2025A&A...704L..18G

Context. Neptune-sized exoplanets (i.e., exo-Neptunes) are fundamental to the study of exoplanet diversity. Their evolution is sculpted by atmospheric escape, often traced by absorption in the H I Lyman-α line at 1216 Å and the He I triplet line at 1.08 μm. On the warm exo-Neptunes HAT-P-11 b, GJ 3470 b and GJ 436 b, H I Lyman-α absorption causes extreme in-transit obscuration of their host stars. This suggests that the He I triplet line absorption would be strong as well, yet it has only been identified on two of these planets.
Aims. We explore processes that had previously been omitted, which might act to attenuate the He I triplet line on warm exo-Neptunes. In particular, we assess the role of vibrationally excited H₂ to remove the He⁺ ion that acts as precursor of the absorbing He(2³S).
Methods. We determined thermal rate coefficients for this chemical process, leveraging the available theoretical and experimental data. The process becomes notably fast at the temperatures expected in the atmospheric layers probed by the He I triplet line.
Results. Our simulations show that this removal process severely mutes the line on GJ 3470 b and leads to the nondetection on GJ 436 b. The overall efficiency of this mechanism is connected to the location in the atmosphere of the H₂-to-H transition and, ultimately, to the amount of high-energy radiation received by the planet. The process will be more significant on small exoplanets than on hotter or more massive ones since, in the latter case, the H₂-to-H transition generally occurs deeper in the atmosphere.
Conclusions. Weak He I triplet line absorption does not necessarily imply the lack of a primordial, H₂-He-dominated atmosphere, an idea to bear in mind when interpreting observations of other small exoplanets.