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Title: Does the radius valley form in 100 Myr or over billions of years?
Plain: Photoevaporation should finish stripping planets within about 100 million years, while core-powered loss takes about a billion. Comparing young and old planet populations can separate them.
Precise: Measure the ratio of sub-Neptunes to super-Earths and the gap location in clusters, associations and field stars with ages $10^7-10^10\ \mathrm{yr}$. Photoevaporation predicts the ratio settles by $\sim 10^8\ \mathrm{yr}$; core-powered mass loss predicts evolution until $\sim 1-2x10^9\ \mathrm{yr}$. The answer is the measured time dependence with enough planets per age bin to distinguish the two.
Settled by: Ages from cluster membership, gyrochronology or asteroseismology for several hundred small planets with precise radii.
Kind: well-posed
Answer type: value
References: (empty)
Literature status: open
Status note: Analyses of 2020-2023 (Berger et al. 2020; David et al. 2021; Christiansen et al. 2023) report the small-planet population changing over Gyr timescales, which favors core-powered mass loss; samples of planets younger than $10^{8}\,\mathrm{yr}$ remain small.
Posed since: (empty)