Insights into the Exoplanet Radius Valley from Host-Star Ages, Activity, Chemistry, and Birth Radius

Abstract

The radius valley, a bimodal feature in the size distribution of close-in small exoplanets, is widely interpreted as a signature of atmospheric loss and therefore provides a key constraint on the formation and atmospheric evolution of these planets. We investigate its dependence on host-star properties using 769 planets orbiting 558 stars, for which we derive stellar ages, chromospheric activity, and Galactic birth radius, together with elemental abundances. We find that the radius valley is not fully established at ages $\sim 3$ Gyr and evolves over gigayear timescales, with its prominence strongly affected by stellar population mixing. The dependence on magnetic activity is non-monotonic: a clear valley is present even among magnetically quiet stars, while highly active systems do not show a systematically stronger depletion. The valley morphology also varies with stellar composition: the valley is strongest in metal-poor stars, weakens near solar metallicity, and partially strengthens again at the highest metallicities. In addition, the valley shows sensitivity to refractory element ratios such as [Mg/Si], while correlations with [C/O] are weaker, indicating a dependence on planetary interior structure. Our results are more consistent with a dominant role for core-powered atmospheric mass loss than with purely irradiation-driven photoevaporation. Finally, the radius valley also depends on the Galactic birth environment, with systems near the estimated solar birth radius $\sim 4.5$ kpc showing a high fraction of Earth-like planets and a well-defined bimodal structure, suggesting that the Solar System formed in a region with a well-developed Earth-sized planet population.

Figures (17)

• Figure 1: Comparison of stellar luminosities derived using different extinction maps. (a) One-to-one comparison of luminosities in logarithmic scale. (b) Probability density distributions of relative luminosity uncertainties.
• Figure 2: Comparison of stellar ages derived in this work and in C25. (a) One-to-one comparison of stellar ages. The dashed line indicates equality. The average age difference is $\sim$0.5 Gyr, with our ages being younger. (b) Probability density distributions of relative age uncertainties, showing overall consistency, with most stars having uncertainties below 40% and a peak near 10%.
• Figure 3: Comparison of planetary radius derived in this work and from the Kepler DR25 catalogue. (a) One-to-one comparison of planetary radius. The dashed line indicates equality. (b) Probability density distributions of relative radius uncertainties.
• Figure 4: Comparison of planetary radius distributions for our sample and previous studies. All samples are restricted to $R_p < 4\,R_\oplus$. Histograms show the counts of planets in each bin, while dashed lines indicate KDE-based smoothed distributions.
• Figure 5: Planet radius-period distribution for the full sample. The blue dashed line shows the support vector machine (SVM) fit to the radius valley, and the shaded blue region indicates the $1\sigma$ uncertainty from bootstrap resampling. Red dotted lines mark $\pm0.2\,R_\oplus$ around the fit, defining the valley region used to classify planets within the valley. Contours represent the two-dimensional density estimated via a kernel density estimator (KDE), highlighting regions where planets are more densely populated.