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 (Prot 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.
D. Cébron, M. L. Bars, P. L. Gal, C. Moutou, J. Leconte, and A. Sauret. Corrigendum to ;Elliptical instability in hot Jupiter systems; ICARUS, Volume 226, Issue 2, November-December 2013, Pages 1642-1653. Icarus, 294:262-262, 2017. [ bib | DOI | PDF version | ADS link ]
The author regret for the corrections and wishes to replace the below
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.
T. Cavalié, O. Venot, F. Selsis, F. Hersant, P. Hartogh, and J. Leconte. Thermochemistry and vertical mixing in the tropospheres of Uranus and Neptune: How convection inhibition can affect the derivation of deep oxygen abundances. Icarus, 291:1-16, 2017. [ bib | DOI | arXiv | PDF version | ADS link ]
Thermochemical models have been used in the past to constrain the deep oxygen abundance in the gas and ice giant planets from tropospheric CO spectroscopic measurements. Knowing the oxygen abundance of these planets is a key to better understand their formation. These models have widely used dry and/or moist adiabats to extrapolate temperatures from the measured values in the upper troposphere down to the level where the thermochemical equilibrium between H2O and CO is established. The mean molecular mass gradient produced by the condensation of H2O stabilizes the atmosphere against convection and results in a vertical thermal profile and H2O distribution that departs significantly from previous estimates. We revisit O/H estimates using an atmospheric structure that accounts for the inhibition of the convection by condensation. We use a thermochemical network and the latest observations of CO in Uranus and Neptune to calculate the internal oxygen enrichment required to satisfy both these new estimates of the thermal profile and the observations. We also present the current limitations of such modeling.
R. Luger, M. Sestovic, E. Kruse, S. L. Grimm, B.-O. Demory, E. Agol, E. Bolmont, D. Fabrycky, C. S. Fernandes, V. Van Grootel, A. Burgasser, M. Gillon, J. G. Ingalls, E. Jehin, S. N. Raymond, F. Selsis, A. H. M. J. Triaud, T. Barclay, G. Barentsen, S. B. Howell, L. Delrez, J. de Wit, D. Foreman-Mackey, D. L. Holdsworth, J. Leconte, S. Lederer, M. Turbet, Y. Almleaky, Z. Benkhaldoun, P. Magain, B. M. Morris, K. Heng, and D. Queloz. A seven-planet resonant chain in TRAPPIST-1. Nature Astronomy, 1:0129, 2017. [ bib | DOI | arXiv | PDF version | ADS link ]
The TRAPPIST-1 system is the first transiting planet system found orbiting an ultracool dwarf star 1 . At least seven planets similar in radius to Earth were previously found to transit this host star 2 . Subsequently, TRAPPIST-1 was observed as part of the K2 mission and, with these new data, we report the measurement of an 18.77 day orbital period for the outermost transiting planet, TRAPPIST-1 h, which was previously unconstrained. This value matches our theoretical expectations based on Laplace relations 3 and places TRAPPIST-1 h as the seventh member of a complex chain, with three-body resonances linking every member. We find that TRAPPIST-1 h has a radius of 0.752 R ⊕ and an equilibrium temperature of 173K. We have also measured the rotational period of the star to be 3.3 days and detected a number of flares consistent with a low-activity, middle-aged, late M dwarf.
F. Forget, T. Bertrand, M. Vangvichith, J. Leconte, E. Millour, and E. Lellouch. A post-new horizons global climate model of Pluto including the N2, CH4 and CO cycles. Icarus, 287:54-71, 2017. [ bib | DOI | PDF version | ADS link ]
We have built a new 3D Global Climate Model (GCM) to simulate Pluto as observed by New Horizons in 2015. All key processes are parametrized on the basis of theoretical equations, including atmospheric dynamics and transport, turbulence, radiative transfer, molecular conduction, as well as phases changes for N2, CH2 and CO. Pluto's climate and ice cycles are found to be very sensitive to model parameters and initial states. Nevertheless, a reference simulation is designed by running a fast, reduced version of the GCM with simplified atmospheric transport for 40,000 Earth years to initialize the surface ice distribution and sub-surface temperatures, from which a 28-Earth-year full GCM simulation is performed. Assuming a topographic depression in a Sputnik-planum (SP)-like crater on the anti-Charon hemisphere, a realistic Pluto is obtained, with most N2 and CO ices accumulated in the crater, methane frost covering both hemispheres except for the equatorial regions, and a surface pressure near 1.1 Pa in 2015 with an increase between 1988 and 2015, as reported from stellar occultations. Temperature profiles are in qualitative agreement with the observations. In particular, a cold atmospheric layer is obtained in the lowest kilometers above Sputnik Planum, as observed by New Horizons's REX experiment. It is shown to result from the combined effect of the topographic depression and N2 daytime sublimation. In the reference simulation with surface N2 ice exclusively present in Sputnik Planum, the global circulation is only forced by radiative heating gradients and remains relatively weak. Surface winds are locally induced by topography slopes and by N2 condensation and sublimation around Sputnik Planum. However, the circulation can be more intense depending on the exact distribution of surface N2 frost. This is illustrated in an alternative simulation with N2 condensing in the South Polar regions and N2 frost covering latitudes between 35degN and 48degN. A global condensation flow is then created, inducing strong surface winds everywhere, a prograde jet in the southern high latitudes, and an equatorial superrotation likely forced by barotropic instabilities in the southern jet. Using realistic parameters, the GCM predict atmospheric concentrations of CO and CH4 in good agreement with the observations. N2 and CO do not condense in the atmosphere, but CH4 ice clouds can form during daytime at low altitude near the regions covered by N2 ice (assuming that nucleation is efficient enough). This global climate model can be used to study many aspects of the Pluto environment. For instance, organic hazes are included in the GCM and analysed in a companion paper (Bertrand and Forget, Icarus, this issue).
