D ec 1 99 9 Probable detection of starlight reflected from the giant exoplanet orbiting

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 1 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 2, 3, 4 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 τ Boötis. Our data give the planet's orbital inclination i = 29 • , 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. A planet orbiting a star scatters back into space some of the starlight it receives. To a distant observer, the ratio of the scattered flux f p from the planet to the direct flux f ⋆ from its star depends on the planet's size and proximity to the star, which determine the amount of light intercepted, and on the scattering properties of the planet's atmosphere. For a planet of radius R p in an orbit of radius a, the planet-to-star brightness ratio is ǫ ≡ f p /f ⋆ = p(λ)(R p /a) 2 , when the planet is observed from 1 above the sub-stellar point. Here p(λ) is the geometric albedo of the atmosphere for light of wavelength λ. The τ Boo planet 2 has a circular orbit with a radius a = 0.0462(M * /1.2 M ⊙) 1/3 AU, derived from Kepler's law with the 3.3 d orbital period and estimated stellar mass. If this is a giant planet, with a Jupiter-like size and albedo, we expect ǫ ∼ 10 −4 ; the scattered starlight will be 10,000 to 20,000 times fainter than the star even when viewed at the most favourable illumination angle α = 0. Although τ Boo and its planet are never separated on the sky by more than 0.003 …