Evolution of the Exoplanet Size Distribution: Forming Large Super-Earths Over Billions of Years

Abstract

The radius valley, a bifurcation in the size distribution of small, close-in exoplanets, is hypothesized to be a signature of planetary atmospheric loss. Such an evolutionary phenomenon should depend on the age of the star–planet system. In this work, we study the temporal evolution of the radius valley using two independent determinations of host star ages among the California–Kepler Survey (CKS) sample. We find evidence for a wide and nearly empty void of planets in the period–radius diagram at the youngest system ages (≲2–3 Gyr) represented in the CKS sample. We show that the orbital period dependence of the radius valley among the younger CKS planets is consistent with that found among those planets with asteroseismically determined host star radii. Relative to previous studies of preferentially older planets, the radius valley determined among the younger planetary sample is shifted to smaller radii. This result is compatible with an atmospheric loss timescale on the order of gigayears for progenitors of the largest observed super-Earths. In support of this interpretation, we show that the planet sizes that appear to be unrepresented at ages ≲2–3 Gyr are likely to correspond to planets with rocky compositions. Our results suggest that the size distribution of close-in exoplanets and the precise location of the radius valley evolve over gigayears.

TOI-2076 and TOI-1807: Two Young, Comoving Planetary Systems within 50 pc Identified by TESS that are Ideal Candidates for Further Follow Up

“…Notably, the theoretical models mentioned also predict larger sizes for sub-Neptunes at earlier times (particularly in the first 100 Myr). Efforts to age-date known exoplanet host stars are providing emerging evidence that the size distribution of small planets continues to evolve over billions of years (Berger et al 2020;Sandoval et al 2021) and that the precise location of the radius gap evolves on similar timescales (David et al 2021). These results are broadly consistent with expectations from the photoevaporation and core-powered mass-loss models.…”

Estimating photoevaporative mass loss of exoplanets with PLATYPOS

“…Planets tend to group into two distinct populations, the so-called super-Earths and sub-Neptunes, with a significant dearth of planets with intermediate radii around 2 R ⊕ . This had been expected from theoretical studies (Lopez et al, 2012;Owen & Wu, 2013) before being observed (David et al, 2021;Fulton et al, 2017;Van Eylen et al, 2018).…”

Creating the Radius Gap without Mass Loss

“…Although gas accretion alone is able to reproduce the observed radius gap, it is likely that some planets, especially those at short orbital periods, undergo post-accretion mass loss, further sculpting the exoplanet radius distribution. In fact, observations suggest a change in the radius distribution of exoplanets across ∼1-2 Gyrs (e.g., Berger et al 2020;David et al 2021).…”

“…It is generally expected that photoevaporative mass loss carves out the gap in the first ∼100 Myrs when the stars remain active whereas core-powered envelope mass loss is expected to carve out the gap over a longer ∼Gyr timescale. There is observational evidence that the radius distribution shifts at a statistically significant level across Gyr timescales (e.g., Berger et al 2020;David et al 2021). However, EUV luminosity is expected to decline more slowly compared to X-ray so that photoevaporative mass loss may operate over ∼Gyrs as well (e.g., King & Wheatley 2021).…”