title: A rocky planet transiting a nearby low-mass star.
abstract: M-dwarf stars--hydrogen-burning stars that are smaller than 60 per cent of the size of the Sun--are the most common class of star in our Galaxy and outnumber Sun-like stars by a ratio of 12:1. Recent results have shown that M dwarfs host Earth-sized planets in great numbers: the average number of M-dwarf planets that are between 0.5 to 1.5 times the size of Earth is at least 1.4 per star. The nearest such planets known to transit their star are 39 parsecs away, too distant for detailed follow-up observations to measure the planetary masses or to study their atmospheres. Here we report observations of GJ 1132b, a planet with a size of 1.2 Earth radii that is transiting a small star 12 parsecs away. Our Doppler mass measurement of GJ 1132b yields a density consistent with an Earth-like bulk composition, similar to the compositions of the six known exoplanets with masses less than six times that of the Earth and precisely measured densities. Receiving 19 times more stellar radiation than the Earth, the planet is too hot to be habitable but is cool enough to support a substantial atmosphere, one that has probably been considerably depleted of hydrogen. Because the host star is nearby and only 21 per cent the radius of the Sun, existing and upcoming telescopes will be able to observe the composition and dynamics of the planetary atmosphere.
authors.affiliation: Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA.|Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA.
authors.forename: Zachory K
authors.initials: ZK
authors.lastname: Berta-Thompson
authors.affiliation: Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA.
authors.forename: Jonathan
authors.initials: J
authors.lastname: Irwin
authors.affiliation: Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA.
authors.forename: David
authors.initials: D
authors.lastname: Charbonneau
authors.affiliation: Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA.
authors.forename: Elisabeth R
authors.initials: ER
authors.lastname: Newton
authors.affiliation: Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA.
authors.forename: Jason A
authors.initials: JA
authors.lastname: Dittmann
authors.affiliation: Observatoire de Genève, Université de Genéve, 51 chemin des Maillettes, 1290 Sauverny, Switzerland.
authors.forename: Nicola
authors.initials: N
authors.lastname: Astudillo-Defru
authors.affiliation: Université Grenoble Alpes, IPAG, F-38000 Grenoble, France.|CNRS, IPAG, F-38000 Grenoble, France.
authors.forename: Xavier
authors.initials: X
authors.lastname: Bonfils
authors.affiliation: Institut d'Astrophysique et de Géophysique, Université de Liège, Allée du 6 Août 17, Bâtiment B5C, 4000 Liège, Belgium.
authors.forename: Michaël
authors.initials: M
authors.lastname: Gillon
authors.affiliation: Institut d'Astrophysique et de Géophysique, Université de Liège, Allée du 6 Août 17, Bâtiment B5C, 4000 Liège, Belgium.
authors.forename: Emmanuël
authors.initials: E
authors.lastname: Jehin
authors.affiliation: Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA.
authors.forename: Antony A
authors.initials: AA
authors.lastname: Stark
authors.affiliation: Institute for Astronomy, University of Hawaii at Manoa, Honolulu, Hawaii 96822, USA.
authors.forename: Brian
authors.initials: B
authors.lastname: Stalder
authors.affiliation: Observatoire de Genève, Université de Genéve, 51 chemin des Maillettes, 1290 Sauverny, Switzerland.|Laboratoire d'Astrophysique de Marseille, UMR 6110 CNRS, Université de Provence, 38 rue Frédéric Joliot-Curie, 13388, Marseille Cedex 13, France.
authors.forename: Francois
authors.initials: F
authors.lastname: Bouchy
authors.affiliation: Université Grenoble Alpes, IPAG, F-38000 Grenoble, France.|CNRS, IPAG, F-38000 Grenoble, France.
authors.forename: Xavier
authors.initials: X
authors.lastname: Delfosse
authors.affiliation: Université Grenoble Alpes, IPAG, F-38000 Grenoble, France.|CNRS, IPAG, F-38000 Grenoble, France.
authors.forename: Thierry
authors.initials: T
authors.lastname: Forveille
authors.affiliation: Observatoire de Genève, Université de Genéve, 51 chemin des Maillettes, 1290 Sauverny, Switzerland.
authors.forename: Christophe
authors.initials: C
authors.lastname: Lovis
authors.affiliation: Observatoire de Genève, Université de Genéve, 51 chemin des Maillettes, 1290 Sauverny, Switzerland.
authors.forename: Michel
authors.initials: M
authors.lastname: Mayor
authors.affiliation: Departamento de Física, Universidade Federal do Rio Grande do Norte, 59072-970 Natal, Rio Grande do Norte, Brazil.
authors.forename: Vasco
authors.initials: V
authors.lastname: Neves
authors.affiliation: Observatoire de Genève, Université de Genéve, 51 chemin des Maillettes, 1290 Sauverny, Switzerland.
authors.forename: Francesco
authors.initials: F
authors.lastname: Pepe
authors.affiliation: Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal.|Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua Campo Alegre, 4169-007 Porto, Portugal.
authors.forename: Nuno C
authors.initials: NC
authors.lastname: Santos
authors.affiliation: Observatoire de Genève, Université de Genéve, 51 chemin des Maillettes, 1290 Sauverny, Switzerland.
authors.forename: Stéphane
authors.initials: S
authors.lastname: Udry
authors.affiliation: Université Grenoble Alpes, IPAG, F-38000 Grenoble, France.|CNRS, IPAG, F-38000 Grenoble, France.
authors.forename: Anaël
authors.initials: A
authors.lastname: Wünsche