Helium in the eroding atmosphere of an exoplanet
J. J. Spake, D. K. Sing, T. M. Evans, A. Oklop\v{c}i\'c, V. Bourrier,, L. Kreidberg, B. V. Rackham, J. Irwin, D. Ehrenreich, A. Wyttenbach, H. R., Wakeford, Y. Zhou, K. L. Chubb, N. Nikolov, J. M. Goyal, G. W. Henry, M. H., Williamson, S. Blumenthal, D. R. Anderson, C. Hellier

TL;DR
This study reports the first detection of helium in an exoplanet's atmosphere, revealing an extended, escaping atmosphere on WASP-107b with significant implications for atmospheric loss and evolution.
Contribution
The paper provides the first observational detection of metastable helium in an exoplanet atmosphere, confirming theoretical predictions about atmospheric escape.
Findings
Detected helium absorption at 10,833 Å with high confidence.
Estimated atmospheric escape rate of 10^10 to 3 x 10^11 g/s.
Indicates a highly extended, eroding atmosphere possibly forming a gas tail.
Abstract
Helium is the second-most abundant element in the Universe after hydrogen and is one of the main constituents of gas-giant planets in our Solar System. Early theoretical models predicted helium to be among the most readily detectable species in the atmospheres of exoplanets, especially in extended and escaping atmospheres. Searches for helium, however, have hitherto been unsuccessful. Here we report observations of helium on an exoplanet, at a confidence level of 4.5 standard deviations. We measured the near- infrared transmission spectrum of the warm gas giant WASP-107b and identified the narrow absorption feature of excited metastable helium at 10,833 angstroms. The amplitude of the feature, in transit depth, is 0.049 +/- 0.011 per cent in a bandpass of 98 angstroms, which is more than five times greater than what could be caused by nominal stellar chromospheric activity. This large…
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