Temperate Earth-sized planets transiting a nearby ultracool dwarf star

Subjects

Abstract

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.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.

from$8.99

All prices are NET prices.

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.

References

  1. 1

    Kirkpatrick, J. D., Henry, T. J. & Simon, D. A. The solar neighborhood. II. The first list of dwarfs with spectral types of M7 and cooler. Astron. J. 109, 797–807 (1995)

  2. 2

    Cantrell, J. R., Henry, T. J. & White, R. J. The solar neighborhood XXIX: the habitable real estate of our nearest stellar neighbours. Astron. J. 146, 99 (2013)

  3. 3

    Andrews, S. M., Wilner, D. J., Hugues, A. M., Qi, C. & Dullemond, C. P. Protoplanetary disk structures in Opiuchus. II. Extension to fainter sources. Astrophys. J. 723, 1241–1254 (2010)

  4. 4

    Liu, Y., Joergens, V., Bayo, A., Nielbock, M. & Wang, H. A homogeneous analysis of disk around brown dwarfs. Astron. Astrophys. 582, A22 (2015)

  5. 5

    Payne, M. J. & Lodato, G. The potential for Earth-mass planet formation around brown dwarfs. Mon. Not. R. Astron. Soc. 381, 1597–1606 (2007)

  6. 6

    Raymond, S. N., Scalo, J. & Meadows, V. S. A decreased probability of habitable planet formation around low-mass stars. Astrophys. J. 669, 606–614 (2007)

  7. 7

    Montgomery, R. & Laughlin, G. Formation and detection of Earth-mass planets around low mass stars. Icarus 202, 1–11 (2009)

  8. 8

    Kopparapu, R. K. et al. Habitable zones around main-sequence stars: new estimates. Astrophys. J. 765, 131 (2013)

  9. 9

    Gillon, M. et al. TRAPPIST: a robotic telescope dedicated to the study of planetary systems. EPJ Web Conf. 11, 06002 (2011)

  10. 10

    Gillon, M., Jehin, E., Fumel, A., Magain, P. & Queloz, D. TRAPPIST-UCDTS: a prototype search for habitable planets transiting ultra-cool stars. EPJ Web Conf. 47, 03001 (2013)

  11. 11

    Liebert, J. & Gizis, J. E. RI photometry of 2MASS-selected late M and L dwarfs. Publ. Astron. Soc. Pacif. 118, 659–670 (2006)

  12. 12

    Costa, E. et al. The solar neighborhood. XVI. Parallaxes from CTIOPI: final results from the 1.5m telescope program. Astron. J. 132, 1234–1247 (2006)

  13. 13

    Filippazzo, J. C. et al. Fundamental parameters and spectral energy distributions of young and field age objects with masses spanning the stellar to planetary regime. Astrophys. J. 810, 158 (2015)

  14. 14

    Gillon, M. et al. The TRAPPIST survey of southern transiting planets. I. Thirty eclipses of the ultra-short period planet WASP-43 b. Astron. Astrophys. 542, A4 (2012)

  15. 15

    Reiners, A. & Basri, G. A volume-limited sample of 63 M7–M9.5 dwarfs. II. Activity, magnetism, and the fade of the rotation-dominated dynamo. Astrophys. J. 710, 924–935 (2010)

  16. 16

    Hosey, A. D. et al. The solar neighbourhood. XXXVI. The long-term photometric variability of nearby red dwarfs in the VRI optical bands. Astron. J. 150, 6 (2015)

  17. 17

    Yu, L. et al. Tests of the planetary hypothesis for PTFO8–8695b. Astrophys. J. 812, 48 (2015)

  18. 18

    Stelzer, B., Marino, A., Micela, G., López-Santiago, J. & Liefke, C. The UV and X-ray activity of the M dwarfs within 10 pc of the Sun. Mon. Not. R. Astron. Soc. 431, 2063–2079 (2013)

