  Giant planets orbiting stars other than the Sun are clearly detectable
through precise radial-velocity measurements of the orbital reflex motion of
the parent star. In the four years since the discovery of the companion to the
star 51 Peg, similar low-amplitude ``Doppler star wobbles'' have revealed the
presence of some 20 planets orbiting nearby solar-type stars. Several of these
newly-discovered planets are very close to their parent stars, in orbits with
periods of only a few days. Being an indirect technique, however, the
reflex-velocity method has little to say about the sizes or compositions of the
planets, and can only place lower limits on their masses. Here we report the
use of high-resolution optical spectroscopy to achieve a probable detection of
the Doppler-shifted signature of starlight reflected from one of these objects,
the giant exoplanet orbiting the star tau Bootis. Our data give the planet's
orbital inclination i=29 degrees, indicating that its mass is some 8 times that
of Jupiter, and suggest strongly that the planet has the size and reflectivity
expected for a gas-giant planet.
