Helium in the eroding atmosphere of an exoplanet

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 absorption signal suggests that WASP-107b has an extended atmosphere that is eroding at a total rate of 10 to 3 × 10 grams per second (0.1-4 per cent of its total mass per billion years), and may have a comet-like tail of gas shaped by radiation pressure.

Confirmation of WASP-107b’s Extended Helium Atmosphere with Keck II/NIRSPEC

“…The result of Spake et al (2018) coincided with a theoretical study of Oklopčić & Hirata (2018), who generated 1D models of escaping atmospheres and demonstrated that observations of He I absorption could be used as a powerful tool for studying atmospheric escape. This is because it suffers little from extinction, can be observed from the ground, and can probe the escaping planetary wind.…”

“…Spake et al (2018) detected an excess absorption of 0.049 ± 0.011% in a 98Åwide bin centered on the helium triplet at 10833Å. This absorption suggested that the planet is losing mass at a rate of 10 10 − 3 × 10 11 g s −1 , or equivalently 0.1 − 4 % of its total mass per Gyr (Spake et al 2018).…”

“…Recently, Spake et al (2018) announced the groundbreaking detection of He I in the extended atmosphere of the sub-Saturn exoplanet WASP-107b, using the G102 grism on Hubble Space Telescope's (HST) Wide Field Camera 3 (WFC3) instrument. Spake et al (2018) detected an excess absorption of 0.049 ± 0.011% in a 98Åwide bin centered on the helium triplet at 10833Å. This absorption suggested that the planet is losing mass at a rate of 10 10 − 3 × 10 11 g s −1 , or equivalently 0.1 − 4 % of its total mass per Gyr (Spake et al 2018).…”

“…Prior to the detection by Spake et al (2018), this absorption signature of a metastable state of neutral helium at 10833Å had been predicted to be a strong feature in exoplanet transmission spectra (Seager & Sasselov 2000;Turner et al 2016;Oklopčić & Hirata 2018). However, despite early efforts (Moutou et al 2003), it went undetected until Spake et al (2018)'s detection.…”

Detection of Na, K, and Hαabsorption in the atmosphere of WASP-52b using ESPRESSO

“…∼0.378% for GTC/OSIRIS in a 16 Å band). This makes WASP-52b one of the very few planets for which the same atomic species has been confirmed by both low-and high-resolution observations, e.g., the Na doublet seen in HD 189733b (Huitson et al 2012;Wyttenbach et al 2015), the He i 1083 nm triplet seen in WASP-107b (Spake et al 2018;Allart et al 2019) and HAT-P-11b (Mansfield et al 2018;Allart et al 2018).…”