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

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 ≲ 3x10⁵ g‧s⁻¹. A net blueshift of the absorption might be explained by high-altitude winds flowing at 3 km‧s⁻¹from day to night-side.