One focus of modern astronomy is to detect temperate terrestrial exoplanets well-suited for atmospheric characterisation. A milestone was recently achieved with the detection of three Earth-sized planets transiting (i.e. passing in front of) a star just 8% the mass of the Sun 12 parsecs away1. Indeed, the transiting configuration of these planets combined with the Jupiter-like size of their host star - named TRAPPIST-1 - makes possible in-depth studies of their atmospheric properties with current and future astronomical facilities1,2,3. Here we report the results of an intensive photometric monitoring campaign of that star from the ground and with the Spitzer Space Telescope. Our observations reveal that at least seven planets with sizes and masses similar to the 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, 9.21, 12.35 days) are near ratios of small integers. This architecture suggests that the planets formed farther from the star and migrated inward4,5. The seven planets have equilibrium temperatures low enough to make possible liquid water on their surfaces6,7,8.

Context. Thanks to remarkable progress, radial velocity surveys are now able to detect terrestrial planets at habitable distance from low-mass stars. Recently, two planets with minimum masses below 10 M ⊕ have been reported in a triple system around the M-type star Gliese 581. These planets are found at orbital distances comparable to the location of the boundaries of the habitable zone of their star. Aims. In this study, we assess the habitability of planets Gl 581c and Gl 581d (assuming that their actual masses are close to their minimum masses) by estimating the locations of the habitable-zone boundaries of the star and discussing the uncertainties affecting their determination. An additional purpose of this paper is to provide simplified formulae for estimating the edges of the habitable zone. These may be used to evaluate the astrobiological potential of terrestrial exoplanets that will hopefully be discovered in the near future. Methods. Using results from radiative-convective atmospheric models and constraints from the evolution of Venus and Mars, we derive theoretical and empirical habitable distances for stars of F, G, K, and M spectral types. Results. Planets Gl 581c and Gl 581d are near to, but outside, what can be considered as the conservative habitable zone. Planet "c" receives 30% more energy from its star than Venus from the Sun, with an increased radiative forcing caused by the spectral energy distribution of Gl 581. This planet is thus unlikely to host liquid water, although its habitability cannot be positively ruled out by theoretical models due to uncertainties affecting cloud properties and cloud cover. Highly reflective clouds covering at least 75% of the day side of the planet could indeed prevent the water reservoir from being entirely vaporized. Irradiation conditions of planet "d" are comparable to those of early Mars, which is known to have hosted surface liquid water. Thanks to the greenhouse effect of CO 2 -ice clouds, also invoked to explain the early Martian climate, planet "d" might be a better candidate for the first exoplanet known to be potentially habitable. A mixture of several greenhouse gases could also maintain habitable conditions on this planet, although the geochemical processes that could stabilize such a super-greenhouse atmosphere are still unknown.Key words. astrobiology -atmospheric effects -stars: planetary systems IntroductionThe M-type star Gl 581 hosts at least 3 planets, which were detected using radial velocity measurements by Bonfils et al. (2005) (planet "b") and Udry et al. (2007) (planets "c" and "d"). The properties of this star and its planets are given in Table 1. Before this discovery, only two exoplanets were known to have a minimum mass below 10 M ⊕ , which is usually considered as a boundary between terrestrial and giant planets, the latter having a significant fraction of their mass in an H 2 -He envelope. The first one was GJ 876d, a very hot planet (P ≤ 2 days) with a minimum mass of 5.9 M ⊕ (Rivera et al. 2005). The other one is OGLE-0...

