【天体物理学】系外惑星から散逸するヘリウムが物語る大気の進化

Helium has been detected in the escaping atmospheric wind of a giant exoplanet for the first time, according to a paper published in this week’s Nature.

Helium is the second-most common element in the Universe and a major component of the gas giants in our Solar System. Although we expect to find helium in gas giants elsewhere, searches until now have been unsuccessful. Exoplanetary atmospheres are normally studied by observing the planets pass in front of their stars and measuring the absorption of light by their outermost atmospheres. Previous searches for escaping atmospheres have tended to look for absorption in the ultraviolet part of the spectrum, which is difficult with current technology and only works for the closest planets.

Jessica Spake and colleagues instead looked for absorption in the near-infrared part of the spectrum of WASP-107b - a gas giant orbiting a small, orange star - using the Hubble Space Telescope. The authors found a signature of excited helium at a wavelength of 10,833 angstrom. The signal’s amplitude suggests that WASP-107b’s atmosphere is leaking away at a rate of 0.1%-4% of the planet’s total mass every billion years. Despite being similarly sized to Jupiter, this exoplanet is eight times less massive - one of the least dense known - and its gravitational pull is scarcely able to retain its atmosphere. With its host star emitting ultraviolet radiation and heating up WASP-107b’s atmospheric gases, they expand rapidly and blast into space like a wind. Finding other planets with helium-rich atmospheres could provide a new way of exploring the formation and evolution of exoplanets.