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Nature 402, 751-755 (16 December 1999) | doi:10.1038/45451; Received 6 July 1999; Accepted 16 November 1999

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

Andrew Collier Cameron1, Keith Horne1, Alan Penny2 & David James1

  1. School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, Fife KY16 9SS, UK
  2. Rutherford Appleton Laboratory, Chilton , Didcot, Oxon OX11 0QX, UK

Correspondence to: Andrew Collier Cameron1 Correspondence and requests for materials should be addressed to A.C.C. (e-mail: Email: andrew.cameron@st-and.ac.uk).

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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 tau 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.