[en] 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.
Disciplines :
Space science, astronomy & astrophysics
Author, co-author :
Berta-Thompson, Zachory K.; 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
Irwin, Jonathan; Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA
Charbonneau, David; Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA
Newton, Elisabeth R.; Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA
Dittmann, Jason A.; Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA
Astudillo-Defru, Nicola; Observatoire de Genève, Université de Genéve, 51 chemin des Maillettes, 1290 Sauverny, Switzerland
Bonfils, Xavier; Université Grenoble Alpes, IPAG, F-38000 Grenoble, France ; CNRS, IPAG, F-38000 Grenoble, France
Jehin, Emmanuel ; Université de Liège > Département d'astrophys., géophysique et océanographie (AGO) > Origines Cosmologiques et Astrophysiques (OrCa)
Stark, Antony A.; Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA
Stalder, Brian; Institute for Astronomy, University of Hawaii at Manoa, Honolulu, Hawaii 96822, USA
Bouchy, Francois; 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
Delfosse, Xavier; Université Grenoble Alpes, IPAG, F-38000 Grenoble, France ; CNRS, IPAG, F-38000 Grenoble, France
Forveille, Thierry; Université Grenoble Alpes, IPAG, F-38000 Grenoble, France ; CNRS, IPAG, F-38000 Grenoble, France
Lovis, Christophe; Observatoire de Genève, Université de Genéve, 51 chemin des Maillettes, 1290 Sauverny, Switzerland
Mayor, Michel; Observatoire de Genève, Université de Genéve, 51 chemin des Maillettes, 1290 Sauverny, Switzerland
Neves, Vasco; Departamento de Física, Universidade Federal do Rio Grande do Norte, 59072-970 Natal, Rio Grande do Norte, Brazil
Pepe, Francesco; Observatoire de Genève, Université de Genéve, 51 chemin des Maillettes, 1290 Sauverny, Switzerland
Santos, Nuno C.; 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
Udry, Stéphane; Observatoire de Genève, Université de Genéve, 51 chemin des Maillettes, 1290 Sauverny, Switzerland
Dressing, C. D. & Charbonneau, D. The occurrence rate of small planets around small stars. Astrophys. J. 767, 95 (2013).
Morton, T. D. & Swift, J. The radius distribution of planets around cool stars. Astrophys. J. 791, 10 (2014).
Dressing, C. D. & Charbonneau, D. The occurrence of potentially habitable planets orbiting M dwarfs estimated from the full Kepler dataset and an empirical measurement of the detection sensitivity. Astrophys. J. 807, 45 (2015).
Muirhead, P. S. et al. Characterizing the cool KOIs. III. KOI 961: a small star with large proper motion and three small planets. Astrophys. J. 747, 144 (2012).
Haywood, R. D. et al. Planets and stellar activity: hide and seek in the CoRoT-7 system. Mon. Not. R. Astron. Soc. 443, 2517-2531 (2014).
Carter, J. A. et al. Kepler-36: a pair of planets with neighboring orbits and dissimilar densities. Science 337, 556-559 (2012).
Dumusque, X. et al. The Kepler-10 planetary system revisited by Harps-N: a hot rocky world and a solid Neptune-mass planet. Astrophys. J. 789, 154 (2014).
Pepe, F. et al. An Earth-sized planet with an Earth-like density. Nature 503, 377-380 (2013).
Howard, A. W. et al. A rocky composition for an Earth-sized exoplanet. Nature 503, 381-384 (2013).
Dressing, C. D. et al. The mass of Kepler-93b and the composition of terrestrial planets. Astrophys. J. 800, 135 (2015).
Motalebi, F. et al. The HARPS-N rocky planet search I. HD 219134 b: a transiting rocky planet in a 4 planet system at 6.5 pc from the Sun. Astron. Astrophys. http://dx.doi.org/10.1051/0004-6361/201526822 (2015).
Irwin, J. M. et al. The MEarth-North and MEarth-South transit surveys: searching for habitable super-Earth exoplanets around nearby M-dwarfs. In 18th Conference Cambridge Work on Cool Stars, Stellar Systems and the Sun (eds van Belle, G. & Harris, H. C.) 767-772 (http://adslabs.org/adsabs/abs/2015csss...18..767I/) (2015).
Berta, Z. K., Irwin, J., Charbonneau, D., Burke, C. J. & Falco, E. E. Transit detection in the MEarth survey of nearby M dwarfs: bridging the clean-first, search-later divide. Astron. J. 144, 145 (2012).
