The Jupiter Twin HD 154345b

We announce the discovery of a twin of Jupiter orbiting the slightly metalpoor ([Fe/H] = −0.1) nearby (d = 18 pc) G8 dwarf HD 154345. This planet has a minimum mass of 0.95 MJup and a 9.2 year, circular orbit with radius 4.2 AU. There is currently little or no evidence for other planets in the system, but smaller or exterior planets cannot yet be ruled out. We also detect a ∼ 9-year activity cycle in this star photometrically and in chromospheric emission. We rule out activity cycles as the source of the radial velocity variations by comparison with other cycling late-G dwarfs. Subject headings: planetary systems — techniques: radial velocities 1 Based on observations obtained at the W. M. Keck Observatory, which is operated jointly by the University