A super-Earth transiting a nearby low-mass star

TL;DR

This paper reports the discovery and characterization of GJ 1214b, a nearby super-Earth with a water-rich composition and an escaping atmosphere, making it a prime target for atmospheric studies.

Contribution

It provides the first detailed characterization of a nearby super-Earth with a water-rich composition and an observable atmosphere.

Findings

GJ 1214b has a mass of 6.55 Earth masses and a radius of 2.68 Earth radii.

The planet's composition is consistent with water and a thin hydrogen-helium envelope.

The atmosphere is likely escaping hydrodynamically, indicating significant evolutionary processes.

Abstract

A decade ago, the detection of the first transiting extrasolar planet provided a direct constraint on its composition and opened the door to spectroscopic investigations of extrasolar planetary atmospheres. As such characterization studies are feasible only for transiting systems that are both nearby and for which the planet-to-star radius ratio is relatively large, nearby small stars have been surveyed intensively. Doppler studies and microlensing have uncovered a population of planets with minimum masses of 1.9-10 times the Earth's mass (M_Earth), called super-Earths. The first constraint on the bulk composition of this novel class of planets was afforded by CoRoT-7b, but the distance and size of its star preclude atmospheric studies in the foreseeable future. Here we report observations of the transiting planet GJ 1214b, which has a mass of 6.55 M_Earth and a radius 2.68 times Earth's radius (R_Earth), indicating that it is intermediate in stature between Earth and the ice giants of the Solar System. We find that the planetary mass and radius are consistent with a composition of primarily water enshrouded by a hydrogen-helium envelope that is only 0.05% of the mass of the planet. The atmosphere is probably escaping hydrodynamically, indicating that it has undergone significant evolution during its history. As the star is small and only 13 parsecs away, the planetary atmosphere is amenable to study with current observatories.