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The signature of orbital motion from the dayside of the planet τ Boötis b


The giant planet orbiting τ Boötis (named τ Boötis b) was amongst the first extrasolar planets to be discovered1. It is one of the brightest exoplanets and one of the nearest to us, with an orbital period of just a few days. Over the course of more than a decade, measurements of its orbital inclination have been announced2 and refuted3, and have hitherto remained elusive4,5,6,7,8. Here we report the detection of carbon monoxide absorption in the thermal dayside spectrum of τ Boötis b. At a spectral resolution of 100,000, we trace the change in the radial velocity of the planet over a large range in phase, determining an orbital inclination of 44.5° ± 1.5° and a mass 5.95 ± 0.28 times that of Jupiter, demonstrating that atmospheric characterization is possible for non-transiting planets. The strong absorption signal points to an atmosphere with a temperature that is decreasing towards higher altitudes, in contrast to the temperature inversion inferred for other highly irradiated planets9,10. This supports the hypothesis that the absorbing compounds believed to cause such atmospheric inversions are destroyed in τ Boötis b by the ultraviolet emission from the active host star11.

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Figure 1: CO signal in the dayside spectrum of the extrasolar planet τ Boötis b.
Figure 2: Comparison of in-trail and out-of-trail cross-correlation values.
Figure 3: The orbital trail of CO absorption.


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We thank the ESO support staff of the Paranal Observatory for their help during the observations. This work is based on observations collected at the European Southern Observatory (186.C-0289). S.A. acknowledges support by a Rubicon fellowship from the Netherlands Organisation for Scientific Research (NWO), and by NSF grant no. 1108595.

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Authors and Affiliations



M.B. led the observations and data analysis, and co-wrote the manuscript. I.A.G.S. conceived the project, contributed to the analysis and co-wrote the manuscript. R.J.d.K. constructed the planet atmosphere models. S.A. conducted the MCMC orbital analysis. J.B., E.J.W.d.M., R.J.d.K. and S.A. discussed the analyses, results and commented on the manuscript.

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Correspondence to Matteo Brogi.

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Brogi, M., Snellen, I., de Kok, R. et al. The signature of orbital motion from the dayside of the planet τ Boötis b. Nature 486, 502–504 (2012).

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