  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.
