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Spectrally resolved helium absorption from the extended atmosphere of a warm Neptune-mass exoplanet

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Science  06 Dec 2019:
eaat5879
DOI: 10.1126/science.aat5879

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Abstract

Stellar heating causes atmospheres of close-in exoplanets to expand and escape. These extended atmospheres are difficult to observe because their main spectral signature—neutral hydrogen at ultraviolet wavelengths—is strongly absorbed by interstellar medium. We report the detection of the near-infrared triplet of neutral helium in the transiting warm Neptune-mass exoplanet HAT-P-11b using ground-based, high-resolution observations. The helium feature is repeatable over two independent transits, with an average absorption depth of 1.08 ± 0.05%. Interpreting absorption spectra with 3D simulations of the planet’s upper atmosphere suggests it extends beyond 5 planetary radii, with a large scale height and a helium mass loss rate ≲ 3×105 g‧s−1. A net blue-shift of the absorption might be explained by high-altitude winds flowing at 3 km‧s−1 from day to night-side.

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