Gillon, M. et al. TRAPPIST: a robotic telescope dedicated to the study of planetary systems. EPJ Web Conf. 11, 06002 (2011).
Stalder, B. et al. PISCO: the Parallel Imager for Southern Cosmology Observations. In Proc. SPIE (eds Ramsay, S. K., McLean, I. S. & Takami, H.) Vol. 9147, 91473Y (2014).
Mayor, M. et al. Setting new standards with HARPS. Messenger 114, 20-24 (2003).
Jao, W.-C. et al. The solar neighborhood XIII: parallax results from the CTIOPI 0.9-m program - stars with mu >= 1″/year (MOTION sample). Astron. J. 129, 1954 (2005).
Delfosse, X. et al. Accurate masses of very low mass stars: IV. Improved mass-luminosity relations. Astron. Astrophys. 364, 217-224 (2000).
Hartman, J. D. et al. HATS-6b: a warm Saturn transiting an early M dwarf star, and a set of empirical relations for characterizing K and M dwarf planet hosts. Astron. J. 149, 166 (2015).
Charbonneau, D. et al. A super-Earth transiting a nearby low-mass star. Nature 462, 891-894 (2009).
Zeng, L. & Sasselov, D. D. A detailed model grid for solid planets from 0.1 through 100 Earth masses. Publ. Astron. Soc. Pacif. 125, 227-239 (2013).
Rogers, L. A. Most 1.6 Earth-radius planets are not rocky. Astrophys. J. 801, 41 (2015).
Lopez, E. D. & Fortney, J. J. Understanding the mass-radius relation for sub-Neptunes: radius as a proxy for composition. Astrophys. J. 792, 1 (2014).
Ballard, S. & Johnson, J. A. The Kepler dichotomy among the M dwarfs: half of systems contain five or more coplanar planets. Preprint at http://adslabs.org/adsabs/abs/2014arXiv1410.4192B/(2014).
Muirhead, P. S. et al. Kepler-445, Kepler-446 and the occurrence of compact multiples orbiting mid-M dwarf stars. Astrophys. J. 801, 18 (2015).
Kasting, J. F., Whitmire, D. P. & Reynolds, R. T. Habitable zones around main sequence stars. Icarus 101, 108-128 (1993).
Luger, R. & Barnes, R. Extreme water loss and abiotic O2 buildup on planets throughout the habitable zones of M dwarfs. Astrobiology 15, 119-143 (2015).
Kreidberg, L. et al. Clouds in the atmosphere of the super-Earth exoplanet GJ 1214b. Nature 505, 69-72 (2014).
Selsis, F., Wordsworth, R. & Forget, F. Thermal phase curves of nontransiting terrestrial exoplanets 1. Characterizing atmospheres. Astron. Astrophys. 532, A1 (2011).
Koll, D. D. B. & Abbot, D. S. Deciphering thermal phase curves of dry, tidally locked terrestrial planets. Astrophys. J. 802, 21 (2015).
Dittmann, J. A., Irwin, J. M., Charbonneau, D. & Berta-Thompson, Z. K. Trigonometric parallaxes for 1507 nearby mid-to-late M dwarfs. Astrophys. J. 784, 156 (2014).
Eggen, O. J. Catalogs of proper-motion stars. I. Stars brighter than visual magnitude 15 and with annual proper motion of 1 arcsec or more. Astrophys. J. 39 (Suppl.), 89 (1979).
Skrutskie, M. F. et al. The two micron all sky survey (2MASS). Astron. J. 131, 1163-1183 (2006).
Winters, J. G. et al. The solar neighborhood. XXXV. Distances to 1404 M dwarf systems within 25 pc in the southern sky. Astron. J. 149, 5 (2014).
Newton, E. R. et al. Near-infrared metallicities, radial velocities, and spectral types for 447 nearby M dwarfs. Astron. J. 147, 20 (2014).
Hawley, S. L., Gizis, J. E. & Reid, N. I. The Palomar/MSU nearby star spectroscopic survey. II. The southern M dwarfs and investigation of magnetic activity. Astron. J. 113, 1458 (1997).
Newton, E. R., Charbonneau, D., Irwin, J. & Mann, A. W. An empirical calibration to estimate cool dwarf fundamental parameters from H-band spectra. Astrophys. J. 800, 85 (2015).
Mann, A. W., Brewer, J. M., Gaidos, E., Lépine, S. & Hilton, E. J. Prospecting in late-type dwarfs: a calibration of infrared and visible spectroscopic metallicities of late K and M dwarfs spanning 1.5 dex. Astron. J. 145, 52 (2013).
