Three Possible Origins for the Gas Layer on GJ 1214b

TL;DR

This study explores possible origins of GJ 1214b's gas layer, analyzing interior models to determine composition and conditions, concluding a significant gas component is likely but composition remains uncertain.

Contribution

It introduces a comprehensive interior structure modeling approach considering three formation scenarios for GJ 1214b's gas layer, highlighting the non-uniqueness of composition inference.

Findings

GJ 1214b likely has a significant gas component.

Most models suggest the planet does not have liquid water.

A low intrinsic luminosity favors a water-rich envelope.

Abstract

We present an analysis of the bulk composition of the MEarth transiting super Earth exoplanet GJ 1214b using planet interior structure models. We consider three possible origins for the gas layer on GJ 1214b: direct accretion of gas from the protoplanetary nebula, sublimation of ices, and outgassing from rocky material. Armed only with measurements of the planet mass (M_p=6.55+/-0.98 M_{earth}), radius (R_p=2.678+/-0.13 R_{earth}), and stellar irradiation level, our main conclusion is that we cannot infer a unique composition. A diverse range of planet interiors fits the measured planet properties. Nonetheless, GJ 1214b's relatively low average density (rho_p=1870+/-400 kg m^{-3}) means that it almost certainly has a significant gas component. Our second major conclusion is that under most conditions we consider GJ 1214b would not have liquid water. Even if the outer envelope is predominantly sublimated water ice, the envelope will likely consist of a super-fluid layer sandwiched between vapor above and plasma (electrically conductive fluid) below at greater depths. In our models, a low intrinsic planet luminosity (<~2TW) is needed for a water envelope on GJ 1214b to pass through the liquid phase.