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.
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We thank M. Everett for gathering the FLWO 1.2-m observations, S. Seager for providing a digital version of the structural models, and D. Sasselov and S. Seager for comments on the manuscript. Support for this work was provided by the David and Lucile Packard Foundation Fellowship for Science and Engineering awarded to D.C., and by the US National Science Foundation under grant number AST-0807690. L.A.B. and D.W.L. acknowledge support from the NASA Kepler mission under cooperative agreement NCC2-1390. M.J.H. acknowledges support by NASA Origins Grant NNX09AB33G. The HARPS observations were gathered under the European Southern Observatory Director’s Discretionary Program 283.C-5022 (A). We thank the Smithsonian Astrophysical Observatory for supporting the MEarth Project at FLWO.
Author Contributions D.C., Z.K.B., J.I., C.J.B., P.N. and E.E.F. gathered and analysed the photometric data from the MEarth observatory, C.L., X.B., L.A.B., S.U., D.Q., F.P., M.M. and C.J.B. gathered and analysed the spectroscopic data from the HARPS instrument, and L.A.B., D.W.L., M.J.H., J.N.W. and P.N. gathered and analysed supplementary photometric and spectroscopic data with the 1.2-m and 1.5-m FLWO telescopes. R.A.M.-C. estimated the hydrodynamic escape rate, and X.B., X.D., T.F., J.I. and P.N. estimated the properties of the parent star. All authors discussed the results and commented on the manuscript. D.C. led the project and wrote the paper.
This file contains Supplementary Table 2.
About this article
The Astronomical Journal (2019)