M. F. Bode, Kisaku Kamiya, Ian A. Bond, P. M. Kilmartin, Yutaka Matsubara, Penny D. Sackett, C. Coutures, Igor Soszyński, Stephen R. Kane, T. Wiȩckowski, Michael D. Albrow, Andrew Williams, J. G. Greenhill, T. R. Britton, O. Szewczyk, Łukasz Wyrzykowski, D. Dominis, M. Desort, R. M. Martin, Fumio Abe, D. M. Bramich, A. C. Gilmore, J. A. R. Caldwell, M. Hoffman, Shota Nakamura, J. J. Calitz, Denis J. Sullivan, Grzegorz Pietrzyński, Susumu Sato, Iain A. Steele, K. Woller, Kailash C. Sahu, P. J. Tristram, Martin Burgdorf, Misao Sasaki, Pascal Fouqué, J. W. Menzies, A. V. Korpela, E. Corrales, D. Kubas, Martin Dominik, T. Sekiguchi, Kimiaki Masuda, David P. Bennett, Colin Snodgrass, S. Brillant, Yoshitaka Itow, C. Okada, Takashi Sako, T. Yoshioka, Philip Yock, K. R. Pollard, Joachim Wambsganss, U. G. Jørgensen, P. J. Meintjes, Bohdan Paczynski, M. Kubiak, Arnaud Cassan, J. B. Marquette, M. Motomura, Andrzej Udalski, Michał K. Szymański, C. Vinter, Kouji Ohnishi, S. Dieters, J. P. Beaulieu, Nicholas J. Rattenbury, John B. Hearnshaw, J. Donatowicz, K. H. Cook, K. M. Hill, Yasushi Muraki, Keith Horne, Institut d'Astrophysique de Paris (IAP), Université Pierre et Marie Curie - Paris 6 (UPMC)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS), Laboratoire d'Astrophysique de l'Observatoire Midi-Pyrénées (LATT), Université Toulouse III - Paul Sabatier (UT3), Université de Toulouse (UT)-Université de Toulouse (UT)-Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire Midi-Pyrénées (OMP), Institut de Recherche pour le Développement (IRD)-Université Toulouse III - Paul Sabatier (UT3), Université de Toulouse (UT)-Université de Toulouse (UT)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National d'Études Spatiales [Toulouse] (CNES)-Centre National de la Recherche Scientifique (CNRS)-Météo-France -Institut de Recherche pour le Développement (IRD)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National d'Études Spatiales [Toulouse] (CNES)-Centre National de la Recherche Scientifique (CNRS)-Météo-France -Centre National de la Recherche Scientifique (CNRS), Astronomische Rechen-Institut [Heidelberg] (ARI), Zentrum für Astronomie der Universität Heidelberg (ZAH), Universität Heidelberg [Heidelberg] = Heidelberg University-Universität Heidelberg [Heidelberg] = Heidelberg University, Département d'Astrophysique, de physique des Particules, de physique Nucléaire et de l'Instrumentation Associée (DAPNIA), Commissariat à l'énergie atomique et aux énergies alternatives (CEA), Centre National de la Recherche Scientifique (CNRS)-Institut national des sciences de l'Univers (INSU - CNRS)-Université Pierre et Marie Curie - Paris 6 (UPMC), Université Fédérale Toulouse Midi-Pyrénées-Université Fédérale Toulouse Midi-Pyrénées-Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire Midi-Pyrénées (OMP), Météo France-Centre National d'Études Spatiales [Toulouse] (CNES)-Université Fédérale Toulouse Midi-Pyrénées-Centre National de la Recherche Scientifique (CNRS)-Institut de Recherche pour le Développement (IRD)-Météo France-Centre National d'Études Spatiales [Toulouse] (CNES)-Centre National de la Recherche Scientifique (CNRS)-Institut de Recherche pour le Développement (IRD)-Centre National de la Recherche Scientifique (CNRS), and Universität Heidelberg [Heidelberg]-Universität Heidelberg [Heidelberg]
Over 170 extrasolar planets have so far been discovered, with a wide range of masses and orbital periods, but until last July no planet of Neptune's mass or less had been detected any more than 0.15 astronomical units (AU) from a normal star. (That's close — Earth is one AU from the Sun). On 11 July 2005 the OGLE Early Warning System recorded a notable event: gravitational lensing of light from a distant object by a foreground star revealed a small planet of about 5.5 Earth masses, orbiting at about 2.6 AU from the foreground star. This is the lowest known mass for an extrasolar planet orbiting a main sequence star, and its detection suggests that cool, sub-Neptune mass planets are more common than gas giants, as predicted by the favoured core accretion theory of planet formation. 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 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 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.