Ever Elusive Exospheres: One Probable Detection and Two Non-Detections of H{\alpha} Transits in Young Systems

TL;DR

This study investigates Hα transits in three young exoplanets to detect atmospheric escape, finding one probable detection in a 17-million-year-old planet and non-detections in older systems, highlighting age as a factor in observability.

Contribution

First detection of Hα excess absorption in a young exoplanet, demonstrating the potential of Hα observations to study atmospheric escape in planets younger than 50 million years.

Findings

Probable Hα detection in HIP 67522b at 17 Myr

No Hα transit signals in HD 63433b and DS Tuc A b

Younger planets (<50 Myr) are more promising for Hα atmospheric escape detection

Abstract

Gaps in the exoplanet population, such as the Neptunian Desert, point to the importance of mass-loss in sculpting the radii of close-in exoplanets. Young planets ($<$500Myr) offer the opportunity to detect such mass-loss while it is still strong, and to test models of the underlying physical processes. We search for evidence of an H$\alpha$ transit in high-resolution spectra of three young planets, HD 63433b (400 Myr), DS TucAb (45 Myr), and HIP 67522b (17 Myr) using HARPS-N, Magellan-PFS, and CHIRON respectively. We validate our method by testing it on several photospheric lines less impacted by stellar variability. We find no evidence of a transit signal for HD 63433b and DS Tuc A b (3$\sigma$ limits of 0.9% and 0.3%, respectively). For HIP 67522b, we detect significant excess absorption (3.44$\pm$0.28%) aligned with the transit time, narrow compared to the stellar line, and blueshifted from the stellar rest frame. In combination, these suggest the signal is from the planet. However, stellar variation in the H$\alpha$ line over the course of the observations is comparable in size to the transit signature and the duration is shorter than the photometric transit, so this detection requires confirmation. Our findings, and other recent results in the literature, suggest that planets younger than 50 Myr are more favorable for the detection of atmospheric escape with H$\alpha$ observations, though older populations might still show escape in other diagnostics.