  19. 19

    Lopez, E. D., Fortney, J. J. & Miller, N. How thermal evolution and mass-loss sculpt populations of super-Earths and sub-Neptunes: application to the Kepler-11 system and beyond. Astrophys. J. 761, 59 (2012)

  20. 20

    Rogers, L. A. Most 1.6 Earth-radius planets are not rocky. Astrophys. J. 801, 41 (2015)

  21. 21

    Wolfgang, A. & Lopez, E. How rocky are they? The composition distribution of Kepler’s sub-Neptune planet candidates within 0.15 AU. Astrophys. J. 806, 183 (2015)

  22. 22

    Seager, S., Kuchner, M., Hier-Majumder, C. A. & Militzer, B. Mass-radius relationships for solid exoplanets. Astrophys. J. 669, 1279–1297 (2007)

  23. 23

    Holman, M. J. & Murray, N. W. The use of transit timing to detect terrestrial-mass extrasolar planets. Science 307, 1288–1291 (2005)

  24. 24

    de Wit, J. & Seager, S. Constraining exoplanet mass from transmission spectroscopy. Science 342, 1473–1477 (2013)

  25. 25

    Kasting, J. F., Whitmire, D. P. & Reynolds, R. T. Habitable zones around main-sequence stars. Icarus 101, 108–128 (1993)

  26. 26

    Leconte, J. et al. 3D climate modelling of close-in land planets: circulation patterns, climate moist instability, and habitability. Astron. Astrophys. 554, A69 (2013)

  27. 27

    Menou, K. Water-trapped world. Astrophys. J. 774, 51 (2013)

  28. 28

    Driscoll, P. E. & Barnes, R. Tidal heating of Earth-like exoplanets around M stars: thermal, magnetic, and orbital evolutions. Astrobiology 15, 739–760 (2015)

  29. 29

    France, K. et al. The ultraviolet radiation environment around M dwarf exoplanet host stars. Astrophys. J. 763, 149 (2013)

  30. 30

    Tian, F. & Ida, S. Water contents of Earth-mass planets around M-dwarfs. Nature Geosci. 8, 177–180 (2015)

  31. 31

    Gizis, J. E. et al. New neighbours from 2MASS: activity and kinematics at the bottom of the main sequence. Astron. J. 120, 1085–1099 (2000)

  32. 32

    Bartlett, J. L. Knowing our neighbours: fundamental properties of nearby stars. Publ. Astron. Soc. Pacif. 119, 828–829 (2007)

  33. 33

    Schmidt, S. J., Cruz, K. L., Bongiorno, B. J., Liebert, J. & Reid, I. N. Activity and kinematics of ultracool dwarfs, including an amazing flare observation. Astron. J. 133, 2258–2273 (2007)

  34. 34

    Lee, K.-G., Berger, E. & Knapp, G. R. Short-term Hα variability in M dwarfs. Astrophys. J. 708, 1482–1491 (2010)

  35. 35

    Rayner, J. T. et al. SpeX: a medium-resolution 0.8–5.5 micron spectrograph and imager for the NASA infrared telescope facility. Publ. Astron. Soc. Pacif. 115, 362–382 (2003)

  36. 36

    Reiners, A. & Basri, G. A volume-limited sample of 63 M7-M9.5 dwarfs. I. Space motion, kinematics age, and lithium. Astrophys. J. 705, 1416–1424 (2009)

  37. 37

    Vacca, W. D., Cushing, M. C. & Rayner, J. T. A method of correcting near-infrared spectra for telluric absorption. Publ. Astron. Soc. Pacif. 115, 389–409 (2003)

  38. 38

    Cushing, M. C., Vacca, W. D. & Rayner, J. T. Spextool: a spectral extraction package for SpeX, a 0.8–5.5 micron cross-dispersed spectrograph. Publ. Astron. Soc. Pacif. 116, 362–376 (2004)

  39. 39

    Rojas-Ayala, B., Covey, K. R., Muirhead, P. S. & Lloyd, J. P. Metallicity and temperature indicators in M dwarf K-band spectra: testing new and updated calibrations with observations of 133 solar neighbourhood M dwarfs. Astrophys. J. 748, 93 (2012)

