The TRAPPIST-1 system is the first transiting planet system found orbiting an ultracool dwarf star 1 . At least seven planets similar in radius to Earth were previously found to transit this host star 2 . Subsequently, TRAPPIST-1 was observed as part of the K2 mission and, with these new data, we report the measurement of an 18.77 day orbital period for the outermost transiting planet, TRAPPIST-1 h, which was previously unconstrained. This value matches our theoretical expectations based on Laplace relations 3 and places TRAPPIST-1 h as the seventh member of a complex chain, with three-body resonances linking every member. We find that TRAPPIST-1 h has a radius of 0.752 R and an equilibrium temperature of 173 K. We have also measured the rotational period of the star to be 3.3 days and detected a number of flares consistent with a low-activity, middle-aged, late M dwarf.

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This paper includes data collected by the K2 mission. Funding for the K2 mission is provided by the NASA (National Space and Aeronautical Administration) Science Mission directorate. This research has made use of NASA’s Astrophysics Data System, the SIMBAD database and VizieR catalogue access tool operated at Centre de Données astronomiques de Strasbourg, Strasbourg, France. The data presented in this paper were obtained from the Mikulski Archive for Space Telescopes. R.L. and E.A. acknowledge support from NASA grant NNX14AK26G and from the NASA Astrobiology Institute’s Virtual Planetary Laboratory Lead Team, funded through the NASA Astrobiology Institute under solicitation NNH12ZDA002C and Cooperative Agreement Number NNA13AA93A. E.A. acknowledges support from NASA grant NNX13AF62G and National Science Foundation grant AST-1615315. E.K. acknowledges support from a National Science Foundation Graduate Student Research Fellowship. B.-O.D. acknowledges support from the Swiss National Science Foundation in the form of a Swiss National Science Foundation Professorship (PP00P2-163967). E.B. acknowledges funding by the European Research Council through ERC grant SPIRE 647383. D.L.H. acknowledges financial support from the UK Science and Technology Facilities Council. M.S. and K.H. acknowledge support from the Swiss National Science Foundation. A.B. acknowledges funding support from the National Science Foundation under award no. AST-1517177 and NASA under grant no. NNX15AI75G. J.L. acknowledges funding from the European Research Council under the European Unions Horizon 2020 research and innovation programme (grant agreement no. 679030/WHIPLASH). M.G., E.J. and V.V.G. are Fonds National de la Recherche Scientifique (F.R.S.-FNRS) Research Associates. S.N.R. thanks the Agence Nationale pour la Recherche for support via grant ANR-13-BS05-0003-002 (grant MOJO). The research leading to these results has received funding from the European Research Council under the FP/2007-2013 European Research Council Grant Agreement no. 336480 and from the Actions de Recherche Concertée (ARC) grant for Concerted Research Actions, financed by the Wallonia–Brussels Federation. S.B.H. wrote science cases to the K2 project office to include TRAPPIST-1 in the campaign 12 field and to make the raw data public upon downlink. D.F.M. is a Sagan Fellow.

Author information


  1. Astronomy Department, University of Washington, Seattle, Washington 98195, USA.

    • Rodrigo Luger
    • , Ethan Kruse
    • , Eric Agol
    • , Daniel Foreman-Mackey
    •  & Brett M. Morris
  2. NASA Astrobiology Institute’s Virtual Planetary Laboratory, Seattle, Washington 98195, USA.

    • Rodrigo Luger
    •  & Eric Agol
  3. University of Bern, Center for Space and Habitability, Sidlerstrasse 5, CH-3012 Bern, Switzerland.

    • Marko Sestovic
    • , Simon L. Grimm
    • , Brice-Olivier Demory
    •  & Kevin Heng
  4. Laboratoire AIM Paris-Saclay, CEA/DRF–CNRS–Université Paris Diderot, IRFU/SAp Centre de Saclay, 91191 Gif-sur-Yvette, France.

    • Emeline Bolmont
  5. Department of Astronomy and Astrophysics, University of Chicago, 5640 South Ellis Avenue, Chicago, Illinois 60637, USA.

