Is π Men c’s Atmosphere Hydrogen-dominated? Insights from a Non-detection of H i Lyα Absorption

Constraining the composition of super-Earth-to-sub-Neptune-sized planets is a priority in order to understand the processes of planetary formation and evolution. π Men c represents a unique target for the atmospheric and compositional characterization of such planets because it is strongly irradiated and its bulk density is consistent with abundant H 2 O. We searched for hydrogen from photodissociating H 2 /H 2 O in π Men c's upper atmosphere through H i Ly α transmission spectroscopy with the Hubble Space Telescope ’s Space Telescope Imaging Spectrograph, but did not detect it. We set 1 σ (3 σ ) upper limits for the effective planet-to-star size ratio R Ly α / R ⋆ = 0.13 (0.24) and 0.12 (0.20) at velocities [−215, −91] km s −1 and [+57, +180] km s −1 , respectively. We reconstructed the stellar spectrum, and estimate that π Men c receives about 1350 erg cm −2 s −1 of 5–912 Å energy, enough to cause rapid atmospheric escape. An interesting scenario to explain the non-detection is that π Men c's atmosphere is dominated by H 2 O or other heavy molecules rather than H 2 /He. According to our models, abundant oxygen results in less extended atmospheres, which transition from neutral to ionized hydrogen closer to the planet. We compare our non-detection to other detection attempts, and tentatively identify two behaviors: planets with densities ≲2 g cm −3 (and likely hydrogen-dominated atmospheres) result in H i Ly α absorption, whereas planets with densities ≳3 g cm −3 (and plausibly non-hydrogen-dominated atmospheres) do not result in measurable absorption. Investigating a sample of strongly irradiated sub-Neptunes may provide some statistical confirmation if it is shown that they do not generally develop extended atmospheres.