  Fomalhaut is a bright star 7.7 parsecs (25 light years) from Earth that
harbors a belt of cold dust with a structure consistent with gravitational
sculpting by an orbiting planet. Here, we present optical observations of an
exoplanet candidate, Fomalhaut b. In the plane of the belt, Fomalhaut b lies
approximately 119 astronomical units (AU) from the star and 18 AU from the
belt, matching predictions. We detect counterclockwise orbital motion using
Hubble Space Telescope observations separated by 1.73 years. Dynamical models
of the interaction between the planet and the belt indicate that the planet's
mass is at most three times that of Jupiter for the belt to avoid gravitational
disruption. The flux detected at 800 nm is also consistent with that of a
planet with mass no greater than a few times that of Jupiter. The brightness at
600 nm and the lack of detection at longer wavelengths suggest that the
detected flux may include starlight reflected off a circumplanetary disk, with
dimension comparable to the orbits of the Galilean satellites. We also observed
variability of unknown origin at 600 nm.
