminosity, planets cannot sustain surface liquid water during the whole orbital period. They completely freeze at apoastron and when approaching periastron an ocean appears around the substellar point. We conclude that the higher the eccentricity and the higher the luminosity of the star, the less reliable the mean flux approximation.

E. Bolmont, S. N. Raymond, J. Leconte, F. Hersant, and A. C. M. Correia. Mercury-T: A new code to study tidally evolving multi-planet systems. Applications to Kepler-62. Astronomy Astrophysics, 583:A116, 2015. [ bib | DOI | arXiv | PDF version | ADS link ]

A large proportion of observed planetary systems contain several planets in a compact orbital configuration, and often harbor at least one close-in object. These systems are then most likely tidally evolving. We investigate how the effects of planet-planet interactions influence the tidal evolution of planets. We introduce for that purpose a new open-source addition to the MercuryN-body code, Mercury-T, which takes into account tides, general relativity and the effect of rotation-induced flattening in order to simulate the dynamical and tidal evolution of multi-planet systems. It uses a standard equilibrium tidal model, the constant time lag model. Besides, the evolution of the radius of several host bodies has been implemented (brown dwarfs, M-dwarfs of mass 0.1 M_&sun;, Sun-like stars, Jupiter). We validate the new code by comparing its output for one-planet systems to the secular equations results. We find that this code does respect the conservation of total angular momentum. We applied this new tool to the planetary system Kepler-62. We find that tides influence the stability of the system in some cases. We also show that while the four inner planets of the systems are likely to have slow rotation rates and small obliquities, the fifth planet could have a fast rotation rate and a high obliquity. This means that the two habitable zone planets of this system, Kepler-62e ad f are likely to have very different climate features, and this of course would influence their potential at hosting surface liquid water.

The code is only available at the CDS via anonymous ftp to <A href=“http://cdsarc.u-strasbg.fr”>http://cdsarc.u-strasbg.fr</A> (ftp://130.79.128.5) or via <A href=“http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/583/A116”>http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/583/A116</A>

J. Leconte, H. Wu, K. Menou, and N. Murray. Asynchronous rotation of Earth-mass planets in the habitable zone of lower-mass stars. Science, 347:632-635, 2015. [ bib | DOI | arXiv | PDF version | ADS link ]

Planets in the habitable zone of lower-mass stars are often assumed to be in a state of tidally synchronized rotation, which would considerably affect their putative habitability. Although thermal tides cause Venus to rotate retrogradely, simple scaling arguments tend to attribute this peculiarity to the massive Venusian atmosphere. Using a global climate model, we show that even a relatively thin atmosphere can drive terrestrial planets rotation away from synchronicity. We derive a more realistic atmospheric tide model that predicts four asynchronous equilibrium spin states, two being stable, when the amplitude of the thermal tide exceeds a threshold that is met for habitable Earth-like planets with a 1-bar atmosphere around stars more massive than ˜0.5 to 0.7 solar mass. Thus, many recently discovered terrestrial planets could exhibit asynchronous spin-orbit rotation, even with a thin atmosphere.

F. Forget and J. Leconte. Possible climates on terrestrial exoplanets. Philosophical Transactions of the Royal Society of London Series A, 372:20130084-20130084, 2014. [ bib | DOI | arXiv | PDF version | ADS link ]

What kind of environment may exist on terrestrial planets around other stars? In spite of the lack of direct observations, it may not be premature to speculate on exoplanetary climates, for instance to optimize future telescopic observations, or to assess the probability of habitable worlds. To first order, climate primarily depends on 1) The atmospheric composition and the volatile inventory; 2) The incident stellar flux; 3) The tidal evolution of the planetary spin, which can notably lock a planet with a permanent night side. The atmospheric composition and mass depends on complex processes which are difficult to model: origins of volatile, atmospheric escape, geochemistry, photochemistry. We discuss physical constraints which can help us to speculate on the possible type of atmosphere, depending on the planet size, its final distance for its star and the star type. Assuming that the atmosphere is known, the possible climates can be explored using Global Climate Models analogous to the ones developed to simulate the Earth as well as the other telluric atmospheres in the solar system. Our experience with Mars, Titan and Venus suggests that realistic climate simulators can be developed by combining components like a “dynamical core”, a radiative transfer solver, a parametrisation of subgrid-scale turbulence and convection, a thermal ground model, and a volatile phase change code. On this basis, we can aspire to build reliable climate predictors for exoplanets. However, whatever the accuracy of the models, predicting the actual climate regime on a specific planet will remain challenging because climate systems are affected by strong positive destabilizing feedbacks (such as runaway glaciations and runaway greenhouse effect). They can drive planets with very similar forcing and volatile inventory to completely different states.

