Sub-Earth

Abstract: Exoplanets at small orbital distances from their host stars are submitted to intense levels of energetic radiations, X-rays and extreme ultraviolet (EUV). Depending on the masses and densities of the planets and on the atmospheric heating efficiencies, the stellar energetic inputs can lead to atmospheric mass loss. These evaporation processes are observable in the ultraviolet during planetary transits. The aim of the present work is to quantify the mass-loss rates (dm/dt), heating efficiencies (eta), and lifetimes for the whole sample of transiting exoplanets, now including hot jupiters, hot neptunes, and hot super-earths. The mass-loss rates and lifetimes are estimated from an "energy diagram" for exoplanets, which compares the planet gravitational potential energy to the stellar X/EUV energy deposited in the atmosphere. We estimate the mass-loss rates of all detected transiting planets to be within 10^6 to 10^13 g/s for various conservative assumptions. High heating efficiencies would imply that hot exoplanets such the gas giants WASP-12b and WASP-17b could be completely evaporated within 1 Gyr. We further show that the heating efficiency can be constrained when dm/dt is inferred from observations and the stellar X/EUV luminosity is known. This leads us to suggest that eta ~ 100% in the atmosphere of the hot jupiter HD209458b, while it could be lower for HD189733b. Simultaneous observations of transits in the ultraviolet and X-rays are necessary to further constrain the exospheric properties of exoplanets.

Abstract: We present an analytical and numerical study of the orbital migration and resonance capture of fictitious two-planet systems with masses in the super-Earth range undergoing Type-I migration. We find that, depending on the flare index and proximity to the central star, the average value of the period ratio, $P_2/P_1$, between both planets may show a significant deviation with respect to the nominal value. For planets trapped in the 2:1 commensurability, offsets may reach values on the order of $0.1$ for orbital periods on the order of $1$ day, while systems in the 3:2 mean-motion resonance (MMR) show much smaller offsets for all values of the semimajor axis. These properties are in good agreement with the observed distribution of near-resonant exoplanets, independent of their detection method. We show that 2:1-resonant systems far from the star, such as HD82943 and HR8799, are characterized by very small resonant offsets, while higher values are typical of systems discovered by Kepler with orbital periods approximately a few days. Conversely, planetary systems in the vicinity of the 3:2 MMR show little offset with no significant dependence on the orbital distance. In conclusion, our results indicate that the distribution of Kepler planetary systems around the 2:1 and 3:2 MMR are consistent with resonant configurations obtained as a consequence of a smooth migration in a laminar flared disk, and no external forces are required to induce the observed offset or its dependence with the commensurability or orbital distance from the star.

Abstract: Magnetic field and plasma measurements in the environments of the planets Mars, Mercury, and Venus are reported.

Abstract: The history of investigation of the solar wind interaction with Mars, Venus, and Mercury is reviewed, and our knowledge on this interaction for each of the three planets is compared. The primary objective is to gain insight into the basic physical mechanisms operative in the earth's magnetosphere from a study of (the somewhat different) magnetospheres of the planets under consideration. Mercury and Venus have significant dipole moments which play an important part in solar wind interaction. The Martian magnetic moment appears to be too weak to influence solar wind interaction. As expected, the bow shock of Mercury and the earth are quite similar since the magnetic moment of each is sufficient to stand off the solar wind. The shocks of Venus and Mars are also similar, but the Venusian shock lies much closer to the planet than the Martian shock. Both Mercury and Venus show evidence of substorm-like field and particle behavior, but with clear differences in the time scale.