Mass Loss by Atmospheric Escape from Extremely Close-in Planets

We explore atmospheric escape from close-in exoplanets with the highest mass loss rates. First, we locate transition stellar X-ray and UV-driven to rapid Roche lobe overflow, which occurs once 10-100 nbar pressure level in atmosphere reaches lobe. Planets enter this regime when ratio of substellar radius polar along visible surface level, that aligns a constant potential, is X/Z~$\gtrsim$~1.2 for Jovian planets (Mp~$\gtrsim$~100 M$_{\Earth}$) X/Z~$\gtrsim$~1.02 sub-Jovian ($M_p \approx$~10--100 M$_{\Earth}$). Around sun-like star, applies orbital periods less than two days radii about 3--14 R$_{\Earth}$. Our results agree properties known transiting can explain parts desert population exoplanets. Second, present detailed numerical simulations planet like Uranus or Neptune orbiting close star support above point interesting qualitative differences between hot Jupiters planets. find Neptunes solar metallicity hydrogen helium envelopes have relatively more extended upper atmospheres typical Jupiters, lower ionization fraction higher abundances escaping molecules. This consistent existing ultraviolet transit observations warm it might provide way use future models distinguish atmospheres.