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

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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 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.

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Figure 1: Greyscale plots showing 145 residual velocity profiles.
Figure 2: Improvement in the fit to the data over the no-planet hypothesis, shown for the simulation of a planet with a wavelength-independent (grey) albedo.
Figure 3: Evidence for a planet in the WHT data, assessed as a function of the planet's orbit velocity Kp.
Figure 4: This periodogram analysis searches for evidence of reflected light from the τ Boo planet over a range of trail values of the orbit velocity Kp and orbit period P.


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This work is based on observations with the 4.2-m William Herschel Telescope at the Observatori del Roque de los Muchachos on La Palma, and used Starlink-supported hardware and software.

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Correspondence to Andrew Collier Cameron.

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Cameron, A., Horne, K., Penny, A. et al. Probable detection of starlight reflected from the giant planet orbiting τ Boötis. Nature 402, 751–755 (1999) doi:10.1038/45451

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