Escaping atmospheres of extrasolar planets

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Science  21 Dec 2018:
Vol. 362, Issue 6421, pp. 1360-1361
DOI: 10.1126/science.aav7010

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The atmospheres of planets orbiting other stars (exoplanets) are windows into their chemical composition and physical properties. For a planet that orbits closely to its star, the intense stellar irradiation can induce substantial atmospheric loss, a phenomenon that can be detected if the planet is transiting through an excess absorption of starlight by gas that is escaping the planet's atmosphere. On pages 1384 and 1388 of this issue, Allart et al. (1) and Nortmann et al. (2), respectively, report two independent measurements of planetary helium with remote, ground-based spectroscopy in the near-infrared. Their findings mark the first time that helium is detected from the ground and is unambiguously associated with the planet's orbital motion. The high spectral resolution of the observations allows direct tracking of helium's velocity and verifies that it trails the planet along its orbit.