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

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Science  21 Dec 2018:
Vol. 362, Issue 6421, pp. 1384-1387
DOI: 10.1126/science.aat5879

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Helium escaping from hot gas giants

Many gas giant exoplanets orbit so close to their host star that they are heated to high temperatures, causing atmospheric gases to escape. Gas giant atmospheres are mostly hydrogen and helium, which are difficult to observe. Two papers have now observed escaping helium in the near-infrared (see the Perspective by Brogi). Allart et al. observed helium in a Neptune-mass exoplanet and performed detailed simulations of its atmosphere, which put constraints on the escape rate. Nortmann et al. found that helium is escaping a Saturn-mass planet, trailing behind it in its orbit. They combined this with observations of several other exoplanets to show that atmospheres are being lost more quickly by exoplanets that are more strongly heated.

Science, this issue p. 1384, p. 1388; see also p. 1360


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 by 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 three-dimensional simulations of the planet’s upper atmosphere suggests that it extends beyond 5 planetary radii, with a large-scale height and a helium mass loss rate of ≲3 × 105 grams per second. A net blue-shift of the absorption might be explained by high-altitude winds flowing at 3 kilometers per second from day to night-side.

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