  40. 40

    Mann, A. W. et al. Prospecting in ultracool dwarfs: measuring the metallicities of mid- and late-M dwarfs. Astron. J. 147, 160 (2014)

  41. 41

    Skrutskie, M. F., Meyer, M. R., Whalen, D. & Hamilton, C. The two micron all sky survey (2MASS). Astron. J. 131, 1163–1183 (2006)

  42. 42

    Cutri, R. M. et al. Vizier online data catalog II/311: WISE all-sky data release. http://vizier.cfa.harvard.edu/viz-bin/VizieR?-source=II/311 (2012)

  43. 43

    Cruz, K. L. et al. Meeting the cool neighbours. IX. The luminosity function of M7–L8 ultracool dwarfs in the field. Astron. J. 133, 439–467 (2007)

  44. 44

    Baraffe, I., Homeier, D., Allard, F. & Chabrier, G. New evolutionary models for pre-main sequence and main sequence low-mass stars down to the hydrogen-burning limit. Astron. Astrophys. 577, A42 (2015)

  45. 45

    Siegler, N., Close, L. M., Mamajeck, E. E. & Freed, M. An adaptive optics survey of M6.0–M7.5 stars: discovery of three very low mass binary system including two probable Hyades member. Astrophys. J. 598, 1265–1276 (2003)

  46. 46

    Siegler, N., Close, L. M., Cruz, K. L., Martín, E. L. & Reid, I. N. Discovery of two very low mass binaries: final results of an adaptive optics survey of nearby M6.0–M7.5 stars. Astrophys. J. 621, 1023–1032 (2005)

  47. 47

    Janson, M. et al. The AstraLux large M-dwarf multiplicity survey. Astrophys. J. 754, 44 (2012)

  48. 48

    Bouy, H. et al. Multiplicity of nearby free-floating ultracool dwarfs: a Hubble Space Telescope WFPC2 search for companions. Astron. J. 126, 1526–1554 (2003)

  49. 49

    Barnes, J. R. et al. Precision radial velocities of 15 M5–M9 dwarfs. Mon. Not. R. Astron. Soc. 439, 3094–3113 (2014)

  50. 50

    Tanner, A. et al. Keck NIRSPEC radial velocity observations of late M-dwarfs. Astrophys. J. 203 (Suppl.), 10 (2012)

  51. 51

    Burgasser, A. J. et al. WISE J072003.20-084651.2: an old and active M9.5 + T5 spectral binary 6 pc from the Sun. Astron. J. 149, 104 (2015)

  52. 52

    Zacharias, N. et al. The second US Naval Observatory CCD astrograph catalog (UCAC2). Astron. J. 127, 3043–3059 (2004)

  53. 53

    Izmailov, I. S. et al. Astrometric CCD observations of visual double stars at the Pulkovo Observatory. Astron. Lett. 36, 349–354 (2010)

  54. 54

    Minkowski, R. L. & Abell, G. O. in Basic Astronomical Data: Stars and Stellar Systems (ed. Strand, K. A. ) 481–487 (Univ. Chicago Press, 1963)

  55. 55

    Jehin, E. et al. TRAPPIST: TRAnsiting Planets and PlanetesImals Small Telescope. The Messenger 145, 2–6 (2011)

  56. 56

    Stetson, P. B. DAOPHOT—a computer program for crowded-field stellar photometry. Publ. Astron. Soc. Pacif. 99, 191–222 (1987)

  57. 57

    Pirard, J.-F. et al. HAWK-I: a new wide-field 1- to 2.5 μm imager for the VLT. Proc. SPIE 5492, 1763–1772 (2004)

  58. 58

    Casali, M. et al. in The New Era of Wide-Field Astronomy (eds Clowes, R., Adamson, A. & Bromage, G. ) 357–363 (ASPC Conf. Series, vol. 232, 2001)

