The giant planet orbiting tau Bootis was among the first extrasolar planetsto be discovered through the reflex motion of its host star. It is one of thebrightest known and most nearby planets with an orbital period of just a fewdays. Over the course of more than a decade, measurements of its orbitalinclination have been announced and refuted, and have subsequently remainedelusive until now. Here we report on the detection of carbon monoxideabsorption in the thermal day-side spectrum of tau Bootis b. At a spectralresolution of R~100,000, we trace the change in the radial velocity of theplanet over a large range in phase, determining an orbital inclination ofi=44.5+-1.5 degrees and a true planet mass of 5.95+-0.28 MJup. This resultextends atmospheric characterisation to non-transiting planets. The strongabsorption signal points to an atmosphere with a temperature that is decreasingtowards higher altitudes. This is a stark contrast to the temperature inversioninvoked for other highly irradiated planets, and supports models in which theabsorbing compounds believed to cause such atmospheric inversions are destroyedby the ultraviolet emission from the active host star.
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