An extrasolar giant planet in an Earth-like orbit. Precise radial velocities of the young star iota Horologii = HR 810

We present 5 1/2 years of radial velocity (RV) monitoring with the ESO CES of the young (ZAMS) G0V star iota Hor. We find a Keplerian signal with a period of 320.1 d indicative of an orbiting planet with minimum mass msin i=2.26 M_Jup. With an eccentricity of 0.161 and a semi-major axis of 0.925 AU this object moves in the most Earth-like orbit found so far among extrasolar planets. iota Hor is the youngest star with a known planet. In addition to the planetary signal we find an excess RV scatter of 27.0 ms^-1 rms exceeding our measurement precision of 17.4 ms^-1 and probably due to stellar activity. Based on observations collected at the European Southern Observatory, La Silla.