    • Daniel Fabrycky
  6. Space Sciences, Technologies and Astrophysics Research Institute, Université de Liège, Allée du 6 Août 19C, B-4000 Liège, Belgium.

    • Catarina S. Fernandes
    • , Valérie Van Grootel
    • , Michaël Gillon
    • , Emmanuël Jehin
    • , Laetitia Delrez
    •  & Pierre Magain
  7. Center for Astrophysics and Space Science, University of California San Diego, La Jolla, California 92093, USA.

    • Adam Burgasser
  8. IPAC, Mail Code 314-6, California Institute of Technology, 1200 East California Boulevard, Pasadena, California 91125, USA.

    • James G. Ingalls
  9. Laboratoire d’Astrophysique de Bordeaux, Université Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France.

    • Sean N. Raymond
    • , Franck Selsis
    •  & Jérémy Leconte
  10. Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK.

    • Amaury H. M. J. Triaud
  11. NASA Ames Research Center, Moffett Field, California 94043, USA.

    • Thomas Barclay
    • , Geert Barentsen
    •  & Steve B. Howell
  12. Bay Area Environmental Research Institute, 25 2nd Street Ste 209 Petaluma, California 94952, USA.

    • Thomas Barclay
    •  & Geert Barentsen
  13. NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA.

    • Thomas Barclay
  14. Cavendish Laboratory, J.J. Thomson Avenue, Cambridge CB3 0HE, UK.

    • Laetitia Delrez
    •  & Didier Queloz
  15. Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA.

    • Julien de Wit
  16. Jeremiah Horrocks Institute, University of Central Lancashire, Preston PR1 2HE, UK.

    • Daniel L. Holdsworth
  17. NASA Johnson Space Center, 2101 NASA Parkway, Houston, Texas 77058, USA.

    • Susan Lederer
  18. Laboratoire de Météorologie Dynamique, Sorbonne Universités, UPMC Université Paris 06, CNRS, 4 place Jussieu, 75005 Paris, France.

    • Martin Turbet
  19. Space and Astronomy Department, Faculty of Science, King Abdulaziz University, 21589 Jeddah, Saudi Arabia.

    • Yaseen Almleaky
  20. King Abdullah Centre for Crescent Observations and Astronomy, Makkah Clock, Mecca 24231, Saudia Arabia.

    • Yaseen Almleaky
  21. LPHEA Laboratory, Oukaïmeden Observatory, Cadi Ayyad University FSSM, BP 2390 Marrakesh, Morocco.

    • Zouhair Benkhaldoun
  22. Observatoire de Genève, Université de Genève, 51 chemin des Maillettes, CH-1290 Sauverny, Switzerland.

    • Didier Queloz


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R.L. and M.S. led the detrending efforts with EVEREST and the Gaissian process-based pipeline, with inputs from E.A., J.G.I, E.K. and D.F.M. R.L. performed the preliminary manual search for transits of TRAPPIST-1 h and the Δχ2 search, with input from E.A, E.K. and D.F.M. S.L.G. took care of the K2 data handling. B.-O.D. led the collaboration, wrote the K2 proposal and performed an independent transit search and Markov Chain Monte Carlo analysis of the K2 dataset. E.A. and D.F. led the dynamics and architecture of the system with inputs from S.N.R. and B.-O.D. E.B. took care of the tidal simulations. C.S.F, V.V.G., A.B., D.L.H. and B.M.M. conducted the work on stellar properties and variability and determined the stellar rotation period. S.N.R. led the formation and migration section. F.S., J.L. and M.T. worked on the atmospheric nature of TRAPPIST-1 h. G.B. and T.B. helped with the handling of the uncalibrated K2 fits files. Figures were prepared by R.L., A.H.M.J.T., J.G.I., E.B and E.K. M.G., E.J., A.H.M.J.T., L.D., J.d.W, S.L., Y.A., Z.B., P.M., K.H. and D.Q. contributed to the discovery and characterization of the TRAPPIST-1 system. All authors participated in the writing and commented on the paper.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Rodrigo Luger.

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