J. Leconte, F. Forget, B. Charnay, R. Wordsworth, and A. Pottier. Increased insolation threshold for runaway greenhouse processes on Earth-like planets. Nature, 504:268-271, 2013. [ bib | DOI | arXiv | PDF version | ADS link ]

The increase in solar luminosity over geological timescales should warm the Earth's climate, increasing water evaporation, which will in turn enhance the atmospheric greenhouse effect. Above a certain critical insolation, this destabilizing greenhouse feedback can `run away' until the oceans have completely evaporated. Through increases in stratospheric humidity, warming may also cause evaporative loss of the oceans to space before the runaway greenhouse state occurs. The critical insolation thresholds for these processes, however, remain uncertain because they have so far been evaluated using one-dimensional models that cannot account for the dynamical and cloud feedback effects that are key stabilizing features of the Earth's climate. Here we use a three-dimensional global climate model to show that the insolation threshold for the runaway greenhouse state to occur is about 375 W m^-2, which is significantly higher than previously thought. Our model is specifically developed to quantify the climate response of Earth-like planets to increased insolation in hot and extremely moist atmospheres. In contrast with previous studies, we find that clouds have a destabilizing feedback effect on the long-term warming. However, subsident, unsaturated regions created by the Hadley circulation have a stabilizing effect that is strong enough to shift the runaway greenhouse limit to higher values of insolation than are inferred from one-dimensional models. Furthermore, because of wavelength-dependent radiative effects, the stratosphere remains sufficiently cold and dry to hamper the escape of atmospheric water, even at large fluxes. This has strong implications for the possibility of liquid water existing on Venus early in its history, and extends the size of the habitable zone around other stars.

J. Leconte, F. Forget, B. Charnay, R. Wordsworth, F. Selsis, E. Millour, and A. Spiga. 3D climate modeling of close-in land planets: Circulation patterns, climate moist bistability, and habitability. Astronomy Astrophysics, 554:A69, 2013. [ bib | DOI | arXiv | PDF version | ADS link ]

The inner edge of the classical habitable zone is often defined by the critical flux needed to trigger the runaway greenhouse instability. This 1D notion of a critical flux, however, may not be all that relevant for inhomogeneously irradiated planets, or when the water content is limited (land planets). Based on results from our 3D global climate model, we present general features of the climate and large-scale circulation on close-in terrestrial planets. We find that the circulation pattern can shift from super-rotation to stellar/anti stellar circulation when the equatorial Rossby deformation radius significantly exceeds the planetary radius, changing the redistribution properties of the atmosphere. Using analytical and numerical arguments, we also demonstrate the presence of systematic biases among mean surface temperatures and among temperature profiles predicted from either 1D or 3D simulations. After including a complete modeling of the water cycle, we further demonstrate that two stable climate regimes can exist for land planets closer than the inner edge of the classical habitable zone. One is the classical runaway state where all the water is vaporized, and the other is a collapsed state where water is captured in permanent cold traps. We identify this “moist” bistability as the result of a competition between the greenhouse effect of water vapor and its condensation on the night side or near the poles, highlighting the dynamical nature of the runaway greenhouse effect. We also present synthetic spectra showing the observable signature of these two states. Taking the example of two prototype planets in this regime, namely Gl 581 c and HD 85512 b, we argue that depending on the rate of water delivery and atmospheric escape during the life of these planets, they could accumulate a significant amount of water ice at their surface. If such a thick ice cap is present, various physical mechanisms observed on Earth (e.g., gravity driven ice flows, geothermal flux) should come into play to produce long-lived liquid water at the edge and/or bottom of the ice cap. Consequently, the habitability of planets at smaller orbital distance than the inner edge of the classical habitable zone cannot be ruled out. Transiting planets in this regime represent promising targets for upcoming exoplanet characterization observatories, such as EChO and JWST.

