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 numbers1,2: 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 star3. The nearest such planets known to transit their star are 39 parsecs away4, 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 densities5,6,7,8,9,10,11. 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.
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References
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
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)
Acknowledgements
We thank the staff at the Cerro Tololo Inter-American Observatory for assistance in the construction and operation of MEarth–South; J. Winn and J. Berta-Thompson for comments on the manuscript; S. Seager and A. Zsom for conversations that improved the work; L. Delrez for her independent analysis of the TRAPPIST data; and J. Eastman, D. Dragomir and R. Siverd for their efforts to observe additional transits. The MEarth Project acknowledges funding from the David and Lucile Packard Fellowship for Science and Engineering, and the National Science Foundation, and a grant from the John Templeton Foundation. The opinions expressed here are those of the authors and do not necessarily reflect the views of the John Templeton Foundation. The development of the PISCO imager was supported by the National Science Foundation. HARPS observations were made with European Southern Observatory (ESO) Telescopes at the La Silla Paranal Observatory. TRAPPIST is a project funded by the Belgian Fund for Scientific Research, with the participation of the Swiss National Science Foundation. Z.K.B.-T. is funded by the MIT Torres Fellowship for Exoplanet Research. X.B., X.D., T.F. and A.W. acknowledge the support of the French Agence Nationale de la Recherche and the European Research Council. M.G. and E.J. are FNRS Research Associates. V.N. acknowledges a CNPq/BJT Post-Doctorate fellowship and partial financial support from the INCT INEspaço. N.C.S. acknowledges the support from the Portuguese National Science Foundation (FCT) as well as the COMPETE program.
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Contributions
The MEarth team (D.C., J.I., Z.K.B.-T., E.R.N. and J.A.D.) discovered the planet, organized the follow-up observations, and led the analysis and interpretation. Z.K.B.-T. analysed the light curve and radial velocity data, and wrote the manuscript. J.I. designed, installed, maintains, and operates the MEarth–South telescope array, identified the first triggered transit event, and substantially contributed to the analysis and interpretation. D.C. leads the MEarth Project, and assisted in analysis and writing the manuscript. E.R.N. determined the metallicity, kinematics, and rotation period of the star. J.A.D. confirmed the star’s trigonometric parallax and helped install the MEarth–South telescopes. The HARPS team (N.A.-D., X.B., F.B., X.D., T.F., C.L., M.M., V.N., F.P., N.C.S., S.U. and A.W.) obtained spectra for Doppler velocimetry, with N.A.-D. and X.B. leading the analysis of those data. M.G. and E.J. gathered photometric observations with TRAPPIST. A.A.S. and B.S. gathered photometric observations with PISCO. All authors read and discussed the manuscript.
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Extended data figures and tables
Extended Data Figure 1 Astrometry of GJ 1132 from MEarth–South.
Measurements of the star GJ 1132’s position in MEarth–South images, along the directions of ecliptic latitude (top) and longitude (bottom). As described elsewhere31, a fitted offset between data gathered at a field rotation of 0° (blue) and 180° (green) has been removed. The published RECONS proper motion17 has been subtracted, and a model fixed to the published 83.07 mas parallax (black line) closely matches the MEarth–South observations.
Extended Data Figure 2 Near-infared spectrum of GJ 1132.
Observations of GJ 1132’s spectrum obtained with the FIRE spectrograph on the Magellan Baade telescope are compared in the z (top left), J (top right), H (bottom left) and K (bottom right) telluric windows to the solar-metallicity composite spectral type standards from ref. 35. The FIRE spectra have been smoothed to match the R = 2,000 resolution of the standards. GJ 1132’s near-infrared spectral type is M4V–M5V.
Extended Data Figure 3 Photometric starspot modulations of GJ 1132.
MEarth–South photometry (with dots representing single pointings and error bars representing ±1σ uncertainty ranges on weighted averages over four-day bins) probes starspots that are rotating in and out of view, and indicates that GJ 1132 has a rotation period of approximately 125 days. The rotational modulation was identified using a methodology similar to that used in previous MEarth work48.
Extended Data Figure 4 Raw transit light curves of GJ 1132b.
Light curves are shown both unbinned (grey points) and in five-minute bins (black bars, representing the ±1σ uncertainty range for the weighted average in each bin), and separated by telescope (row) and transit event (column). Model curves are shown, with the Gaussian process noise model conditioned on the observations, for parameters sampled from the posterior (green) and for the maximum likelihood parameters (blue). This is the complete set of light-curve data behind the transit parameter fits.
Supplementary information
Supplementary Data 1
This text file contains photometric observations of transits of GJ 1132b from the spring of 2015. Observations with different telescopes or on different nights are presented as separate light curves. (TXT 516 kb)
Supplementary Data 2
This text file contains radial velocity observations of GJ 1132, collected with HARPS in the spring and summer of 2015, which appear in Figure 2. (TXT 1 kb)
Supplementary Data 3
This text file contains the curated compilation of transiting exoplanets that appears in Figure 3. Data were downloaded from the NASA Exoplanet Archive on 25 August 2015, and minor modifications were made to correct egregious errors in the stellar/planetary parameters of some systems. (TXT 396 kb)
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Berta-Thompson, Z., Irwin, J., Charbonneau, D. et al. A rocky planet transiting a nearby low-mass star. Nature 527, 204–207 (2015). https://doi.org/10.1038/nature15762
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DOI: https://doi.org/10.1038/nature15762
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