For extrasolar planets discovered using the radial velocity method1, the spectral characterization of the host star leads to a mass estimate of the star and subsequently of the orbiting planet. If the orbital velocity of the planet could be determined, the masses of both star and planet could be calculated using Newton’s law of gravity, just as in the case of stellar double-line eclipsing binaries. Here we report high-dispersion ground-based spectroscopy of a transit of the extrasolar planet HD 209458b. We see a significant wavelength shift in absorption lines from carbon monoxide in the planet’s atmosphere, which we conclude arises from a change in the radial component of the planet’s orbital velocity. The masses of the star and planet are 1.00 ± 0.22MSun and 0.64 ± 0.09MJup respectively. A blueshift of the carbon monoxide signal of approximately 2 km s−1 with respect to the systemic velocity of the host star suggests the presence of a strong wind flowing from the irradiated dayside to the non-irradiated nightside of the planet within the 0.01–0.1 mbar atmospheric pressure range probed by these observations. The strength of the carbon monoxide signal suggests a carbon monoxide mixing ratio of (1–3) × 10−3 in this planet’s upper atmosphere.
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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 (383.C-0045A). S.A. acknowledges support by a Rubicon fellowship from the Netherlands Organisation for Scientific Research (NWO).
The authors declare no competing financial interests.
This file contains Supplementary Information comprising: Observational set-up; Data reduction and analysis; The model transmission spectra; The cross-correlation analysis; Supplementary Figures S1-S2 with legends and References. (PDF 782 kb)
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Snellen, I., de Kok, R., de Mooij, E. et al. The orbital motion, absolute mass and high-altitude winds of exoplanet HD 209458b. Nature 465, 1049–1051 (2010). https://doi.org/10.1038/nature09111
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