Creating the Radius Gap without Mass Loss

Abstract

The observed exoplanet population features a gap in the radius distribution that separates the smaller super-Earths (≲1.7 Earth radii) from the larger sub-Neptunes (∼1.7–4 Earth radii). While mass-loss theories can explain many of the observed features of this radius valley, it is difficult to reconcile them with the potentially rising population of terrestrials beyond orbital periods of ∼30 days. We investigate the ability of gas accretion during the gas-poor phase of disk evolution to reproduce both the location of the observed radius gap and the existence of long-period terrestrial planets. Updating the analytic scaling relations of gas accretion rate accounting for the shrinking of the bound radius by hydrodynamic effects and deriving a more realistic disk temperature profile, we find that the late-stage gas accretion alone is able to carve out the observed radius gap, with slopes R gap ∝ P −0.096 and R gap ∝ M ⋆ 0.15 for top-heavy; and R gap ∝ P −0.089 and R gap ∝ M ⋆ 0.22 for bottom-heavy core mass distributions, in good agreement with observations. The general morphology of the primordial radius gap is stable against a range of disk gas density and disk accretion rate with the latter affecting mostly the population of large planets (≳3–4 R ⊕). The peaks and valleys in the radius distribution were likely set in place primordially while post-formation mass loss further tunes the exoplanetary population. We provide potential observational tests that may be possible with TESS, PLATO, and Roman Space Telescope.

Small Planets around Cool Dwarfs: Enhanced Formation Efficiency of Super-Earths around M Dwarfs

“…In general, we predict the fraction of disks that can create inner planets up to their isolation masses 1-2M ⊕ to drop around stars M å < 0.3-0.5M e ; it remains possible to create smaller cores. Because this is about the mass below which short-period planets would emerge rocky (Lee & Connors 2021;Lee et al 2022), our model expects a fall in the occurrence rate of mini-Neptunes around low-mass stars but not necessarily that of terrestrial planets, in line with recent observational evidence (e.g., Brady & Bean 2022;Ment & Charbonneau 2023). A caveat to this statement is that we have limited our analysis to pebble accretion, which sets the initial mass of planetary cores.…”

Detection of Atmospheric Escape from Four Young Mini-Neptunes

“…A now-famous "radius gap" or "Fulton gap" (Fulton et al 2017;Fulton & Petigura 2018) separates the smaller, denser super-Earths (1-1.7 R ⊕ ) from the larger, less dense mini-Neptunes (2-3 R ⊕ ). The conventional explanation for this gap is that mini-Neptunes have a primordial hydrogen/helium atmosphere comprising ∼1% of their total mass that significantly inflates their radii, whereas super-Earths have either lost their hydrogen-rich atmospheres or never acquired them in the first place (e.g., Lee & Connors 2021;Lee et al 2022). If most super-Earths initially formed with hydrogen-rich envelopes, they could have been stripped away by intense X-ray and extreme UV (XUV) irradiation from the young star (photoevaporative mass loss; e.g., Owen & Wu 2017;Mills & Mazeh 2017).…”

Conclusive Evidence for a Population of Water Worlds around M Dwarfs Remains Elusive

“…Two successful mass-loss models are X-ray or extreme ultraviolet (EUV) photoevaporation, which relies on high-energy stellar flux (e.g., Owen & Wu 2013;Lopez & Fortney 2013), and core-powered mass loss, which calls upon remnant thermal energy from formation and bolometric stellar luminosity (e.g., Ginzburg et al 2018;Gupta & Schlichting 2019). Other models may also explain the radius gap via atmospheric escape due to giant impacts (e.g., Inamdar & Schlichting 2016;Wyatt et al 2020) or through gaseous accretion of primordial atmospheres (e.g., Lee & Connors 2021;Lee et al 2022).…”