The Gaia-Kepler Stellar Properties Catalog. II. Planet Radius Demographics as a Function of Stellar Mass and Age

Abstract: Studies of exoplanet demographics require large samples and precise constraints on exoplanet host stars. Using the homogeneous Kepler stellar properties derived using Gaia Data Release 2 by Berger et al. (2020), we re-compute Kepler planet radii and incident fluxes and investigate their distributions with stellar mass and age. We measure the stellar mass dependence of the planet radius valley to be $d \log R_{\mathrm{p}}$/$d \log M_\star = 0.26^{+0.21}_{-0.16}$, consistent with the slope predicted by a planet mass dependence on stellar mass ($0.24-0.35$) and core-powered mass-loss (0.33). We also find first evidence of a stellar age dependence of the planet populations straddling the radius valley. Specifically, we determine that the fraction of super-Earths ($1-1.8 \mathrm{R_\oplus}$) to sub-Neptunes ($1.8-3.5 \mathrm{R_\oplus}$) increases from $0.61 \pm 0.09$ at young ages (< 1 Gyr) to $1.00 \pm 0.10$ at old ages (> 1 Gyr), consistent with the prediction by core-powered mass-loss that the mechanism shaping the radius valley operates over Gyr timescales. Additionally, we find a tentative decrease in the radii of relatively cool ($F_{\mathrm{p}} < 150 \mathrm{F_\oplus}$) sub-Neptunes over Gyr timescales, which suggests that these planets may possess H/He envelopes instead of higher mean molecular weight atmospheres. We confirm the existence of planets within the hot sub-Neptunian "desert" ($2.2 < R_{\mathrm{p}} < 3.8 \mathrm{R_\oplus}$, $F_{\mathrm{p}} > 650 \mathrm{F_\oplus}$) and show that these planets are preferentially orbiting more evolved stars compared to other planets at similar incident fluxes. In addition, we identify candidates for cool ($F_{\mathrm{p}} < 20 \mathrm{F_\oplus}$) inflated Jupiters, present a revised list of habitable zone candidates, and find that the ages of single- and multiple-transiting planet systems are statistically indistinguishable.

• The paper demonstrates a dependence of the planet radius valley on stellar mass with a quantified slope of 0.26, aligning with predictions from atmospheric loss models.
• The paper reveals an age-related trend with an increase in super-Earth prevalence over sub-Neptunes in systems older than 1 Gyr, supporting core-powered mass-loss processes.
• The paper employs rigorous data selection from Gaia DR2 and Kepler, ensuring reliable recalculations of planet radii and incident fluxes to assess planetary evolution.

Gaia-Kepler Stellar Properties Catalog II: Exoplanet Analysis

The study by Berger et al. presents a comprehensive investigation into the demographics of exoplanets using the Gaia-Kepler Stellar Properties Catalog. This paper focuses on the distributions of planet radii and incident fluxes as functions of stellar mass and age. The primary objectives are to analyze the dependence of the planet radius valley—a decrease in frequency at approximately 1.9 $R_{\oplus}$—on these stellar parameters, and to assess implications for theories such as photoevaporation and core-powered mass-loss.

Methodology

Utilizing data from Gaia DR2 and revised stellar properties from Kepler, the study recalculates planet radii and incident fluxes. The authors introduce a stringent selection process to ensure accuracy, including data cross-matching and exclusion criteria to preclude binary systems and unreliable measurements.

Results

One of the salient findings of the paper is the dependence of the planet radius valley on stellar mass. The authors quantify this effect with a slope of 0.26, with intervals that intersect with the predicted values from photoevaporation and core-powered mass-loss models, thus supporting both theories but not providing definitive proof for either.

A remarkable component of the study is the identification of an age-related trend in planet demographics. The paper reports a significant increase in the prevalence of super-Earths relative to sub-Neptunes as stellar systems age beyond 1 Gyr. The implication is consistent with predictions of core-powered mass-loss operating on Gyr timescales.

Additional Observations

• Stellar Evolution Impact: The study finds that planets within the so-called "hot sub-Neptunian desert" are frequently hosted by more evolved stars, suggesting post-formation orbital migration or changes due to stellar age.
• Inflated Jupiter Candidates: The research identifies possible "cool, inflated Jupiters" in systems with young stars, questioning the established paradigms on atmospheric retention and core cooling post-formation.
• Systems Evolution: The ages of multiple transiting and single transiting planet systems are reported to be statistically indistinguishable, indicating that dynamical interactions possibly occur on much shorter timescales.

Implications

The findings significantly contribute to the understanding of how planetary characteristics are influenced by stellar properties, particularly providing insights into the evolutionary processes shaping planetary systems. Notably, the age correlations found align well with the core-powered mass-loss hypothesis, suggesting that Gyr-scale internal processes in planets play a crucial role in their observed state. However, the continuous overlap of observational evidence with photoevaporation demands further inquiry and data precision.

Conclusion

The research presented sets a firm groundwork to further explore planetary system evolution via both empirical data and theoretical modeling. As the authors suggest, future advancements in stellar characterization and detailed observations from upcoming missions such as TESS could enhance our understanding and discriminator between the competing theories on planetary demographics. This work underscores the need for large, precise datasets to resolve long-standing questions about exoplanet formation and evolution.