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

TL;DR

This study detects neutral helium in the extended atmosphere of exoplanet HAT-P-11b, revealing atmospheric escape and wind dynamics through high-resolution ground-based spectroscopy.

Contribution

First ground-based detection of helium in a warm Neptune's extended atmosphere, providing insights into atmospheric escape and wind patterns.

Findings

Helium absorption depth of 1.08% detected over two transits.

Atmosphere extends beyond 5 planetary radii with high scale height.

Helium mass loss rate estimated to be ≤ 3x10^5 g/s.

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 =< 3x10^5 g/s. A net blue-shift of the absorption might be explained by high-altitude winds flowing at 3 km/s from day to night-side.