Atmospheric evaporation key to planetary evolution
While we are constantly learning more about the processes that lead to planet formation, many aspects remain unclear. This is especially true for exoplanets – planets that orbit around stars other than the sun – which appear to have volatile atmospheres. Close proximity to stars makes exoplanet upper atmospheres vulnerable to mass-loss through evaporation caused by heating. “At the start of the project, atmospheric evaporation was understood almost exclusively for hydrogen (H) and helium (He)-rich envelopes, with no reliable models for highly irradiated or heavy-element-dominated atmospheres,” explains PEVAP project coordinator James Owen from Imperial College(opens in new window) of Science, Technology and Medicine in the United Kingdom. “We therefore lacked both a predictive physical theory of evaporation and a way to use it as a diagnostic of how and where planets formed. The PEVAP project was designed to close precisely these gaps.”
Next-generation evaporation models
To achieve its aims, PEVAP, which was supported by the European Research Council(opens in new window), brought together expertise in planetary hydrodynamics, radiative transfer, stellar high-energy emission and exoplanet demographics, centred at Imperial. “Methodologically, we developed next-generation evaporation models,” says Owen. “These were used to solve the coupled hydrodynamics, thermodynamics and chemistry of planetary outflows (i.e. atmospheric escape).” In parallel, the project team linked these models to planetary analyses. Using large suites of evolutionary calculations, the team predicted how evaporation might influence the distribution of planets. “This approach made it possible to use evaporation as a powerful probe of planet formation,” explains Owen.
Diagnostics of past atmospheric escape
A key result has been to show how evaporation can quantitatively explain the detailed structure of the close-in exoplanet population. “We showed that the relative numbers of super-Earths (exoplanets with masses higher than Earth’s but substantially below those of Neptune or Uranus) can be reproduced if many close-in planets formed with modest, H/He envelopes on solid cores,” notes Owen. “More broadly, we have provided the first robust constraints on the birth distribution of core masses and envelope fractions for close-in planets, placing limits on where they must have accreted their gas and how much migration is allowed,” adds Owen. “Together, these findings turn the observed exoplanet population into a calibrated diagnostic of past atmospheric escape.”
Exploring connections to planetary habitability
The project has been successful in reframing atmospheric evaporation as a key tool for understanding planet formation, and showing that many close-in super-Earths are the stripped remnants of once sub-Neptune-like planets. “This has clarified that the dominant mode of formation for close-in planets involves modest gas accretion followed by strong high-energy irradiation, rather than in situ formation of bare rocky planets like the Solar System’s terrestrials,” says Owen. A key next step will be to generalise the evaporation–inference framework to a wider range of interstellar objects. “Extending our models to heavy-element-dominated and water-rich atmospheres will allow us to interpret upcoming James Webb Space Telescope(opens in new window) and Ariel(opens in new window) data in a physically consistent evolutionary context,” notes Owen. Owen and his team also plan to combine improved evaporation models with formation simulations, tightening constraints on where planets formed relative to ice lines and how much solid and gas they accreted. “A further avenue is to explore connections to planetary habitability,” Owen adds. “By mapping where evaporation removes primordial envelopes but allows secondary atmospheres to persist, we can identify regimes favourable to temperate, potentially rocky worlds.”
