Helium escape in context: Comparative signatures of four close-in exoplanets
Observations of escaping atmospheres on close-in exoplanets show a wide range in the strength and morphology of He I 10830 A and H I absorption. Scaling relations attempt to link the He I signal to XUV irradiation, mass loss, and bulk planetary parameters. We test these relations with a comparative analysis of HD209458b, HD189733b, HD149026b, and GJ1214b using a 1D hydrodynamic, multi-species, full-atmosphere escape model. For the benchmark HD209458b, our previously validated solution reproduces the observed He I and Ha transit depths without imposing composition constraints. HD189733b exhibits comparable He I depths, but the broadest reported profiles require ~12 km/s of additional non-thermal broadening, whereas more recent measurements are narrower, consistent with our predictions. For HD149026b, despite similar system properties, our model shows that higher gravity suppresses escape and enhances diffusive separation, depleting helium at high altitudes and yielding extremely weak He I absorption. For the sub-Neptune GJ1214b, H/He-only models overestimate He I absorption; including H2 and its ions (H2+, H3+, HeH+) lowers the escape rate and modifies the ion/electron balance, reducing the metastable helium densities. Compared against scaling relations, HD189733b observations and our HD149026b prediction fall below the trend, whereas some observations of HD209458b and GJ1214b are consistent; however, the observed transit depths are variable. Across all targets, we find diffusive separation of helium and hydrogen, which may explain why sub-solar He/H ratios are often required in simplified models. We conclude that interpreting He I and Ha absorption requires first-principles models that include self-consistent temperature and velocity profiles, multi-species transport, and molecular chemistry.