The California-Kepler Survey. X. The Radius Gap as a Function of Stellar Mass, Metallicity, and Age

Abstract

In 2017, the California-Kepler Survey (CKS) published its first data release (DR1) of high-resolution optical spectra of 1305 planet hosts. Refined CKS planet radii revealed that small planets are bifurcated into two distinct populations, super-Earths (smaller than 1.5 R ⊕) and sub-Neptunes (between 2.0 and 4.0 R ⊕), with few planets in between (the “radius gap”). Several theoretical models of the radius gap predict variation with stellar mass, but testing these predictions is challenging with CKS DR1 due to its limited M ⋆ range of 0.8–1.4 M ⊙. Here we present CKS DR2 with 411 additional spectra and derived properties focusing on stars of 0.5–0.8 M ⊙. We found that the radius gap follows R p ∝ P m with m = −0.10 ± 0.03, consistent with predictions of X-ray and ultraviolet- and core-powered mass-loss mechanisms. We found no evidence that m varies with M ⋆. We observed a correlation between the average sub-Neptune size and M ⋆. Over 0.5–1.4 M ⊙, the average sub-Neptune grows from 2.1 to 2.6 R ⊕, following R p ∝ M ⋆ α with α = 0.25 ± 0.03. In contrast, there is no detectable change for super-Earths. These M ⋆–R p trends suggest that protoplanetary disks can efficiently produce cores up to a threshold mass of M c , which grows linearly with stellar mass according to M c ≈ 10 M ⊕(M ⋆/M ⊙). There is no significant correlation between sub-Neptune size and stellar metallicity (over −0.5 to +0.5 dex), suggesting a weak relationship between planet envelope opacity and stellar metallicity. Finally, there is no significant variation in sub-Neptune size with stellar age (over 1–10 Gyr), which suggests that the majority of envelope contraction concludes after ∼1 Gyr.

Transit Hunt for Young and Maturing Exoplanets (THYME). VIII. A Pleiades-age Association Harboring Two Transiting Planetary Systems from Kepler

“…The LAMOST (corrected) velocity is −32 ± 4 km s −1 , which is consistent with the value predicted for membership (;−27 km s −1 ). A more precise measurement from the CKScool project Petigura et al (2022) yielded an RV of −27.15 ± 0.10 km s −1 , an excellent match for the association. The corresponding high-resolution spectrum from Petigura et al (2022) shows weak lithium (<40 mÅ), but this is consistent with 100 Myr for its spectral type (Baraffe et al 2015).…”

“…A more precise measurement from the CKScool project Petigura et al (2022) yielded an RV of −27.15 ± 0.10 km s −1 , an excellent match for the association. The corresponding high-resolution spectrum from Petigura et al (2022) shows weak lithium (<40 mÅ), but this is consistent with 100 Myr for its spectral type (Baraffe et al 2015). As we show in Figures 1, 6, and 7, Kepler-970 lands on the expected CMD for a Pleiades-aged star, and is near other likely members both in Galactic position and proper motion.…”

Kepler and the Behemoth: Three Mini-Neptunes in a 40 Million Year Old Association

“…One specific question that seems within reach would be to clarify whether enough young stars have been searched for the dearth of young hot Jupiters to be significant. Since the hot Jupiter occurrence rate is strongly dependent on stellar mass and metallicity (Petigura et al 2018(Petigura et al , 2022, particular care would be needed to select a sample of well-studied FGK dwarfs for the measurement, likely using stars in Sco OB2, Cep-Her, and Orion. For demographic studies focused on how mini-Neptune sizes evolve, the combined K2 and Kepler dataset would be the better primary source.…”

MIRECLE: Science Yield for a Mid-IR Explorer-Class Mission to Study Non-Transiting Rocky Planets Orbiting the Nearest M-Stars Using Planetary Infrared Excess

“…At orbital periods less than 50 days, planets below 1.6 Earth radii are thought to be predominantly rocky (Rogers 2015), but it's not known if they formed that size or were once larger and subsequently lost their primordial atmospheres. The Kepler observatory has shown us that there is a gap in the occurrence rate of planets between 1.5 and 2.0 Earth radii (e.g., Fulton et al 2017;Petigura et al 2022). This paucity of planets supports the idea that a sub-population of close-in sub-Neptune planets undergo a relatively quick process of atmospheric mass loss that leaves them with a rocky core measuring up to 1.6 Earth radii and potentially a thin, high-meanmolecular-weight atmosphere (i.e., super Earths).…”