The Evolutionary Track of H/He Envelopes of the Observed Population of Sub-Neptunes and Super-Earths

• Published in: The Astrophysical journal Vol. 898; no. 2; pp. 104 - 109
• Main Authors: Estrela, Raissa, Swain, Mark R., Gupta, Akash, Sotin, Christophe, Valio, Adriana
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 01.08.2020; IOP Publishing
• Subjects: Astrophysics; Envelopes; Exoplanet atmospheres; Exoplanet structure; Exoplanets; Extrasolar planets; Extrasolar rocky planets; Mini Neptunes; Planetary evolution; Planetary mass; Planets; Stellar planets; Terrestrial planets
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/ab9a4d

The observational detection of a localized reduction in the small planet occurrence rate, sometimes termed a "gap," is an exciting discovery because of the implications for planet evolutionary history. This gap appears to define a transition region in which sub-Neptune planets are believed to have lost their H/He envelopes, potentially by photoevaporation or core powered mass loss, and have thus been transformed into bare core terrestrial planets. Here we investigate the transition between sub-Neptunes and super-Earths using a real sample of observed small close-in planets and applying envelope evolution models of the H/He envelope together with the mass-radius diagram and a photoevaporation model. We find that photoevaporation can explain the H/He envelope loss of most super-Earths in 100 Myr, although an additional loss mechanism appears necessary in some planets. We explore the possibility that these planets' families have different core masses and find a continuum in the primordial population of the strongly irradiated super-Earths and the sub-Neptunes. Our analysis also shows that close-orbiting sub-Neptunes with R < 3.5 R⊕ typically lose ∼30% of their primordial envelope.