Temperate Earth-sized planets transiting a nearby ultracool dwarf star



Star-like objects with effective temperatures of less than 2,700 kelvin are referred to as ‘ultracool dwarfs’1. This heterogeneous group includes stars of extremely low mass as well as brown dwarfs (substellar objects not massive enough to sustain hydrogen fusion), and represents about 15 per cent of the population of astronomical objects near the Sun2. Core-accretion theory predicts that, given the small masses of these ultracool dwarfs, and the small sizes of their protoplanetary disks3,4, there should be a large but hitherto undetected population of terrestrial planets orbiting them5—ranging from metal-rich Mercury-sized planets6 to more hospitable volatile-rich Earth-sized planets7. Here we report observations of three short-period Earth-sized planets transiting an ultracool dwarf star only 12 parsecs away. The inner two planets receive four times and two times the irradiation of Earth, respectively, placing them close to the inner edge of the habitable zone of the star8. Our data suggest that 11 orbits remain possible for the third planet, the most likely resulting in irradiation significantly less than that received by Earth. The infrared brightness of the host star, combined with its Jupiter-like size, offers the possibility of thoroughly characterizing the components of this nearby planetary system.

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Figure 1: Transit photometry of the TRAPPIST-1 planets.
Figure 2: Masses of host stars and equilibrium temperatures of known sub-Neptune-sized exoplanets.
Figure 3: Potential for characterizing the atmospheres of known transiting sub-Neptune-sized exoplanets.


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TRAPPIST is funded by the Belgian Fund for Scientific Research (FRS–FNRS) under grant FRFC 2.5.594.09.F, with the participation of the Swiss Fund for Scientific Research. The research leading to our results was funded in part by the European Research Council (ERC) under the FP/2007-2013 ERC grant 336480, and through an Action de Recherche Concertée (ARC) grant financed by the Wallonia-Brussels Federation. Our work was also supported in part by NASA under contract NNX15AI75G. UKIRT is supported by NASA and operated under an agreement among the University of Hawaii, the University of Arizona, and Lockheed Martin Advanced Technology Center; operations are enabled through the cooperation of the East Asian Observatory. The facilities at the Indian Astronomical Observatory (IAO) and the Consortium for Research Excellence, Support and Training (CREST) are operated by the Indian Institute of Astrophysics, Bangalore. M.G., E.J. and V.V.G. are FRS–FNRS research associates. L.D. and C.O. are FRS–FNRS PhD students. We thank V. Mégevand, the ASTELCO telescope team, S. Sohy, V. Chantry, and A. Fumel for their contributions to the TRAPPIST project; the Infrared Telescope Facility (IRTF) operators B. Cabreira and D. Griep for assistance with the SpeX observations; UKIRT staff scientists W. Varricatt & T. Kerr, telescope operators S. Benigni, E. Moore and T. Carroll, and Cambridge Astronomy Survey Unit (CASU) scientists G. Madsen and M. Irwin for assistance with UKIRT observations; the European Southern Observatory (ESO) astronomers A. Smette and G. Hau for providing us with the best possible VLT data; and the staff of IAO (in Hanle) and CREST (in Hosakote) for making observations with the HCT possible. Ad.B. and D.B.G. are visiting astronomers at the IRTF, which is operated by the University of Hawaii under Cooperative Agreement NNX-08AE38A with NASA’s Science Mission Directorate, Planetary Astronomy Program.

Author information




The TRAPPIST team (M.G., E.J., L.D., A.B., C.O. and P.M.) discovered the planets. M.G. leads the exoplanet program of TRAPPIST, set up and organized the ultracool-dwarf transit survey, planned and analysed part of the observations, led their scientific exploitation, and wrote most of the manuscript. E.J. manages the maintenance and operations of the TRAPPIST telescope. S.M.L. obtained the director’s discretionary time on UKIRT, and managed, with E.J., the preparation of the UKIRT observations. L.D. and C.O. scheduled and carried out some of the TRAPPIST observations. L.D. and A.B. analysed some photometric observations. J.d.W. led the study of the amenability of the planets for detailed atmospheric characterization. V.V.G. checked the physical parameters of the star. A.J.B. checked the spectral type of the star and determined its metallicity. B.-O.D. took charge of the dynamical simulations. D.B.G. acquired the SpeX spectra. D.K.S. gathered the HCT observations. S.M.L., A.H.M.J.T., P.M. and D.Q. helped to write the manuscript. A.H.M.J.T. prepared most of the figures.

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Correspondence to Michaël Gillon.

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Extended data figures and tables

Extended Data Figure 1 Raw TRAPPIST-1 transit light curves.

The light curves are shown in chronological order from top to bottom and left to right, with unbinned data shown as cyan dots, and binned 0.005-day (7.2-minute) intervals shown as black dots with error bars. The error bars are the standard errors of the mean of the measurements in the bins. The best-fit transit-plus-baseline models are overplotted (red line). The light curves are phased for the mid-transit time and shifted along the y axis for clarity. For the dual transit of 11 December 2015, the light curve is phased for the mid-transit time of planet TRAPPIST-1c. T1b, TRAPPIST-1b; T1c, TRAPPIST-c; T1d, TRAPPIST-1d. Source data

Extended Data Figure 2 De-trended TRAPPIST-1 transit light curves.

The details are as in Extended Data Fig. 1, except that the light curves shown here are divided by the best-fit baseline model to highlight the transit signatures. Source data

Extended Data Figure 3 Near-infrared spectra of TRAPPIST-1.

a, Comparison of TRAPPIST-1’s near-infrared spectrum (black)—obtained with the spectrograph IRTF/SpeX35—with that of the M8-type standard LHS132 (red). b, Cross-dispersed IRTF/SpeX spectrum of TRAPPIST-1 in the 2.17–2.35-μm region. Na i, Ca i and CO features are labelled. Additional structure primarily originates from overlapping H2O bands. The spectrum is normalized at 2.2 μm. Fλ, spectral flux density; fλ, normalized spectra flux density.

Extended Data Figure 4 Flare events in the TRAPPIST 2015 photometry.

The photometric measurements are shown unbinned (cyan dots) and binned per 7.2-minute interval (black dots). For each interval, the error bars are the standard error of the mean. Source data

Extended Data Figure 5 Photometric variability of TRAPPIST-1.

a, Global light curve of the star as measured by TRAPPIST. The photometric measurements are shown unbinned (cyan dots) and binned per night (black dots with error bars (±s.e.m.)). This light curve is compared with that of the comparison star 2MASS J23063445 − 0507511, shifted along the y axis for clarity. b, The same light curve for TRAPPIST-1, folded on the period P = 1.40 days and binned by 10-minute intervals (error bars indicate ±s.e.m.). For clarity, two consecutive periods are shown. Source data

Extended Data Table 1 TRAPPIST-1 transit light curves
Extended Data Table 2 Quadratic limb-darkening coefficients
Extended Data Table 3 Posterior likelihoods of the orbital solutions for TRAPPIST-1d
Extended Data Table 4 Individual mid-transit timings measured for the TRAPPIST-1 planets

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Gillon, M., Jehin, E., Lederer, S. et al. Temperate Earth-sized planets transiting a nearby ultracool dwarf star. Nature 533, 221–224 (2016). https://doi.org/10.1038/nature17448

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