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.
J. Leconte, F. Selsis, F. Hersant, and T. Guillot. Condensation-inhibited convection in hydrogen-rich atmospheres . Stability against double-diffusive processes and thermal profiles for Jupiter, Saturn, Uranus, and Neptune. Astronomy Astrophysics, 598:A98, 2017. [ bib | DOI | arXiv | PDF version | ADS link ]
In an atmosphere, a cloud condensation region is characterized by a strong vertical gradient in the abundance of the related condensing species. On Earth, the ensuing gradient of mean molecular weight has relatively few dynamical consequences because N2 is heavier than water vapor, so that only the release of latent heat significantly impacts convection. On the contrary, in a hydrogen dominated atmosphere (e.g., giant planets), all condensing species are significantly heavier than the background gas. This can stabilize the atmosphere against convection near a cloud deck if the enrichment in the given species exceeds a critical threshold. This raises two questions. What is transporting energy in such a stabilized layer, and how affected can the thermal profile of giant planets be? To answer these questions, we first carry out a linear analysis of the convective and double-diffusive instabilities in a condensable medium showing that an efficient condensation can suppress double-diffusive convection. This suggests that a stable radiative layer can form near a cloud condensation level, leading to an increase in the temperature of the deep adiabat. Then, we investigate the impact of the condensation of the most abundant species (water) with a steady-state atmosphere model. Compared to standard models, the temperature increase can reach several hundred degrees at the quenching depth of key chemical tracers. Overall, this effect could have many implications for our understanding of the dynamical and chemical state of the atmosphere of giant planets, for their future observations (with Juno for example), and for their internal evolution.
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; Teff 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.
A. Chiavassa, A. Caldas, F. Selsis, J. Leconte, P. Von Paris, P. Bordé, Z. Magic, R. Collet, and M. Asplund. Measuring stellar granulation during planet transits. Astronomy Astrophysics, 597:A94, 2017. [ bib | DOI | arXiv | PDF version | ADS link ]
Context. Stellar activity and convection-related surface structures might cause bias in planet detection and characterization that use these transits. Surface convection simulations help to quantify the granulation signal. <BR /> Aims: We used realistic three-dimensional (3D) radiative hydrodynamical (RHD) simulations from the Stagger grid and synthetic images computed with the radiative transfer code Optim3D to model the transits of three prototype planets: a hot Jupiter, a hot Neptune, and a terrestrial planet. <BR /> Methods: We computed intensity maps from RHD simulations of the Sun and a K-dwarf star at different wavelength bands from optical to far-infrared that cover the range of several ground- and space-based telescopes which observe exoplanet transits. We modeled the transit using synthetic stellar-disk images obtained with a spherical-tile imaging method and emulated the temporal variation of the granulation intensity generating random images covering a granulation time-series of 13.3 h. We measured the contribution of the stellar granulation on the light curves during the planet transit. <BR /> Results: We identified two types of granulation noise that act simultaneously during the planet transit: (I) the intrinsic change in the granulation pattern with timescale (e.g., 10 min for solar-type stars assumed in this work) is smaller than the usual planet transit ( hours as in our prototype cases); and (II) the fact that the transiting planet occults isolated regions of the photosphere that differ in local surface brightness as a result of convective-related surface structures. First, we showed that our modeling approach returns granulation timescale fluctuations that are comparable with what has been observed for the Sun. Then, our statistical approach shows that the granulation pattern of solar and K-dwarf-type stars have a non-negligible effect of the light curve depth during the transit, and, consequentially on the determination of the planet transit parameters such as the planet radius (up to 0.90% and 0.47% for terrestrial and gaseous planets, respectively). We also showed that larger (or smaller) orbital inclination angles with respect to values corresponding to transit at the stellar center display a shallower transit depth and longer ingress and egress times, but also granulation fluctuations that are correlated to the center-to-limb variation: they increase (or decrease) the value of the inclination, which amplifies the fluctuations. The granulation noise appears to be correlated among the different wavelength ranges either in the visible or in the infrared regions. <BR /> Conclusions: The prospects for planet detection and characterization with transiting methods are excellent with access to large amounts of data for stars. The granulation has to be considered as an intrinsic uncertainty (as a result of stellar variability) on the precise measurements of exoplanet transits of planets. The full characterization of the granulation is essential for determining the degree of uncertainty on the planet parameters. In this context, the use of 3D RHD simulations is important to measure the convection-related fluctuations. This can be achieved by performing precise and continuous observations of stellar photometry and radial velocity, as we explained with RHD simulations, before, after, and during the transit periods.