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 (EOH) 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.
T. Fouchet, T. K. Greathouse, A. Spiga, L. N. Fletcher, S. Guerlet, J. Leconte, and G. S. Orton. Stratospheric aftermath of the 2010 Storm on Saturn as observed by the TEXES instrument. I. Temperature structure. Icarus, 277:196-214, 2016. [ bib | DOI | arXiv | PDF version | ADS link ]
We report on spectroscopic observations of Saturn's stratosphere in July 2011 with the Texas Echelon Cross Echelle Spectrograph (TEXES) mounted on the NASA InfraRed Telescope Facility (IRTF). The observations, targeting several lines of the CH4ν4 band and the H2 S(1) quadrupolar line, were designed to determine how Saturn's stratospheric thermal structure was disturbed by the 2010 Great White Spot. A study of Cassini Composite Infrared Spectrometer (CIRS) spectra had already shown the presence of a large stratospheric disturbance centered at a pressure of 2 hPa, nicknamed the beacon B0, and a tail of warm air at lower pressures (Fletcher et al. [2012] Icarus 221, 560-586). Our observations confirm that the beacon B0 vertical structure determined by CIRS, with a maximum temperature of 180 1 K at 2 hPa, is overlain by a temperature decrease up to the 0.2-hPa pressure level. Our retrieved maximum temperature of 180 1 K is colder than that derived by CIRS (200 1 K), a difference that may be quantitatively explained by terrestrial atmospheric smearing. We propose a scenario for the formation of the beacon based on the saturation of gravity waves emitted by the GWS. Our observations also reveal that the tail is a planet-encircling disturbance in Saturn's upper stratosphere, oscillating between 0.2 and 0.02 hPa, showing a distinct wavenumber-2 pattern. We propose that this pattern in the upper stratosphere is either the signature of thermal tides generated by the presence of the warm beacon in the mid-stratosphere, or the signature of Rossby wave activity.
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.
S. Fromang, J. Leconte, and K. Heng. Shear-driven instabilities and shocks in the atmospheres of hot Jupiters. Astronomy Astrophysics, 591:A144, 2016. [ bib | DOI | arXiv | PDF version | ADS link ]
Context. General circulation models of the atmosphere of hot Jupiters have shown the existence of a supersonic eastward equatorial jet. These results have been obtained using numerical schemes that filter out vertically propagating sound waves and assume vertical hydrostatic equilibrium, or were acquired with fully compressive codes that use large dissipative coefficients. <BR /> Aims: We remove these two limitations and investigate the effects of compressibility on the atmospheric dynamics by solving the standard Euler equations. <BR /> Methods: This was done by means of a series of simulations performed in the framework of the equatorial β-plane approximation using the finite-volume shock-capturing code RAMSES. <BR /> Results: At low resolution, we recover the classical results described in the literature: we find a strong and steady supersonic equatorial jet of a few km s-1 that displays no signature of shocks. We next show that the jet zonal velocity depends significantly on the grid meridional resolution. When this resolution is fine enough to properly resolve the jet, the latter is subject to a Kelvin-Helmholtz instability. The jet zonal mean velocity displays regular oscillations with a typical timescale of a few days and a significant amplitude of about 15% of the jet velocity. We also find compelling evidence for the development of a vertical shear instability at pressure levels of a few bars. It seems to be responsible for an increased downward kinetic energy flux that significantly affects the temperature of the deep atmosphere and appears to act as a form of drag on the equatorial jet. This instability also creates velocity fluctuations that propagate upward and steepen into weak shocks at pressure levels of a few mbars. <BR /> Conclusions: We conclude that hot-Jupiter equatorial jets are potentially unstable to both a barotropic Kelvin-Helmholtz instability and a vertical shear instability. Upon confirmation using more realistic models, these two instabilities could result in significant time variability of the atmospheric winds, may provide a small-scale dissipation mechanism in the flow, and might have consequences for the internal evolution of hot Jupiters.
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.
P. von Paris, P. Gratier, P. Bordé, J. Leconte, and F. Selsis. Inferring asymmetric limb cloudiness on exoplanets from transit light curves. Astronomy Astrophysics, 589:A52, 2016. [ bib | DOI | arXiv | PDF version | ADS link ]
Context. Clouds have been shown to be present in many exoplanetary atmospheres. Cloud formation modeling predicts considerable inhomogeneities of cloud cover, consistent with optical phase curve observations. However, optical phase curves cannot resolve some existing degeneracies between cloud location and cloud optical properties. <BR /> Aims: We present a conceptually simple technique for detecting inhomogeneous cloud cover on exoplanets. Such an inhomogeneous cloud cover produces an asymmetric primary transit of the planet in front of the host star. Asymmetric transits produce characteristic residuals that are different from standard symmetric models. Furthermore, bisector spans can be used to determine asymmetries in the transit light curve. <BR /> Methods: We apply a model of asymmetric transits to the light curves of HAT-P-7b, Kepler-7b, and HD 209458b and search for possible cloud signatures. The nearly uninterrupted Kepler photometry is particularly well suited for this method since it allows for a very high time resolution. <BR /> Results: We do not find any statistically sound cloud signature in the data of the considered planets. For HAT-P-7b, a tentative detection of an asymmetric cloud cover is found, consistent with analysis of the optical phase curve. Based on Bayesian probability arguments, a symmetric model with an offset in the transit ephemeris is still the most viable model. This work demonstrates that for suitable targets, namely low-gravity planets around bright stars, the method can be used to constrain cloud cover characteristics and is thus a helpful additional tool for the study of exoplanetary atmospheres.