Exoplanetology: Exoplanets & Exomoons

Understanding What Helium Absorption Tells Us About Atmospheric Escape From Exoplanets

By Keith Cowing

Status Report

astro-ph.EP

January 15, 2025

Filed under astro-ph.EP, atmosphere, atmospheric escape, exoplanet, helium

Understanding What Helium Absorption Tells Us About Atmospheric Escape From Exoplanets — Top: Scaled equivalent width as a function of mass-loss rate, for Jupiters, Neptunes and Sub-Neptunes. Each dot corresponds to an exoplanet with a mass and radius randomly selected amongst the values in Table 1. A random stellar spectra and orbital separation are assigned to each planet. The colours scale with the ratio of XEUV to FUV fluxes and the marker size scales with the planet’s gravity. Following the convention introduced in Figure 7, squares indicate those planets for which 𝑅XUV is larger than the hot sonic radius, while diamonds represent planets for which 𝑅XUV is larger than the cold sonic radius (see Sections 3.2 and 4.1 for more details). The red horizontal line indicates a typical detection threshold (see text for the details on the calculation). Bottom: Histogram of the frequency of exoplanets found around each stellar type (M, K, G and F). The blue columns indicate the total number of exoplanets, while the red columns show the number of detected exoplanets, meaning those ones above the detection threshold. — astro-ph.EP

Atmospheric escape is now considered the major contributing factor in shaping the demographic of detected exoplanets.

However, inferences about the exoplanet populations strongly depend on the accuracy of the models. Direct observational tests of atmospheric models are still in their infancy. Helium escape from planetary atmospheres has rapidly become the primary observational probe, already observed in ≳20 exoplanets.

Grounding our understanding in the basic physics of atmospheric escape, we present a new theoretical model to predict the excess absorption from the helium absorption line. We constrain the atmosphere properties, such as mass-loss rates and outflow temperatures, by implementing a Parker wind solution with an energy limited evaporating outflow.

Importantly, we self-consistently link the mass-loss rates and outflow temperatures, which are critical to understanding helium absorption as the triplet-level population is typically exponentially sensitive to temperature. Furthermore, helium absorption is typically optically thin and the absorption is dominated far from the planet.

Therefore, the absorption depth is not a measure of the size of the helium outflow. Our results indicate that for planets with a detectable signal, typically the helium triplet population in the atmosphere rapidly approaches a statistical equilibrium between populations by recombination and depopulation caused by electron collisions.

We suggest that excess helium absorption can be quantified by a scaled equivalent width, which is positively correlated with the mass loss rate. We also show that the helium absorption scales with incident radiation, particularly with the XEUV to FUV flux ratios.

Giulia Ballabio, James E. Owen

Comments: 15 pages, 12 figures, accepted for publication by MNRAS
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2501.06149 [astro-ph.EP] (or arXiv:2501.06149v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2501.06149
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Submission history
From: Giulia Ballabio Dr
[v1] Fri, 10 Jan 2025 18:19:50 UTC (16,903 KB)
https://arxiv.org/abs/2501.06149
Astrobiology,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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