The HARPS search for southern extra-solar planets

We report the discovery of a Neptune-mass planet around Gl 581 (M3V, M = 0. 31 M-circle dot), 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 K-1 = 13.2 ms ms(-1). The resulting minimum mass of the planet (M-2 sin i) is only 0.052) M-Jup = 0.97 M-Nep = 16.6 M-Earth 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 properties match a simple mass scaling of their counterparts around solar-type stars.