Three Possible Origins for the Gas Layer on GJ 1214b

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  • 16 December 2009
Abstract
We present an analysis of the interior composition of the new MEarth transiting super Earth exoplanet GJ 1214b. The relatively low average planet density (\rho_p=1870\pm400 kg m^{-3}) means that GJ 1214b almost certainly has a significant gas component. Based on the mass and radius alone, (M_p=6.55\pm0.98 M_{earth}, R_p=2.678\pm0.13 R_{earth}) we cannot infer a unique composition of the interior or the gas layer. In this paper we explore the range of possible origins and compositions for the gas layer by considering three end-member scenarios that account for the observed mass and radius: a mini Neptune, a water planet, or an outgassed super Earth. If GJ 1214b is a mini Neptune or outgassed super Earth, a hydrogen-rich envelope accounting for ~< 1% of the planet mass could account for the planet radius (in contrast to Neptune's 5-15%). Alternatively, the measured mass and radius are also consistent with a water planet scenario in which GJ 1214b is composed of at least 47% H2O by mass and is surrounded by a massive steam atmosphere. We find that in this scenario, given our water EOS and reasonable assumptions for the planet albedo and interior luminosity, GJ 1214b would be a so-called hot-ocean planet with an outer steam atmosphere that transitions continuously to a superfluid without passing through the liquid phase. Regardless of the true nature of GJ 1214b, the relatively low density means it is certainly a new kind of planet with no solar system analogs.

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