F. Forget, R. Wordsworth, E. Millour, J.-B. Madeleine, L. Kerber, J. Leconte, E. Marcq, and R. M. Haberle. 3D modelling of the early martian climate under a denser CO₂ atmosphere: Temperatures and CO₂ ice clouds. Icarus, 222:81-99, 2013. [ bib | DOI | arXiv | PDF version | ADS link ]

On the basis of geological evidence, it is often stated that the early martian climate was warm enough for liquid water to flow on the surface thanks to the greenhouse effect of a thick atmosphere. We present 3D global climate simulations of the early martian climate performed assuming a faint young Sun and a CO₂ atmosphere with surface pressure between 0.1 and 7 bars. The model includes a detailed radiative transfer model using revised CO₂ gas collision induced absorption properties, and a parameterisation of the CO₂ ice cloud microphysical and radiative properties. A wide range of possible climates is explored using various values of obliquities, orbital parameters, cloud microphysic parameters, atmospheric dust loading, and surface properties. Unlike on present day Mars, for pressures higher than a fraction of a bar, surface temperatures vary with altitude because of the adiabatic cooling and warming of the atmosphere when it moves vertically. In most simulations, CO₂ ice clouds cover a major part of the planet. Previous studies had suggested that they could have warmed the planet thanks to their scattering greenhouse effect. However, even assuming parameters that maximize this effect, it does not exceed +15 K. Combined with the revised CO₂ spectroscopy and the impact of surface CO₂ ice on the planetary albedo, we find that a CO₂ atmosphere could not have raised the annual mean temperature above 0 degC anywhere on the planet. The collapse of the atmosphere into permanent CO₂ ice caps is predicted for pressures higher than 3 bar, or conversely at pressure lower than 1 bar if the obliquity is low enough. Summertime diurnal mean surface temperatures above 0 degC (a condition which could have allowed rivers and lakes to form) are predicted for obliquity larger than 40deg at high latitudes but not in locations where most valley networks or layered sedimentary units are observed. In the absence of other warming mechanisms, our climate model results are thus consistent with a cold early Mars scenario in which nonclimatic mechanisms must occur to explain the evidence for liquid water. In a companion paper by Wordsworth et al. we simulate the hydrological cycle on such a planet and discuss how this could have happened in more detail.

R. Heller, R. Barnes, and J. Leconte. Habitability of Extrasolar Planets and Tidal Spin Evolution. Origins of Life and Evolution of the Biosphere, 41:539-543, 2011. [ bib | DOI | arXiv | PDF version | ADS link ]

Stellar radiation has conservatively been used as the key constraint to planetary habitability. We review here the effects of tides, exerted by the host star on the planet, on the evolution of the planetary spin. Tides initially drive the rotation period and the orientation of the rotation axis into an equilibrium state but do not necessarily lead to synchronous rotation. As tides also circularize the orbit, eventually the rotation period does equal the orbital period and one hemisphere will be permanently irradiated by the star. Furthermore, the rotational axis will become perpendicular to the orbit, i.e. the planetary surface will not experience seasonal variations of the insolation. We illustrate here how tides alter the spins of planets in the traditional habitable zone. As an example, we show that, neglecting perturbations due to other companions, the Super-Earth Gl581d performs two rotations per orbit and that any primordial obliquity has been eroded.