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Context. The atmosphere of hot Jupiters can be probed by primary transit and secondary eclipse spectroscopy. Owing to the intense UV irradiation, mixing, and circulation, their chemical composition is maintained out of equilibrium and must be modeled with kinetic models. Aims. Our purpose is to release a chemical network and the associated rate coefficients, developed for the temperature and pressure range relevant to hot Jupiters atmospheres. Using this network, we study the vertical atmospheric composition of the two hot Jupiters (HD 209458b and HD 189733b) with a model that includes photolyses and vertical mixing, and we produce synthetic spectra. Methods. The chemical scheme has been derived from applied combustion models that were methodically validated over a range of temperatures and pressures typical of the atmospheric layers influencing the observations of hot Jupiters. We compared the predictions obtained from this scheme with equilibrium calculations, with different schemes available in the literature that contain N-bearing species, and with previously published photochemical models. Results. Compared to other chemical schemes that were not subjected to the same systematic validation, we find significant differences whenever nonequilibrium processes take place (photodissociations or vertical mixing). The deviations from the equilibrium, hence the sensitivity to the network, are larger for HD 189733b, since we assume a cooler atmosphere than for HD 209458b. We found that the abundances of NH 3 and HCN can vary by two orders of magnitude depending on the network, demonstrating the importance of comprehensive experimental validation. A spectral feature of NH 3 at 10.5 μm is sensitive to these abundance variations and thus to the chemical scheme.Conclusions. Due to the influence of the kinetics, we recommend using a validated scheme to model the chemistry of exoplanet atmospheres. The network we release is robust for temperatures within 300-2500 K and pressures from 10 mbar up to a few hundred bars, for species made of C, H, O, and N. It is validated for species up to 2 carbon atoms and for the main nitrogen species (NH 3 , HCN, N 2 , NO x ). Although the influence of the kinetic scheme on the hot Jupiters spectra remains within the current observational error bars (with the exception of NH 3 ), it will become more important for atmospheres that are cooler or subjected to higher UV fluxes, because they depart more from equilibrium.

Context. A study of the mass loss enhancement for very close "Hot Jupiters" due to the gravitational field of the host star is presented. Aims. The influence of the proximity to a planet of the Roche lobe boundary on the critical temperature for blow-off conditions for estimating the increase of the mass loss rate through hydrodynamic blow-off for close-in exoplanets is investigated. Methods. We consider the gravitational potential for a star and a planet along the line that joins their mass centers and the energy balance equation for an evaporating planetary atmosphere including the effect of the stellar tidal force on atmospheric escape. Results. By studying the effect of the Roche lobe on the atmospheric loss from short-periodic gas giants we derived reasonably accurate approximate formulas to estimate atmospheric loss enhancement due to the action of tidal forces on a "Hot Jupiter" and to calculate the critical temperature for the onset of "geometrical blow-off", which are valid for any physical values of the Roche lobe radial distance. Using these formulas, we found that the stellar tidal forces can enhance the hydrodynamic evaporation rate from TreS-1 and OGLE-TR-56b by about 2 fold, while for HD 209458b we found an enhancement of about 50%. For similar exoplanets which are closer to their host star than OGLE-TR-56b, the mass loss enhancement can be even larger. Moreover, we showed that the effect of the Roche lobe allows "Hot Jupiters" to reach blow-off conditions at temperatures which are less than expected due to the stellar X-ray and EUV heating.

We present a novel chemical database for gas-phase astrochemistry. Named the KInetic Database for Astrochemistry (KIDA), this database consists of gas-phase reactions with rate coefficients and uncertainties that will be vetted to the greatest extent possible. Submissions of measured and calculated rate coefficients are welcome, and will be studied by experts before inclusion into the database. Besides providing kinetic information for the interstellar medium, KIDA is planned to contain such data for planetary atmospheres and for circumstellar envelopes. Each year, a subset of the reactions in the database (kida.uva) will be provided as a network for the simulation of the chemistry of dense interstellar clouds with temperatures between 10 K and 300 K. We also provide a code, named Nahoon, to study the timedependent gas-phase chemistry of 0D and 1D interstellar sources.

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The TRAPPIST-1 system is the first transiting planet system found orbiting an ultra-cool dwarf star. At least seven planets similar to Earth in radius and in mass were previously found to transit this host star. Subsequently, TRAPPIST-1 was observed as part of the K2 mission and, with these new data, we report the measurement of an 18.77 d orbital period for the outermost planet, TRAPPIST-1h, which was unconstrained until now. This value matches our theoretical expectations based on Laplace relations and places TRAPPIST-1h as the seventh member of a complex chain, with three-body resonances linking every member. We find that TRAPPIST-1h has a radius of 0.727 Earth radii and an equilibrium temperature of 173 K. We have also measured the rotational period of the star at 3.3 d and detected a number of flares consistent with a low-activity, middle-aged, late M dwarf.Comment: 42 pages, 8 figures, 2 table