  We announce the discovery of a twin of Jupiter orbiting the slightly
metal-poor ([Fe/H] = -0.1) nearby (d = 18 pc) G8 dwarf HD 154345. This planet
has a minimum mass of 0.95 Jupiter masses 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.
