ar X iv : a st ro - p h / 04 08 58 7 v 2 1 5 Se p 20 04 A Neptune – Mass Planet Orbiting the Nearby M Dwarf GJ

We report precise Doppler measurements of GJ 436 (M2.5V) obtained at Keck Observatory. The velocities reveal a planetary companion with orbital period of 2.644 d, eccentricity of 0.12 (consistent with zero) and velocity semiamplitude of K = 18.1 m s . The minimum mass (M sin i ) for the planet is 0.067 MJUP = 1.2 MNEP = 21 MEARTH, making it the lowest mass exoplanet yet found around a main sequence star and the first candidate in the Neptune mass domain. GJ 436 (Mass = 0.41 M⊙) is only the second M dwarf found to harbor a planet, joining the two–planet system around GJ 876. The low mass of the planet raises questions about its constitution, with possible compositions of primarily H and He gas, ice/rock, or rock–dominated. The implied semi– major axis is a = 0.028 AU = 14 stellar radii, raising issues of planet formation, migration, and tidal coupling with the star. GJ 436 is > 3 Gyr old, based on both kinematic and chromospheric diagnostics. The star exhibits no photometric variability on the 2.644-day Doppler period to a limiting amplitude of 0.0004 mag, supporting the planetary interpretation of the Doppler periodicity. Photometric Based on observations obtained at the W.M. Keck Observatory, which is operated jointly by the University of California and the California Institute of Technology. Keck time has been granted by both NASA and the University of California. Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Rd NW, Washington DC, USA 20015-1305 UCO/Lick Observatory, University of California at Santa Cruz, Santa Cruz, CA, USA 95064 Department of Astronomy, University of California, Berkeley, CA USA 94720 Department of Physcis and Astronomy, San Francisco State University, San Francisco, CA, USA 94132 Center of Excellence in Information Systems, Tennessee State University, 330 10th Avenue North, Nashville, TN 37203; Also Senior Research Associate, Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235 Space Science Division, 245-3 NASA Ames Research Center, Moffett Field, CA 94035-1000 USA