E. Bolmont, S. N. Raymond, and J. Leconte. Tidal evolution of planets around brown dwarfs. Astronomy Astrophysics, 535:A94, 2011. [ bib | DOI | arXiv | PDF version | ADS link ]

Context. The tidal evolution of planets orbiting brown dwarfs (BDs) presents an interesting case study because BDs' terrestrial planet forming region is located extremely close-in. In fact, the habitable zones of BDs range from roughly 0.001 to 0.03 AU and for the lowest-mass BDs are located interior to the Roche limit. <BR /> Aims: In contrast with stars, BDs spin up as they age. Thus, the corotation distance moves inward. This has important implications for the tidal evolution of planets around BDs. <BR /> Methods: We used a standard equilibrium tidal model to compute the orbital evolution of a large ensemble of planet-BD systems. We tested the effect of numerous parameters such as the initial semi-major axis and eccentricity, the rotation period of the BD, the masses of both the BD and planet, and the tidal dissipation factors. <BR /> Results: We find that all planets that form at or beyond the corotation distance and with initial eccentricities smaller than ˜0.1 are repelled from the BD. Some planets initially interior to corotation can survive if their inward tidal evolution is slower than the BD's spin evolution, but most initially close-in planets fall onto the BD. <BR /> Conclusions: We find that the most important parameter for the tidal evolution is the initial orbital distance with respect to the corotation distance. Some planets can survive in the habitable zone for Gyr timescales, although in many cases the habitable zone moves inward past the planet's orbit in just tens to hundreds of Myr. Surviving planets can have orbital periods of less than 10 days (as small as 10 h), so they could be observable by transit.

R. Heller, J. Leconte, and R. Barnes. Tidal obliquity evolution of potentially habitable planets. Astronomy Astrophysics, 528:A27, 2011. [ bib | DOI | arXiv | PDF version | ADS link ]

Context. Stellar insolation has been used as the main constraint on a planet's potential habitability. However, as more Earth-like planets are discovered around low-mass stars (LMSs), a re-examination of the role of tides on the habitability of exoplanets has begun. Those studies have yet to consider the misalignment between a planet's rotational axis and the orbital plane normal, i.e. the planetary obliquity. <BR /> Aims: This paper considers the constraints on habitability arising from tidal processes due to the planet's spin orientation and rate. Since tidal processes are far from being understood we seek to understand differences between commonly used tidal models. <BR /> Methods: We apply two equilibrium tide theories - a constant-phase-lag model and a constant-time-lag model - to compute the obliquity evolution of terrestrial planets orbiting in the habitable zones around LMSs. The time for the obliquity to decrease from an Earth-like obliquity of 23.5deg to 5deg, the “tilt erosion time”, is compared to the traditional insolation habitable zone (IHZ) in the parameter space spanned by the semi-major axis a, the eccentricity e, and the stellar mass M_s. We also compute tidal heating and equilibrium rotation caused by obliquity tides as further constraints on habitability. The Super-Earth Gl581 d and the planet candidate Gl581 g are studied as examples for these tidal processes. <BR /> Results: Earth-like obliquities of terrestrial planets in the IHZ around stars with masses 0.25 M_&sun; are eroded in less than 0.1 Gyr. Only terrestrial planets orbiting stars with masses 0.9 M_&sun; experience tilt erosion times larger than 1 Gyr throughout the IHZ. Tilt erosion times for terrestrial planets in highly eccentric orbits inside the IHZ of solar-like stars can be 10 Gyr. Terrestrial planets in the IHZ of stars with masses 0.25 M_&sun; undergo significant tidal heating due to obliquity tides, whereas in the IHZ of stars with masses 0.5 M_&sun; they require additional sources of heat to drive tectonic activity. The predictions of the two tidal models diverge significantly for e 0.3. In our two-body simulations, Gl581 d's obliquity is eroded to 0deg and its rotation period reached its equilibrium state of half its orbital period in 0.1 Gyr. Tidal surface heating on the putative Gl581 g is 150 mW/m² as long as its eccentricity is smaller than 0.3. <BR /> Conclusions: Obliquity tides modify the concept of the habitable zone. Tilt erosion of terrestrial planets orbiting LMSs should be included by atmospheric modelers. Tidal heating needs to be considered by geologists.