Letter to the Editor

The HARPS search for southern extra-solar planets

VI. A Neptune-mass planet around the nearby M dwarf Gl 581

¹ Laboratoire d'Astrophysique, Observatoire de Grenoble, BP 53, 38041 Grenoble Cedex 9, France
² Observatoire de Genève, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland e-mail: Xavier.Bonfils@obs.unige.ch
³ Canada-France-Hawaii Telescope Corporation, 65-1238 Mamalahoa Highway, Kamuela, HI96743, Hawaii, USA
⁴ Laboratoire d'Astrophysique de Marseille, Traverse du Siphon, 13013 Marseille, France
⁵ Service d'Aéronomie du CNRS, BP 3, 91371 Verrières-le-Buisson, France

Received: 22 August 2005
Accepted: 24 September 2005

Abstract

We report the discovery of a Neptune-mass planet around Gl 581 (M3V, ), based on precise Doppler measurements with the HARPS spectrograph at La Silla Observatory. The radial velocities reveal a circular orbit of period P = 5.366 days and semi-amplitude K₁ = 13.2 m s^-1. The resulting minimum mass of the planet () is only 0.052  = 0.97  = 16.6   making Gl 581b one of the lightest extra-solar planet known to date. The Gl 581 planetary system is only the third centered on an M dwarf, joining the Gl 876 three-planet system and the lone planet around Gl 436. Its discovery reinforces the emerging tendency of such planets to be of low mass, and found at short orbital periods. The statistical properties of the planets orbiting M dwarfs do not seem to match a simple mass scaling of their counterparts around solar-type stars.