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

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2020
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Astrophysical Journal
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7
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Estrela R.
Swain M.R.
Gupta A.
Sotin C.
Valio A.
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© 2020. The American Astronomical Society. All rights reserved..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.
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