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. Beaulieu^1,4 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 (M) 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 stars^{1, 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 M planetary companion at a separation of au from a M M-dwarf star, where M 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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