  59. 59

    Eastman, J., Siverd, R. & Gaudi, B. S. Achieving better than 1 minute accuracy in the heliocentric and barycentric Julian dates. Publ. Astron. Soc. Pacif. 122, 935–946 (2010)

  60. 60

    Mandel, K. & Agol, E. Analytic light curves for planetary transit searches. Astrophys. J. 580, L171–L175 (2002)

  61. 61

    Schwarz, G. Estimating the dimension of a model. Ann. Stat. 6, 461–464 (1978)

  62. 62

    Claret, A. & Bloemen, S. Gravity and limb-darkening coefficients for the Kepler, CoRoT, Spitzer, uvby, UBVRIJHK, and Sloan photometric systems. Astron. Astrophys. 529, A75 (2011)

  63. 63

    Gelman, A. & Rubin., D. B. Inference from iterative simulation using multiple sequences. Stat. Sci. 7, 457–472 (1992)

  64. 64

    Seager, S. & Mallén-Ornelas, G. A unique solution of planet and star parameters from an extrasolar planet transit light curve. Astrophys. J. 585, 1038–1055 (2003)

  65. 65

    Agol, E., Steffen, J., Sari, R. & Clarkson, W. On detecting terrestrial planets with timing of giant planet transits. Mon. Not. R. Astron. Soc. 359, 567–579 (2005)

  66. 66

    Davenport, J. R. A. et al. Kepler flares II: the temporal morphology of white-light flares on GJ 1243. Astrophys. J. 797, 122 (2014)

  67. 67

    Scargle, J. D. Studies in astronomical time series analysis. II. Statistical aspects of spectral analysis of unevenly spaced data. Astrophys. J. 263, 835–853 (1982)

  68. 68

    Goldreich, P. & Soter, S. Q in the solar system. Icarus 5, 375–389 (1966)

  69. 69

    Murray, C. D. & Dermott, S. F. Solar System Dynamics (Cambridge Univ. Press, 2001)

  70. 70

    Limbach, M. A. & Turner, E. L. The orbital eccentricity—multiplicity relation and the solar system. Proc. Natl Acad. Sci. USA 112, 20–24 (2015)

  71. 71

    Chambers, J. E. A hybrid symplectic integrator that permits close encounters between massive bodies. Mon. Not. R. Astron. Soc. 304, 793–799 (1999)

  72. 72

    Charbonneau, D. et al. A super-Earth transiting a nearby low-mass star. Nature 462, 891–894 (2009)

  73. 73

    Miller-Ricci, E., Seager, S. & Sasselov, D. The atmospheric signatures of super-Earths: how to distinguish between hydrogen-rich and hydrogen-poor atmospheres. Astrophys. J. 690, 1056–1067 (2009)

  74. 74

    Han, E. et al. The exoplanet orbit database. II. Updates to exoplanet.org. Publ. Astron. Soc. Pacif. 126, 827–837 (2014)

  75. 75

    Kreidberg, L. et al. Clouds in the atmosphere of the super-Earth exoplanet GJ 1214b. Nature 505, 69–72 (2014)

  76. 76

    Berta, Z. K. et al. The flat transmission spectrum of the super-Earth GJ 1214b from Wide Field Camera 3 on the Hubble Space Telescope. Astrophys. J. 747, 35 (2012)

  77. 77

    Snellen, I. A. G., de Kock, R. J., de Mooij, E. J. W. & Albrecht, S. The orbital motion, absolute mass and high-altitude winds of exoplanet HD 209458b. Nature 465, 1049–1051 (2010)

  78. 78

    Rodler, F. & López-Morales, M. Feasibility studies for the detection of O2 in an Earth-like exoplanet. Astrophys. J. 781, 54 (2014)

Download references

Acknowledgements

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

Affiliations

Authors

Contributions

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.

Corresponding author

Correspondence to Michaël Gillon.

Ethics declarations

Competing interests

The authors declare no competing financial interests.

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

PowerPoint slides

Source data

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

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

Download citation

Further reading

Comments

By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.