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Detection of Carbon Monoxide and Water Absorption Lines in an Exoplanet Atmosphere

High-Resolution Spectrum of an Exoplanet Unlike most of the extrasolar planets we know about, the four planets around the star HR 8799 were detected directly. Konopacky et al. (p. 1398, published online 14 March; see the Perspective by Marley) obtained a high-resolution spectrum of one of the planets that reveals both water and carbon monoxide but not methane in the planet's atmosphere. The atmospheric carbon-to-oxygen ratio, which traces the process of planet formation, is greater than that of the host star, providing clues to how the planets formed. A high-resolution spectrum of an exoplanet reveals molecular lines that provide clues about the planet’s formation. [Also see Perspective by Marley] Determining the atmospheric structure and chemical composition of an exoplanet remains a formidable goal. Fortunately, advancements in the study of exoplanets and their atmospheres have come in the form of direct imaging—spatially resolving the planet from its parent star—which enables high-resolution spectroscopy of self-luminous planets in jovian-like orbits. Here, we present a spectrum with numerous, well-resolved molecular lines from both water and carbon monoxide from a massive planet orbiting less than 40 astronomical units from the star HR 8799. These data reveal the planet’s chemical composition, atmospheric structure, and surface gravity, confirming that it is indeed a young planet. The spectral lines suggest an atmospheric carbon-to-oxygen ratio that is greater than that of the host star, providing hints about the planet’s formation.