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

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 ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:msubsup> <mml:mrow> <mml:mi>R</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">HK</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> </mml:mrow> </mml:msubsup> </mml:math> ), and Galactic birth radii, together with elemental abundances. We find that the radius valley is not fully established at ages ≲ 3 Gyr and evolves over gigayear timescales, with its prominence strongly affected by stellar population mixing. The dependence on magnetic activity is nonmonotonic: a clear valley is present even among magnetically quiet stars, while highly active systems do not show 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 ( R birth ≃ 4.5 ± 0.4 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.