THERMAL EVOLUTION AND STRUCTURE MODELS OF THE TRANSITING SUPER-EARTH GJ 1214b

Abstract

The planet GJ 1214b is the second known super-Earth with a measured mass and radius. Orbiting a quiet M-star, it receives considerably less mass-loss driving X-ray and UV radiation than CoRoT-7b, so that the interior may be quite dissimilar in composition, including the possibility of a large fraction of water. We model the interior of GJ 1214b assuming a two-layer (envelope+rock core) structure where the envelope material is either H/He, pure water, or a mixture of H/He and H 2 O. Within this framework we perform models of the thermal evolution and contraction of the planet. We discuss possible compositions that are consistent with M p = 6.55 M ⊕ , R p = 2.678 R ⊕ , an age τ = 3 − 10 Gyr, and the irradiation level of the atmosphere. These conditions require that if water exists in the interior, it must remain in a fluid state, with important consequences for magnetic field generation. These conditions also require the atmosphere to have a deep isothermal region extending down to 80−800 bar, depending on composition. Our results bolster the suggestion of a metal-enriched H/He atmosphere for the planet, as we find water-world models that lack an H/He atmosphere to require an implausibly large water-to-rock ratio of more than 6:1. We instead favor a H/He/H 2 O envelope with high water mass fraction (∼ 0.5−0.85), similar to recent models of the deep envelope of Uranus and Neptune. Even with these high water mass fractions in the H/He envelope, generally the bulk composition of the planet can have subsolar water:rock ratios. Dry, water-enriched, and pure water envelope models differ to an observationally significant level in their tidal Love numbers k 2 of respectively ∼ 0.018, ∼ 0.15, and ∼ 0.7. Subject headings: planets and satellites: general -planets and satellites: individual(GJ 1214b)

THE OPTICAL AND NEAR-INFRARED TRANSMISSION SPECTRUM OF THE SUPER-EARTH GJ 1214b: FURTHER EVIDENCE FOR A METAL-RICH ATMOSPHERE

“…Our current knowledge of GJ 1214b's mass and radius indicates that the planet must have an atmosphere (Rogers & Seager 2010b;Nettelmann et al 2011). Our interpretation of the featureless transmission spectrum for the planet is that its atmosphere must either have at least 70% H 2 O by mass or optically thick high-altitude clouds or haze.…”

“…Its determined mass (6.5 M ⊕ ) and radius (2.65 R ⊕ ) put it firmly in the degenerate region of parameter space for interior and atmosphere mass and compositions. The only thing that can be said with a high degree of confidence based on just the mass and radius of the planet is that it must have a substantial atmosphere because it is too large to be composed solely of solid material (Rogers & Seager 2010b;Nettelmann et al 2011). Rogers & Seager (2010b) identified three possible origins for the gas layer on GJ 1214b and they presented distinct interior structure models based on these scenarios that are consistent with its measured mass and radius.…”

Ground-based transit observations of the super-Earth GJ 1214 b

“…The recent theoretical predictions for the atmosphere of GJ 1214 b currently offer the most plausible models that fit the planet's mass, radius, and irradiation level: a rocky or an icy core with a nebular hydrogen-helium envelope, that is, a miniNeptune, a rocky planet with an outgassed hydrogen atmosphere, or a core with a heavy (up to 45% of the planet mass) hot water vapor envelope with high molecular mass (Rogers & Seager 2010;Nettelmann et al 2011), as well as a planet with a cloudy or hazy atmosphere, with a high mean molecular mass composition (Morley et al 2013).…”

THE FLAT TRANSMISSION SPECTRUM OF THE SUPER-EARTH GJ1214b FROM WIDE FIELD CAMERA 3 ON THEHUBBLE SPACE TELESCOPE

“…Rogers & Seager (2010) have proposed three general scenarios consistent with GJ1214b's large radius, where the planet could (i) have accreted and maintained a nebular H 2 /He envelope atop an ice and rock core, (ii) consist of a rocky planet with an H 2 -rich envelope that formed by recent outgassing, or (iii) contain a large fraction of water in its interior surrounded by a dense H 2 -depleted, H 2 O-rich atmosphere. Detailed thermal evolution calculations by Nettelmann et al (2011) disfavor this last model on the basis that it would require unreasonably large bulk water-to-rock ratios, arguing for at least a partial H 2 /He envelope, albeit one that might be heavily enriched in H 2 O relative to the primordial nebula.…”