# Strong HI Lyman-$\alpha$ variations from the 11 Gyr-old host star Kepler-444: a planetary origin ?

## TL;DR

This study investigates the variable Lyman-alpha flux from the old star Kepler-444, exploring potential planetary atmospheric signatures and stellar activity, with implications for exoplanet atmospheric evolution and the presence of water-rich exoplanets.

## Contribution

It presents the first detection of significant Lyman-alpha flux variations in Kepler-444, proposing planetary exospheres or stellar activity as possible causes, and estimates the star's XUV irradiation after 11 billion years.

## Key findings

- Detected ~20% flux variations during transits of Kepler-444e and f.
- Observed ~40% flux variations out of transit, possibly due to stellar activity.
- Estimated the star's XUV flux, suggesting moderate atmospheric mass loss over 11 Gyr.

## 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 Ly-$\alpha$ line. We detect significant flux variations during the transits of both Kepler-444e 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+-1.0Gyr) 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-444e 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 out-of-transit variations, however, would require the presence of a yet-undetected Kepler-444g 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 Ly-$\alpha$ 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 11.2Gyr. Follow-up of the system at XUV wavelengths will be required to assess this tantalizing possibility.