[P-CG19] Exoplanet

[PCG19-12] On the Physics of Atmospheric Escape

★Invited Papers

*Antonio Garcia Munoz¹ (1.CEA Paris-Saclay)

Keywords:Exoplanets, Atmospheres, Escape, Radiation, Solar system

The evolution of a planet is partly dictated by how much of its atmosphere is lost to space over timescales of Gyears. Considerations of this type have been fundamental to retrace the history since formation of the solar system planets. The discovery in the last three decades of more than 5000 exoplanets, many of them with masses and sizes (and presumably compositions) very unlike those of the solar system planets, has brought atmospheric escape back onto the spotlight. Conveniently, many close-in exoplanets are losing their atmospheres in ways that can be probed as it occurs through a variety of diagnostic spectral lines. Our understanding of planet evolution depends strongly on the connection between such observations and the complex physics that occurs at high altitude in the atmospheres, and for which we often lack any other information. Establishing this connection in a physically-motivated way is particularly timely as a new generation of telescopes and instruments is enabling the characterization of ever smaller exoplanets susceptible to catastrophic mass loss. This review will cover the physical principles of atmospheric escape and how they are affected by both the planet and stellar properties. I will discuss the main diagnostic lines that are used to probe atmospheric escape from H/He-dominated atmospheres with space-borne telescopes (typically, far-ultraviolet lines that include H I Lyman-α, resonance lines of O I and C II, and lines of heavier metals such as Mg and Fe) and with ground-based telescopes (more prominently, lines in the Balmer and Paschen series of H I, and the He I triplet at 1.08 μm). I will elaborate on the significance of these detections to constrain the dynamics and energy budget of the low-density atmospheres, and touch upon concepts such as chemical disequilibrium, opacity and mass fractionation. I will then use these ideas to delve into the more complex problem of atmospheric escape from non-H/He-dominated atmospheres, which are notably more challenging to characterize observationally. For them, the presence of e.g. H₂O or CO₂ molecules introduces new possibilities such as non-local-thermodynamic equilibrium in rovibrational bands that may affect the energy budget and therefore the long-term stability of the atmospheres. I will finish the review with a perspective of where we stand “model-wise”, and how we can benefit from other astrophysical fields, and what we can expect to learn about atmospheric escape and planetary evolution in general from ongoing and future observational surveys.