title: A super-Earth transiting a nearby low-mass star.
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. Because 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[symbol:see text]), called super-Earths. The first constraint on the bulk composition of this novel class of planets was afforded by CoRoT-7b (refs 8, 9), 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 6.55M[symbol:see text]), and a radius 2.68 times Earth's radius (R[symbol:see text]), 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. The star is small and only 13 parsecs away, so the planetary atmosphere is amenable to study with current observatories.
authors.affiliation: Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA. dcharbonneau@cfa.harvard.edu
authors.forename: David
authors.initials: D
authors.lastname: Charbonneau
authors.forename: Zachory K
authors.initials: ZK
authors.lastname: Berta
authors.forename: Jonathan
authors.initials: J
authors.lastname: Irwin
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Burke
authors.forename: Philip
authors.initials: P
authors.lastname: Nutzman
authors.forename: Lars A
authors.initials: LA
authors.lastname: Buchhave
authors.forename: Christophe
authors.initials: C
authors.lastname: Lovis
authors.forename: Xavier
authors.initials: X
authors.lastname: Bonfils
authors.forename: David W
authors.initials: DW
authors.lastname: Latham
authors.forename: Stéphane
authors.initials: S
authors.lastname: Udry
authors.forename: Ruth A
authors.initials: RA
authors.lastname: Murray-Clay
authors.forename: Matthew J
authors.initials: MJ
authors.lastname: Holman
authors.forename: Emilio E
authors.initials: EE
authors.lastname: Falco
authors.forename: Joshua N
authors.initials: JN
authors.lastname: Winn
authors.forename: Didier
authors.initials: D
authors.lastname: Queloz
authors.forename: Francesco
authors.initials: F
authors.lastname: Pepe
authors.forename: Michel
authors.initials: M
authors.lastname: Mayor
authors.forename: Xavier
authors.initials: X
authors.lastname: Delfosse
authors.forename: Thierry
authors.initials: T
authors.lastname: Forveille