M dwarf stars, which have masses less than 60 per cent that of the Sun, make up 75 per cent of the population of the stars in the Galaxy1. The atmospheres of orbiting Earth-sized planets are observationally accessible via transmission spectroscopy when the planets pass in front of these stars2,3. Statistical results suggest that the nearest transiting Earth-sized planet in the liquid-water, habitable zone of an M dwarf star is probably around 10.5 parsecs away4. A temperate planet has been discovered orbiting Proxima Centauri, the closest M dwarf5, but it probably does not transit and its true mass is unknown. Seven Earth-sized planets transit the very low-mass star TRAPPIST-1, which is 12 parsecs away6,7, but their masses and, particularly, their densities are poorly constrained. Here we report observations of LHS 1140b, a planet with a radius of 1.4 Earth radii transiting a small, cool star (LHS 1140) 12 parsecs away. We measure the mass of the planet to be 6.6 times that of Earth, consistent with a rocky bulk composition. LHS 1140b receives an insolation of 0.46 times that of Earth, placing it within the liquid-water, habitable zone8. With 90 per cent confidence, we place an upper limit on the orbital eccentricity of 0.29. The circular orbit is unlikely to be the result of tides and therefore was probably present at formation. Given its large surface gravity and cool insolation, the planet may have retained its atmosphere despite the greater luminosity (compared to the present-day) of its host star in its youth9,10. Because LHS 1140 is nearby, telescopes currently under construction might be able to search for specific atmospheric gases in the future2,3.
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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. The MEarth team acknowledges funding from the David and Lucille Packard Fellowship for Science and Engineering (awarded to D.C.). This material is based on work supported by the National Science Foundation under grants AST-0807690, AST-1109468, AST-1004488 (Alan T. Waterman Award) and AST-1616624. This publication was made possible through the support of a grant from the John Templeton Foundation and NASA XRP Program #NNX15AC90G. The opinions expressed in this publication are those of the authors and do not necessarily reflect the views of the John Templeton Foundation. HARPS observations were made with European Southern Observatory (ESO) telescopes under observing programs 191.C-0873 and 198.C-0838. This work was performed in part under contract with the Jet Propulsion Laboratory (JPL) funded by NASA through the Sagan Fellowship Program executed by the NASA Exoplanet Science Institute. E.R.N. is supported by an NSF Astronomy and Astrophysics Postdoctoral Fellowship under award AST-1602597. N.C.S. acknowledges support from Fundação para a Ciência e a Tecnologia (FCT) through national funds and by FEDER through COMPETE2020 by grants UID/FIS/04434/2013&POCI-01-0145-FEDER-007672 and PTDC/FIS-AST/1526/2014&POCI-01-0145-FEDER-016886. N.C.S. was also supported by FCT through Investigador FCT contract reference IF/00169/2012/CP0150/CT0002. X.B., X.D. and T.F. acknowledge the support of the INSU/PNP (Programme national de planétologie) and INSU/PNPS (Programme national de physique stellaire). X.B., J.-M.A. and A.W. acknowledge funding from the European Research Council under ERC Grant Agreement no. 337591-ExTrA. We thank A. Vanderburg for backseat MCMCing. This publication makes use of data products from the Two Micron All Sky Survey (2MASS), which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by NASA and the National Science Foundation. This publication makes use of data products from the Wide-field Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and the JPL/California Institute of Technology, funded by NASA. This research has made extensive use of the NASA Astrophysics Data System (ADS), and the SIMBAD database, operated at CDS, Strasbourg, France.
The authors declare no competing financial interests.
Reviewer Information Nature thanks A. Hatzes and the other anonymous reviewer(s) for their contribution to the peer review of this work.
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Extended data figures and tables
The top three light curves are MEarth-South trigger observations whereas the bottom three are targeted transit observations. Light curves are offset and shown in alternating colours for clarity. We have corrected for the effects of air mass for all light curves and have fitted a linear trend in time to both full-transit observations (the bottom two light curves).
a, Periodogram of our window function for our radial-velocity observations. We see large peaks at approximately 18 days and its harmonics. We also see large peaks at around 40 days and at the stellar rotational period of 131 days (red dashed line). The 24.73712-day orbital period of LHS 1140b is indicated by the black dashed line. b, Periodogram for our HARPS radial-velocity observations. We see substantial power near the 24.73712-day orbital period of LHS 1140b (black dashed line), as well as broad power at longer periods associated with stellar rotation (131 days, red dashed line) and at shorter periods associated with the window function of our observations.
Extended Data Figure 3 Radial-velocity (RV) observations of LHS 1140 phased to our best-fitting sinusoid for the stellar activity signal.
The signal from LHS 1140b has not been removed. Grey data points are copies of the purple data points and the red line represents our best-fitting model. We find a radial-velocity amplitude of 4.26 ± 0.60 m s−1 due to stellar activity coupled with rotation, comparable to the radial-velocity amplitude of LHS 1140b. RV = 0 corresponds to the radial velocity of the host star.
a, Periodogram of our residual radial velocities after subtracting our best-fitting sinusoid for the stellar activity signal. The highest peak in this dataset is located at the orbital period of LHS 1140b (black dashed line) and the broad power located at long periods has been suppressed. b, Periodogram of our residual radial velocities after subtracting our best-fitting model that includes stellar radial-velocity variation as well as the orbit of LHS 1140b. We see no additional substantial peaks in our radial velocities, with the small peak located near P = 18 days due to the window function of our observations.
Extended Data Figure 5 HARPS radial-velocity (ΔRV) measurements fitted with a Gaussian process model.
Our radial-velocity observations (points; error bars are 1σ) and best-fitting Gaussian-process-based model (line with shaded 1σ error regions). The high cadence and adequate observational strategy allows us to identify the orbital signature of the planet by eye, and the Gaussian-process-based model naturally incorporates the uncertainty that arises from the activity-driven signals of the host star in our final determination of planetary mass. The residuals obtained after subtracting the model from the data are shown for the bottom two panels. ΔRV = 0 corresponds to the radial velocity of the host star.
Extended Data Figure 6 Marginalized posterior distributions of the radial-velocity model parameters.
The solid lines over-plotted on the histograms are kernel density estimations of the marginal distributions. These smooth, Gaussian-shaped posterior distributions indicate the good convergence of the MCMC chain. Here we show the parameters of the Gaussian process (A, E, R and S), the orbital period of LHS 1140b (Pb), its radial velocity (relative to the host star; RV0), radial-velocity semi-amplitude (Kb) and time of mid-transit (t0,b), the parameters and , where eb is the eccentricity of LHS 1140b and ωb is its argument of periastron, and a white-noise error term (σs; in units of metres per second) that is added in quadrature to each HARPS radial-velocity data point. The top left is histogram is for A (the amplitude of the Gaussian process).
IRTF/SPeX spectrum of LHS 1140 (black), with M4V (blue) and M5V (red) spectra over-plotted. We classify LHS 1140 as an M4.5V star on the basis on its near-infrared spectrum.
The plot shows MEarth-South photometry of LHS 1140. We find that LHS 1140 has photometric modulation due to stellar rotation and the asymmetric distribution of starspots, with a period of 131 days. Data from the two telescopes monitoring LHS 1140 are coloured in red and blue, and a sinusoid with a 131-day period is over-plotted in black. We find that the amplitude of variation increased between 2014 and 2016. Error bars, 1σ.
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Dittmann, J., Irwin, J., Charbonneau, D. et al. A temperate rocky super-Earth transiting a nearby cool star. Nature 544, 333–336 (2017). https://doi.org/10.1038/nature22055
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