Letter

Nature 439, 437-440 (26 January 2006) | doi:10.1038/nature04441; Received 28 September 2005; Accepted 14 November 2005

Discovery of a cool planet of 5.5 Earth masses through gravitational microlensing

J.-P. Beaulieu1,4, D. P. Bennett1,3,5, P. Fouqué1,6, A. Williams1,7, M. Dominik1,8, U. G. Jørgensen1,9, D. Kubas1,10, A. Cassan1,4, C. Coutures1,11, J. Greenhill1,12, K. Hill1,12, J. Menzies1,13, P. D. Sackett1,14, M. Albrow1,15, S. Brillant1,10, J. A. R. Caldwell1,16, J. J. Calitz1,17, K. H. Cook1,18, E. Corrales1,4, M. Desort1,4, S. Dieters1,12, D. Dominis1,19, J. Donatowicz1,20, M. Hoffman1,19, S. Kane1,21, J.-B. Marquette1,4, R. Martin1,7, P. Meintjes1,17, K. Pollard1,15, K. Sahu1,22, C. Vinter1,9, J. Wambsganss1,23, K. Woller1,9, K. Horne1,8, I. Steele1,24, D. M. Bramich1,8,24, M. Burgdorf1,24, C. Snodgrass1,25, M. Bode1,24, A. Udalski2,26, M. K. Szyman acuteski2,26, M. Kubiak2,26, T. Wie cedilckowski2,26, G. Pietrzyn acuteski2,26,27, I. Soszyn acuteski2,26,27, O. Szewczyk2,26, L stroke. Wyrzykowski2,26,28, B. Paczyn acuteski2,29, F. Abe3,30, I. A. Bond3,31, T. R. Britton3,15,32, A. C. Gilmore3,15, J. B. Hearnshaw3,15, Y. Itow3,30, K. Kamiya3,30, P. M. Kilmartin3,15, A. V. Korpela3,33, K. Masuda3,30, Y. Matsubara3,30, M. Motomura3,30, Y. Muraki3,30, S. Nakamura3,30, C. Okada3,30, K. Ohnishi3,34, N. J. Rattenbury3,28, T. Sako3,30, S. Sato3,35, M. Sasaki3,30, T. Sekiguchi3,30, D. J. Sullivan3,33, P. J. Tristram3,32, P. C. M. Yock3,32 & T. Yoshioka3,30

