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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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.
The authors declare no competing financial interests.
Extended data figures and tables
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.
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.
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.
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.
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)
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)
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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