Discovery of a planet orbiting a binary star system from gravitational microlensing

The properties of the recently discovered(1,2) extrasolar planets were not anticipated by theoretical work on the formation of planetary systems, most models for which were developed to explain our Solar System. Indeed, the observational technique used to detect these planets (measurement of radial-velocity shifts in stellar spectral lines) do not yet have the sensitivity to detect planetary systems like our own(3). Here we report observations and modelling of the gravitational microlensing event MACHO-97-BLG-41. We infer that the lens system consists of a planet of about 3 Jupiter masses orbiting a binary stellar system consisting of a late-K dwarf star and an M dwarf. The stars are separated by similar to 1.8 astronomical units (1 AU is the Earth-Sun distance), and the planet is orbiting them at a distance of about 7 AU. We had expected to find first the microlensing signature of jovian planets around single stars, so this result suggests that such planets orbiting short-period binary stars may be common.