Issue A&A Volume 507, Number 1, November III 2009 Page(s) 487 - 494 Section Planets and planetary systems DOI 10.1051/0004-6361/200912172 Published online 24 September 2009

A&A 507, 487-494 (2009)
DOI: 10.1051/0004-6361/200912172

The HARPS search for southern extra-solar planets

XVIII. An Earth-mass planet in the GJ 581 planetary system

M. Mayor¹, X. Bonfils^{2, 3}, T. Forveille², X. Delfosse², S. Udry¹, J.-L. Bertaux⁴, H. Beust², F. Bouchy⁵, C. Lovis¹, F. Pepe¹, C. Perrier², D. Queloz¹, and N. C. Santos<sup>1, 6

1</sup>  Observatoire de Genève, Université de Genève, 51 ch. des Maillettes, 1290 Sauverny, Switzerland
    e-mail: michel.mayor@obs.unige.ch
²  Laboratoire d'Astrophysique, Observatoire de Grenoble, Université J. Fourier, CNRS (UMR5571), BP 53, 38041 Grenoble, Cedex 9, France
³  Centro de Astronomia e Astrofísica da Universidade de Lisboa, Observatório Astronómico de Lisboa, Tapada da Ajuda, 1349-018 Lisboa, Portugal
⁴  Service d'Aéronomie du CNRS, BP 3, 91371 Verrières-le-Buisson, France
⁵  Institut d'Astrophysique de Paris, CNRS, Université Pierre et Marie Curie, 98bis Bd Arago, 75014 Paris, France
⁶  Centro de Astrofísica, Universidade do Porto, Rua das Estrelas, P4150-762 Porto, Portugal

Received 24 March 2009 / Accepted 14 June 2009

Abstract
The GJ 581 planetary system is already known to harbour three planets, including two super-Earth planets that straddle its habitable zone. We report the detection of an additional planet – GJ 581e – with a minimum mass of 1.9 . With a period of 3.15 days, it is the innermost planet of the system and has a ~5% transit probability.
We also correct our previous confusion about the orbital period of GJ 581d (the outermost planet) with a one-year alias, benefitting from an extended time span and many more measurements. The revised period is 66.8 days, and positions the semi-major axis inside the habitable zone of the low mass star.
The dynamical stability of the 4-planet system imposes an upper bound on the orbital plane inclination. The planets cannot be more massive than approximately 1.6 times their minimum mass.

Key words: planetary systems -- stars: late-type -- techniques: radial velocities -- stars: individual: GJ 581

© ESO 2009

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