Year: 2022
Type: article
Abstract: We explore atmospheric escape from close-in exoplanets with the highest mass-loss rates. First, we locate the transition from stellar X-ray and UV-driven escape to rapid Roche lobe overflow, which occurs once the 10–100 nbar pressure level in the atmosphere reaches the Roche lobe. Planets enter this regime when the ratio of the substellar radius to the polar radius along the visible surface pressure level, which aligns with a surface of constant Roche potential, is X/Z ≳ 1.2 for Jovian planets ( M p ≳ 100 M ⊕ ) and X/Z ≳ 1.02 for sub-Jovian planets ( M p ≈ 10–100 M ⊕ ). Around a Sun-like star, this regime applies to orbital periods of less than two days for planets with radii of about 3–14R ⊕ . Our results agree with the properties of known transiting planets and can explain parts of the sub-Jovian desert in the population of known exoplanets. Second, we present detailed numerical simulations of atmospheric escape from a planet like Uranus or Neptune orbiting close to a Sun-like star that support the results above and point to interesting qualitative differences between hot Jupiters and sub-Jovian planets. We find that hot Neptunes with solar-metallicity hydrogen and helium envelopes have relatively more extended upper atmospheres than typical hot Jupiters, with a lower ionization fraction and higher abundances of escaping molecules. This is consistent with existing ultraviolet transit observations of warm Neptunes, and it might provide a way to use future observations and models to distinguish solar-metallicity atmospheres from higher-metallicity atmospheres.
Source: The Astrophysical Journal
Authors: Tommi T. Koskinen, Panayotis Lavvas, Chenliang Huang, Galen Bergsten, Rachel B. Fernandes, Mitchell E. Young
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $4,499
Awards: G5492458037, THE DISCOVERY OF EXOPLANETS HAS LED TO THE DEVELOPMENT OF SOPHISTICATED MODELS BY NUMEROUS GROUPS FOR THEIR LOWER AND UPPER ATMOSPHERES; HOWEVER MIDDLE ATMOSPHERES HAVE RECEIVED SCANT ATTENTION DESPITE THEIR IMPORTANCE TO ATMOSPHERIC CHEMISTRY ESCAPE AND EVOLUTION AND TO THE INTERPRETATION OF EXOPLANET OBSERVATIONS. AS ON SOLAR SYSTEM PLANETS WE EXPECT THE THERMAL STRUCTURE OF MIDDLE ATMOSPHERES OF EXOPLANETS TO BE DOMINATED BY RADIATIVE BALANCE; HOWEVER RADIATIVE TRANSFER IN THE MIDDLE ATMOSPHERE IS COMPLICATED BY THE BREAKDOWN OF LTE WHICH AFFECTS BOTH THE THERMAL COOLING RATES AND THE THERMALIZATION OF ABSORBED STELLAR RADIATION. MOREOVER THE UPPER LEVELS OF THE MIDDLE ATMOSPHERE ARE INCREASINGLY AFFECTED BY THERMAL CONDUCTION OF ENERGY DOWNWARD FROM THE HOT UPPER ATMOSPHERE. WE PROPOSE TO DEVELOP MODELS FOR THE ENERGY BALANCE IN EXOPLANET MIDDLE ATMOSPHERES THAT PROPERLY INCORPORATE NON-LTE POPULATIONS OF MOLECULAR ENERGY LEVELS. THE CALCULATED TEMPERATURE PROFILES WILL BE CONSTRAINED BY EXOPLANET TRANSIT OBSERVATIONS PARTICULARLY OF THE STRONG ALKALI LINES THAT PROBE THE MIDDLE ATMOSPHERE AND ARE SENSITIVE TO THE TEMPERATURE PROFILE. THE EFFECT OF THE TEMPERATURE PROFILES ON PHOTOCHEMISTRY WHICH OCCURS PRIMARILY IN THE MIDDLE ATMOSPHERE WILL BE INVESTIGATED. THE IMPORTANCE OF THE MIDDLE ATMOSPHERE THERMAL STRUCTURE FOR ATMOSPHERIC ESCAPE THROUGH DISSOCIATION OF H2 AND OTHER MOLECULES IN THE MIDDLE ATMOSPHERE WILL BE DETERMINED AS WELL AS THE IMPACT ON THE PRODUCTION OF HAZES AND HIGH ALTITUDE CLOUDS., G6686480456, G7719629023
OpenAlex ID: W4221154330