Ever Elusive Exospheres: One Probable Detection and Two Non-Detections of Hα Transits in Young Systems

TL;DR

This study searches for Hα transit signals as evidence of atmospheric escape in three young exoplanets using high-resolution spectroscopy from HARPS-N, Magellan-PFS, and CHIRON. It validates the method on photospheric lines less affected by stellar variability and reports non-detections for HD 63433b and DS Tuc Ab (3σ limits of $0.9\%$ and $0.3\%$). For HIP 67522b, it finds a significant excess absorption of $3.44 \pm 0.28\%$ aligned with transit time, but stellar variability and a shorter transit duration require independent confirmation. Together with other recent work, the results suggest Hα escape is most detectable in planets younger than approximately 50 Myr, though older populations may still reveal escape via other tracers.

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 ($<$500Myr) offer the opportunity to detect such mass-loss while it is still strong, and to test models of the underlying physical processes. We search for evidence of an H$α$ 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$σ$ limits of 0.9% and 0.3%, respectively). For HIP 67522b, we detect significant excess absorption (3.44$\pm$0.28%) aligned with the transit time, narrow compared to the stellar line, and blueshifted from the stellar rest frame. In combination, these suggest the signal is from the planet. However, stellar variation in the H$α$ 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$α$ observations, though older populations might still show escape in other diagnostics.

Figures (1)

• Figure 1: The young planet population in period-radius space. The color denotes the age of the planet. HIP 67522 b is marked by a diamond, DS Tuc A b by a cross, and HD 63433 b by a star. The contours indicate the relative occurrence of planets inferred from the Kepler distribution Dattilo2023. The young planet population shown is not corrected for completeness, although young planets are known to be larger than their older counterparts even after accounting detection biases Vach2024.