Mann, A. W. et al. Prospecting in ultracool dwarfs: measuring the metallicities of mid- and late-M dwarfs. Astron. J. 147, 160 (2014).
Boyajian, T. S. et al. Stellar diameters and temperatures. II. Main-sequence K- and M-stars. Astrophys. J. 757, 112 (2012).
Dotter, A. et al. The Dartmouth stellar evolution database. Astrophys. J. 178 (Suppl.), 89-101 (2008).
Mann, A. W., Feiden, G. A., Gaidos, E., Boyajian, T. & von Braun, K. How to constrain your M dwarf: measuring effective temperature, bolometric luminosity, mass, and radius. Astrophys. J. 804, 64 (2015).
Leggett, S. K., Allard, F., Geballe, T. R., Hauschildt, P. H. & Schweitzer, A. Infrared spectra and spectral energy distributions of late M and L dwarfs. Astrophys. J. 548, 908-918 (2001).
Pecaut, M. J. & Mamajek, E. E. Intrinsic colors, temperatures, and bolometric corrections of pre-main-sequence stars. Astrophys. J. 208 (Suppl.), 9 (2013).
West, A. A. et al. Constraining the age-activity relation for cool stars: the Sloan digital sky survey data release 5 low-mass star spectroscopic sample. Astron. J. 135, 785-795 (2008).
Walkowicz, L. M. & Hawley, S. L. Tracers of chromospheric structure. I. Observations of Ca II K and Hα in M dwarfs. Astron. J. 137, 3297-3313 (2009).
Bonfils, X. et al. The HARPS search for southern extra-solar planets. XXXI. The M-dwarf sample. Astron. Astrophys. 549, A109 (2013).
Irwin, J. et al. On the angular momentum evolution of fully-convective stars: rotation periods for field M-dwarfs from the MEarth transit survey. Astrophys. J. 727, 56 (2010).
Benedict, G. F. et al. Photometry of Proxima Centauri and Barnard's Star using Hubble space telescope fine guidance sensor 3: a search for periodic variations. Astron. J. 116, 429-439 (1998).
Feltzing, S. & Bensby, T. The galactic stellar disc. Phys. Scr. 2008, T133 (2008).
Kiraga, M. & Stepien, K. Age-rotation-activity relations for M dwarf stars. Acta Astron. 57, 149-172 (2007).
Mamajek, E. E. & Hillenbrand, L. A. Improved age estimation for solar-type dwarfs using activity-rotation diagnostics. Astrophys. J. 687, 1264 (2008).
Goldreich, P. & Soter, S. Q in the solar system. Icarus 5, 375-389 (1966).
Mandel, K. & Agol, E. Analytic light curves for planetary transit searches. Astrophys. J. 580, L171-L175 (2002).
Claret, A., Hauschildt, P. H. & Witte, S. New limb-darkening coefficients for PHOENIX/1D model atmospheres. Astron. Astrophys. 546, A14 (2012).
Ambikasaran, S., Foreman-Mackey, D., Greengard, L., Hogg, D. W. & O'Neil, M. Fast direct methods for Gaussian processes and the analysis of NASA Kepler mission data. Preprint at http://arxiv.org/abs/1403.6015 (2014).
Gibson, N. P. et al. A Gaussian process framework for modelling instrumental systematics: application to transmission spectroscopy. Mon. Not. R. Astron. Soc. 419, 2683-2694 (2012).
Foreman-Mackey, D., Hogg, D. W., Lang, D. & Goodman, J. Emcee: the MCMC hammer. Publ. Astron. Soc. Pacif. 125, 306-312 (2013).
Goodman, J. & Weare, J. Ensemble samplers with affine invariance. Comm. App. Math. Comp. Sci. 5, 65-80 (2010).
Tokovinin, A. et al. CHIRON - a fiber fed spectrometer for precise radial velocities. Publ. Astron. Soc. Pacif. 125, 1336-1347 (2013).
Bouchy, F., Pepe, F. & Queloz, D. Fundamental photon noise limit to radial velocity measurements. Astron. Astrophys. 374, 733-739 (2001).
Astudillo-Defru, N. et al. The HARPS search for southern extra-solar planets. Astron. Astrophys. 575, A119 (2015).
Akeson, R. L. et al. The NASA exoplanet archive: data and tools for exoplanet research. Publ. Astron. Soc. Pacif. 125, 989-999 (2013).
