Mass Loss by Atmospheric Escape from Extremely Close-in Planets

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 (Mp ≳ 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.

The Upper Edge of the Neptune Desert Is Stable Against Photoevaporation

“…This process can lead to mass-loss rates far exceeding those from photoevaporation, and is predicted to be consequential for planets on exceptionally short-period orbits (Jackson et al 2017). While it likely plays a role for the closest-in Neptune desert planets, the RLO massloss rate drops off precipitously with orbital distance, so it cannot explain the entirety of the desert (Koskinen et al 2022).…”

“…If close-in gas-giant planets formed in situ (e.g., Batygin et al 2016) or arrived at their present-day locations relatively early (∼10 Myr) via disk migration (e.g., Ida & Lin 2008), they might have experienced a period of strong photoevaporation (e.g., Murray-Clay et al 2009). Hot Neptunes typically have lower gravitational potentials than their Jovian counterparts, making them more susceptible to photoevaporation and/or Roche lobe overflow (Kurokawa & Nakamoto 2014;Valsecchi et al 2015;Koskinen et al 2022). This means that Jupiters could survive at orbital separations where most Neptunes would be destroyed.…”

The Hubble/STIS Near-ultraviolet Transmission Spectrum of HD 189733b

“…The reference simula-tion, on which the spectrum in Figure 4 is based on, assumes an isolated planet. We also ran a second simulation that includes Roche lobe overflow (see, Koskinen et al 2022) but this had negligible effect on the modeled spectrum, in line with the expectation that Roche lobe overflow does not significantly enhance the escape rate or upper atmosphere densities on HD 189733 b.…”

The Colorado Ultraviolet Transit Experiment Mission Overview

“…Depending on the temperature profile in the atmosphere, species like Si, Mg, and Fe are expected to condense to form clouds in the lower atmosphere; however, the calculations indicate that strong mixing, either by turbulence or global circulation, can inhibit cloud formation or allow for these species to be present in the upper atmosphere where they can escape (Koskinen et al 2013b;Cubillos et al 2020;Koskinen et al 2022). The comparison of continuum and atomic line absorption therefore acts as a diagnostic of cloud formation, elemental abundances and mass loss on close-in exoplanets (Cubillos et al 2020;Lothringer et al 2020).…”