Résumé du preprint DAPNIA-05-341

Discovery of a cool planet of 5.5 Earth masses through gravitational microlensing
J.-P. Beaulieu, D. P. Bennett, P. Fouqué, A. Williams, M. Dominik1, U. G. Jørgensen, D. Kubas, A. Cassan, C. Coutures, J. Greenhi, K. Hill, J. Menzies, P.D. Sackett, M. Albrow1, S. Brillant, J.A.R. Caldwel, J. J. Calitz, K. H. Cook, E. Corrales, M. Desort, S. Dieters, D. Dominis, J. Donatowicz, M. Hoffman, S. Kane, J.-B. Marquette, R. Martin, P. Meintjes, K. Pollard, K. Sahu, C. Vinter, J. Wambsganss, K. Woller, K. Horne, I. Steele, D. M. Bramich, M. Burgdorf, C. Snodgrass, M. Bode, A. Udalski, M.K. Szymański, M. Kubiak, T. Więckowski, G. Pietrzyński, I. Soszyński, O. Szewczyk, Ł. Wyrzykowski, B. Paczyński, F. Abe, I. A. Bond, T. R. Britton, A. C. Gilmore, J. B. Hearnshaw, Y. Itow, K. Kamiya, P. M. Kilmartin, A. V. Korpela, K. Ohnishi, Y. Matsubara, M. Motomura, Y. Muraki, S. Nakamura, C. Okada, K. Ohnishi, N. J. Rattenbury, T. Sako, S. Sato, M. Sasaki, T. Sekiguchi, D. J. Sullivan, P. J. Tristram, P. C. M. Yock, T. Yoshioka
 The favoured theoretical explanation for planetary systems formation is the core-accretion model in which 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 of our Galaxy, this model favours the formation of Earth to Neptune mass planets in a few million years with orbital sizes of 1 to 10 AU, which is consistent with the small number of detections of giant planets with M-dwarf host stars1-4. More than 170 extrasolar planets have been discovered so far with a wide range of masses and orbital periods, but planets of Neptunes mass or less have only been detected at separations of less than 0.15 AU.  Here we report the discovery of a   Earth-mass planetary companion at a separation of   from a   M-dwarf star, which is the lens star for gravitational microlensing event OGLE 2005-BLG-390. This is the lowest mass ever reported for an extrasolar planet orbiting a main sequence star, although the error bars overlap those for the mass of GJ876d5. This detection suggests that such cool, sub-Neptune mass planets may be common, as predicted by the core accretion theory.


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