Don't panic, but Earth is slowly losing its atmosphere - very slowly, at a rate so small that the Sun will balloon into a red giant and engulf our planet long before its atmosphere is whisked away. Atmospheric escape is an unavoidable reality for any planet encased in gas, but around our calm, middle-aged star, atmospheric loss is minimal.

Other planets in distant star systems are not as lucky: pummeled by fierce radiation and howling stellar winds, many exoplanets will lose most or all of their atmospheres. A planet's ability to hold on to its atmosphere shapes its habitability, the possibility of harboring long-lived oceans, and other critical factors. With exoplanet habitability a persistently hot topic in astronomy, it's critical to know which planets are most likely to have atmospheres.

What's Radius Got to Do with It?
To approach this question, a team led by Laura Chin (Boston University) investigated how a rocky exoplanet's size affects its ability to hang on to its atmosphere. Using fluid dynamics models, Chin and collaborators simulated the atmospheric loss due to stellar radiation and stellar winds. High-energy radiation ionizes atmospheric particles, allowing them to be picked up and carried away by the magnetized stellar wind, and imparts enough energy to some particles to allow them to escape the planet's gravitational pull.

The team modeled Venus-like exoplanets with radii ranging from half of Venus's radius (RV; roughly the size of Mars) to 2.25 RV (roughly the limit beyond which exoplanets are gaseous rather than rocky). Similar to Venus, the simulated planets do not have global magnetic fields, and their atmospheres are mainly carbon dioxide. Because the smallest stars in our galaxy - M dwarfs - are both the most numerous stars and are likely to host small, rocky planets, the team used stellar properties similar to those of M dwarfs: powerful, dense stellar winds and searing ultraviolet light.

A Surprising Finding
How quickly a planet without a global magnetic field loses its atmosphere through interactions with stellar winds is a combination of the planet's gravitational pull, which determines how snugly its atmosphere clings to the planet, and the planet's size, which determines the area that gets hit by the wind. Intriguingly, the combination of these factors creates a sweet spot for atmospheric escape: rocky planets with radii around 0.75 RV have the highest rates of atmospheric escape.

The increase in the escape rate for planets with radii from 0.5 to 0.75 RV appears to be driven by the increase in the planet's size; a larger planet's atmosphere absorbs more starlight (i.e., energy), creating more ions that can be stolen away by a magnetized stellar wind and boosting the escape rate. Above 0.75 RV, the decrease in the escape rate is driven by the increase in the planet's gravitational pull, which makes it harder for atoms to depart the atmosphere.

This result contrasts with expectations that the atmospheric escape rate is largest for the smallest planets. Future surveys of rocky planets around M-dwarf stars may be able to search for a connection between planet size and atmosphere thickness, testing this finding.

Research Report:"Role of Planetary Radius on Atmospheric Escape of Rocky Exoplanets,"

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