Probable detection of starlight reflected from the giant planet orbiting τ Boötis

In the four years following the discovery of a planet orbiting the star 51 Pegasi, about 20 other planets have been detected through their influence on the radial velocities of lines in the stellar spectra. The orbital motion of the planet is detected through the smaller ‘reflex motion’ of the star, which can be measured using high-precision spectroscopy. This indirect technique cannot investigate the radius or composition of the planet, and can place only a lower limit on its mass. Here we report the probable detection of Doppler-shifted starlight reflected from the planet known to orbit τ Boötis with a period of just a few days. We find that the orbital inclination is about i = 29°, from which we infer that the mass is about eight times that of Jupiter. The planet has the size and reflectivity expected for a gas-giant planet.