Composition and fate of short-period super-Earths

Abstract

Context. The discovery of CoRoT-7b, a planet of a radius 1.68 ± 0.09 R ⊕ , a mass 4.8 ± 0.8 M ⊕ , and an orbital period of 0.854 days demonstrates that small planets can orbit extremely close to their star. Aims. Several questions arise concerning this planet, in particular concerning its possible composition, and fate. Methods. We use knowledge of hot Jupiters, mass loss estimates and models for the interior structure and evolution of planets to understand its composition, structure and evolution. Results. The inferred mass and radius of CoRoT-7b are consistent with a rocky planet that would be significantly depleted in iron relative to the Earth. However, a one sigma increase in mass (5.6 M ⊕ ) and one sigma decrease in size (1.59 R ⊕ ) would make the planet compatible with an Earth-like composition (33% iron, 67% silicates). Alternatively, it is possible that CoRoT-7b contains a significant amount of volatiles. For a planet made of an Earth-like interior and an outer volatile-rich vapour envelope, an equally good fit to the measured mass and radius is found for a mass of the vapour envelope equal to 3% (and up to 10% at most) of the planetary mass. Because of its intense irradiation and small size, we determine that the planet cannot possess an envelope of hydrogen and helium of more than 1/10 000 of its total mass. We show that a relatively significant mass loss ∼10 11 g s −1 is to be expected and that it should prevail independently of the planet's composition. This is because to first order, the hydrodynamical escape rate is independent of the mean molecular mass of the atmosphere, and because given the intense irradiation, even a bare rocky planet would be expected to possess an equilibrium vapour atmosphere thick enough to capture stellar UV photons. Clearly, this escape rate rules out the possibility that a hydrogen-helium envelope is present, as it would escape in only ∼1 Ma. A water vapour atmosphere would escape in ∼1 Ga, indicating that this is a plausible scenario. The origin of CoRoT-7b cannot be inferred from the present observations: It may have always had a rocky composition; it may be the remnant of a Uranus-like ice giant, or a gas giant with a small core that has been stripped of its gaseous envelope. Conclusions. With high enough sensitivity, spectroscopic transit observations of CoRoT-7 should constrain the composition of the evaporating flow and therefore allow us to distinguish between a rocky planet and a volatile-rich vapour planet. In addition, the theoretical tools developed in this study are applicable to any short-period transiting super-Earth and will be important to understanding their origins.

Spectroscopy of planetary atmospheres in our Galaxy

“…We have no information about their composition, but models suggest that they could exhibit silicate compounds in the gaseous/liquid phase (Léger et al 2011, Valencia et al 2010Rouan et al 2011). At lower temperatures (700-1000 K), as in the case of Kepler 11b, f (Lissauer et al 2011) and Kepler 20b, c, e, f (Gautier et al 2012), silicates do not evaporate.…”

Tidal Venuses: Triggering a Climate Catastrophe via Tidal Heating

“…Their formulation is complicated, and rather than reproduce it here, the reader is referred to their paper. We do not include the recent models put forth by Swift et al (2012), as they did not include formulae, but they did find that their results are consistent with those of Fortney et al (2007) and Valencia et al (2010) for rock and iron planets.…”

“…D3-D4 are analytic fits to complex models and, hence, predict slightly different values for R p for a planet made of rock (f ice = 0 and f rock = 1). Valencia et al (2007) considered planets in the range 1-10 M Earth and parameterized the radius in terms of f ice (see also Valencia et al, 2010). They found The last relationship we review is that of Grasset et al (2009), who cast their model in terms only of f ice .…”

The silicate and carbon-rich models of CoRoT-7b, Kepler-9d and Kepler-10b

“…The degeneracy problems of bulk compositions of low-mass exoplanets can be partly overcome if they are very close to their host stars because some chemical constituents may be ruled out a priori. Valencia et al (2010) discussed the evolution of close-in low-mass planets like CoRoT-7b. Due to the intense stellar irradiation and its small size, it is unlikely to possess an envelope of hydrogen and helium of more than 1/10000 of (Torres et al 2011), Kepler-10b (Batalha et al 2011, and CoRoT-7b (Léger et al 2009 Host Star Age (Ga) 11.9 ± 4.5 3.0 ± 1.0 1.5 +0.8 Holman et al (2010).…”