Signatures of the core-powered mass-loss mechanism in the exoplanet population: dependence on stellar properties and observational predictions

Abstract

Recent studies have shown that atmospheric mass-loss powered by the cooling luminosity of a planet's core can explain the observed radius valley separating super-Earths and sub-Neptunes, even without photoevaporation. In this work, we investigate the dependence of this core-powered mass-loss mechanism on stellar mass (M * ), metallicity (Z * ) and age (τ * ). Without making any changes to the underlying planet population, we find that the core-powered mass-loss model yields a shift in the radius valley to larger planet sizes around more massive stars with a slope given by d logR p /d logM * 0.35, in agreement with observations. To first order, this slope is driven by the dependence of core-powered mass-loss on the bolometric luminosity of the host star and is given by d logR p /d logM * (3α − 2)/36 0.36, where (L * /L ) = (M * /M ) α is the stellar mass-luminosity relation and α 5 for the CKS dataset. We therefore find, contrary to photoevaporation models, no evidence for a correlation between planet and stellar mass. In addition, we show that the location of the radius valley is, to first order, independent of stellar age and metallicity. In contrast, assuming that the atmospheric opacity scales linearly with stellar metallicity, we determine that that the size of sub-Neptune population increases with metallicity and decreases with age with a slope given by d logR p /d logZ * 0.1 and d logR p /d logτ * −0.1, respectively. This implies that the abundance of super-Earths relative to sub-Neptunes increases with age but decreases with stellar metallicity. We conclude with a series of observational tests that can differentiate between core-powered mass-loss and photoevaporation models.

WASP-107b’s Density Is Even Lower: A Case Study for the Physics of Planetary Gas Envelope Accretion and Orbital Migration

“…Our constraints on the envelope metallicity therefore rule out the dusty accretion scenario at any orbital distance in disks with uniform dust-to-gas mass ratio. Furthermore, as mentioned above (Section 5.4), the observationally inferred GCR provides only a lower limit on the amount of gas accreted by the core, because mass loss powered by stellar irradiation and the planet's leftover heat from formation can pare down the envelope mass fraction and is believed to sculpt the radius distribution of small planets (Owen & Wu 2017;Ginzburg et al 2018;Gupta & Schlichting 2020). Reaching a higher GCR at formation would necessitate an even higher metallicity.…”

The New Generation Planetary Population Synthesis (NGPPS)

“…While originally explained by photoevaporation, alternative processes have also been hypothesized to produce the pattern. Debated mechanisms include core-powered mass loss, where the core's internal luminosity removes the planetary atmosphere (Ginzburg et al 2016(Ginzburg et al , 2018Gupta & Schlichting 2020); different formation pathways of planets above and below the gap (Zeng et al 2019); and planetesimal impacts (e.g., Liu et al 2015;Wyatt et al 2019).…”

A super-Earth and a sub-Neptune orbiting the bright, quiet M3 dwarf TOI-1266

“…We note, however, that the origin for the radius valley feature is currently unconstrained and that other studies advocate, for instance, for core-powered mass loss as the driving mechanism for this pattern A49, page 16 of 21 B.-O. Demory et al: A super-Earth and a sub-Neptune orbiting the M3V TOI-1266 (Ginzburg et al 2018;Gupta & Schlichting 2020). The recent discovery of a change in the fraction of planets above and below the valley over ∼ Gyr timescales has indeed been interpreted in favour of the core-powered mass loss scenario for some solartype stars (Berger et al 2020), although there is evidence that the formation pathway may be different in low-mass stars (Cloutier & Menou 2020).…”