Composition and fate of short-period super-Earths

The case of CoRoT-7b

¹ Observatoire de la Côte d'Azur, Université de Nice-Sophia Antipolis, CNRS UMR 6202, BP 4229, 06304 Nice Cedex 4, France e-mail: valencia@oca.eu
² Dept. of Earth and Planetary Sciences, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8551, Japan
³ Institut für Physik, Universität Rostock, 18051 Rostock, Germany

Received: 7 July 2009
Accepted: 7 February 2010

Abstract

Context. The discovery of CoRoT-7b, a planet of a radius 1.68±0.09 , a mass 4.8±0.8 , 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 ) and one sigma decrease in size (1.59 ) 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 ~10¹¹ 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.