Strong H i Lyman-α variations from an 11 Gyr-old host star: a planetary origin?

Abstract

Kepler-444 provides a unique opportunity to probe the atmospheric composition and evolution of a compact system of exoplanets smaller than the Earth. Five planets transit this bright K star at close orbital distances, but they are too small for their putative lower atmosphere to be probed at optical/infrared wavelengths. We used the Space Telescope Imaging Spectrograph instrument onboard the Hubble Space Telescope to search for the signature of the planet's upper atmospheres at six independent epochs in the Lyman-α line. We detect significant flux variations during the transits of both Kepler-444 e and f (∼20%), and also at a time when none of the known planets was transiting (∼40%). Variability in the transition region and corona of the host star might be the source of these variations. Yet, their amplitude over short time scales (∼2-3 hours) is surprisingly strong for this old (11.2±1.0 Gyr) and apparently quiet main-sequence star. Alternatively, we show that the in-transits variations could be explained by absorption from neutral hydrogen exospheres trailing the two outer planets (Kepler-444 e and f). They would have to contain substantial amounts of water to replenish such hydrogen exospheres, which would reveal them as the first confirmed ocean-planets. The outof-transit variations, however, would require the presence of a yet-undetected Kepler-444 g at larger orbital distance, casting doubt on the planetary origin scenario. Using HARPS-N observations in the sodium doublet, we derived the properties of two Interstellar Medium clouds along the line-of-sight toward Kepler-444. This allowed us to reconstruct the stellar Lyman-α line profile and to estimate the XUV irradiation from the star, which would still allow for a moderate mass loss from the outer planets after 11.2 Gyr. Follow-up of the system at XUV wavelengths will be required to assess this tantalizing possibility.

Hubble PanCET: an extended upper atmosphere of neutral hydrogen around the warm Neptune GJ 3470b

“…Wood et al 2005). This strengthens the trend noted in Bourrier et al (2017a) that M dwarfs show no self-reversal in their core, while K dwarfs sometimes show double-peaked profiles and G-type stars always do. K dwarfs might thus have transition regions, where the core of the Lyman-α line is formed (Vernazza et al 1981), that have intermediate structures in between those of G-type stars and M dwarfs.…”

The high-energy environment and atmospheric escape of the mini-Neptune K2-18 b

“…However, to date, no evidence for extended atmo-Article number, page 1 of 5 arXiv:2001.04532v2 [astro-ph.EP] 20 Jan 2020 A&A proofs: manuscript no. 37327 spheres has been found for planets smaller than Neptune, as nondetections were reported for the super-Earths 55 Cnc e (Ehrenreich et al 2012), HD 97658 b (Bourrier et al 2017c), GJ 1132 b (Waalkes et al 2019), and π Men c (García Muñoz et al 2019), and marginal detections were reported for the small rocky-planet systems in TRAPPIST-1 (Bourrier et al 2017a) and Kepler-444 (Bourrier et al 2017b).…”

The XUV irradiation and likely atmospheric escape of the super-Earth π Men c

“…It also showed a non-detection at Lyman-α (Ehrenreich et al 2012). Lyman-α observations of Kepler-444 show variations that might be associated with hydrogen escape from its rocky planets (Bourrier et al 2017b).…”