'Super-Earths' are extrasolar planets about two to ten times the mass of the Earth, too small to be considered 'Jupiters'. Observations from the MEarth Project — using two 40-cm (16-inch) telescopes that will eventually be part of an eight-telescope array — have now identified a super-Earth (GJ 1214b) transiting a nearby low mass star. GJ 1214b has a mass 6.55 times that of the Earth and a radius of 2.68 'Earths'. As the star is small and only 13 parsecs away, the planetary atmosphere is available for direct study with current observatories. A population of extrasolar planets has been uncovered with minimum masses of 1.9–10 times the Earth's mass, called super-Earths, but atmospheric studies can be precluded by the distance and size of their stars. Here, observations of the transiting planet GJ 1214b are reported; it has a mass 6.55 times that of the Earth and a radius 2.68 times the Earth's radius. The star is small and only 13 parsecs away, permitting the study of the planetary atmosphere with current observatories. A decade ago, the detection of the first1,2 transiting extrasolar planet provided a direct constraint on its composition and opened the door to spectroscopic investigations of extrasolar planetary atmospheres3. 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 studies4,5,6 and microlensing7 have uncovered a population of planets with minimum masses of 1.9–10 times the Earth's mass (M⊕), 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⊕ and a radius 2.68 times Earth's radius (R⊕), 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.

A super-Earth transiting a nearby low-mass star