  1. PLANET/RoboNet Collaboration (http://planet.iap.fr and http://www.astro.livjm.ac.uk/RoboNet/)
  2. OGLE Collaboration (http://ogle.astrouw.edu.pl)
  3. MOA Collaboration (http://www.physics.auckland.ac.nz/moa)
  4. Institut d'Astrophysique de Paris, CNRS, Université Pierre et Marie Curie UMR7095, 98bis Boulevard Arago, 75014 Paris, France
  5. University of Notre Dame, Department of Physics, Notre Dame, Indiana 46556-5670, USA
  6. Observatoire Midi-Pyrénées, Laboratoire d'Astrophysique, UMR 5572, Université Paul Sabatier—Toulouse 3, 14 avenue Edouard Belin, 31400 Toulouse, France
  7. Perth Observatory, Walnut Road, Bickley, Perth, WA 6076, Australia
  8. Scottish Universities Physics Alliance, University of St Andrews, School of Physics and Astronomy, North Haugh, St Andrews KY16 9SS, UK
  9. Niels Bohr Institutet, Astronomisk Observatorium, Juliane Maries Vej 30, 2100 København Ø, Denmark
  10. European Southern Observatory, Casilla 19001, Santiago 19, Chile
  11. CEA DAPNIA/SPP Saclay, 91191 Gif-sur-Yvette cedex, France
  12. University of Tasmania, School of Mathematics and Physics, Private Bag 37, Hobart, TAS 7001, Australia
  13. South African Astronomical Observatory, PO Box 9, Observatory 7935, South Africa
  14. Research School of Astronomy and Astrophysics, Australian National University, Mt Stromlo Observatory, Weston Creek, ACT 2611, Australia
  15. University of Canterbury, Department of Physics and Astronomy, Private Bag 4800, Christchurch 8020, New Zealand
  16. McDonald Observatory, 16120 St Hwy Spur 78 #2, Fort Davis, Texas 79734, USA
  17. Boyden Observatory, University of the Free State, Department of Physics, PO Box 339, Bloemfontein 9300, South Africa
  18. Lawrence Livermore National Laboratory, IGPP, PO Box 808, Livermore, California 94551, USA
  19. Universität Potsdam, Institut für Physik, Am Neuen Palais 10, 14469 Potsdam, Astrophysikalisches Institut Potsdam, An der Sternwarte 16, D-14482, Potsdam, Germany
  20. Technische Universität Wien, Wiedner Hauptstrasse 8 / 020 B.A. 1040 Wien, Austria
  21. Department of Astronomy, University of Florida, 211 Bryant Space Science Center, Gainesville, Florida 32611-2055, USA
  22. Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, Maryland 21218, USA
  23. Astronomisches Rechen-Institut (ARI), Zentrum für Astronomie, Universität Heidelberg, Mönchhofstrasse 12–14, 69120 Heidelberg, Germany
  24. Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead CH41 1LD, UK
  25. Astronomy and Planetary Science Division, Department of Physics, Queen's University Belfast, Belfast, UK
  26. Obserwatorium Astronomiczne Uniwersytetu Warszawskiego, Aleje Ujazdowskie 4, 00-478 Warszawa, Poland
  27. Universidad de Concepcion, Departamento de Fisica, Casilla 160–C, Concepcion, Chile
  28. Jodrell Bank Observatory, The University of Manchester, Macclesfield, Cheshire SK11 9DL, UK
  29. Princeton University Observatory, Peyton Hall, Princeton, New Jersey 08544, USA
  30. Solar-Terrestrial Environment Laboratory, Nagoya University, Nagoya 464-860, Japan
  31. Institute for Information and Mathematical Sciences, Massey University, Private Bag 102-904, Auckland, New Zealand
  32. Department of Physics, University of Auckland, Private Bag 92019, Auckland, New Zealand
  33. School of Chemical and Physical Sciences, Victoria University, PO Box 600, Wellington, New Zealand
  34. Nagano National College of Technology, Nagano 381-8550, Japan
  35. Department of Astrophysics, Faculty of Science, Nagoya University, Nagoya 464-860, Japan

Correspondence to: J.-P. Beaulieu1,4 Correspondence and requests for materials should be addressed to J.P.B. (Email: beaulieu@iap.fr) or D.P.B. (Email: bennett@nd.edu). The photometric data set is available at planet.iap.fr and ogle.astrouw.edu.pl.

In the favoured core-accretion model of formation of planetary systems, solid planetesimals accumulate to build up planetary cores, which then accrete nebular gas if they are sufficiently massive. Around M-dwarf stars (the most common stars in our Galaxy), this model favours the formation of Earth-mass (Mcircle plus) to Neptune-mass planets with orbital radii of 1 to 10 astronomical units (au), which is consistent with the small number of gas giant planets known to orbit M-dwarf host stars1, 2, 3, 4. More than 170 extrasolar planets have been discovered with a wide range of masses and orbital periods, but planets of Neptune's mass or less have not hitherto been detected at separations of more than 0.15 au from normal stars. Here we report the discovery of a Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, or to obtain a text description, please contact npg@nature.comMcircle plus planetary companion at a separation of Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, or to obtain a text description, please contact npg@nature.comau from a Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, or to obtain a text description, please contact npg@nature.comMcircle dot M-dwarf star, where Mcircle dot refers to a solar mass. (We propose to name it OGLE-2005-BLG-390Lb, indicating a planetary mass companion to the lens star of the microlensing event.) The mass is lower than that of GJ876d (ref. 5), although the error bars overlap. Our detection suggests that such cool, sub-Neptune-mass planets may be more common than gas giant planets, as predicted by the core accretion theory.

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