The Radius Valley Is a Fossil of Timing

01The evidence

• Exoplanet statistics identify photoevaporation driven by high-energy stellar photons as the leading explanation for the radius valley, with core-powered mass loss as the main alternative.Exoplanet statistics and theoretical implications
• Well-characterised planets in the radius-density plane show the gap clearly, and its sharpness is one of the strongest constraints on any stripping model.Radius-density plane

02The tension

Steady-state stripping models reproduce the valley by tuning the efficiency of mass loss. That tuning is not independently constrained, and the models predict a valley at a fixed radius regardless of the system's history, which makes the sharpness a coincidence of parameters rather than a consequence.

03The theory

Photoevaporation only works while the planet is still losing mass efficiently, which is the first few hundred million years, and only if the disk is gone so that the atmosphere is not replenished. The window is narrow and its timing depends on the star.

If a planet's envelope is stripped during the window, it ends up as a bare core below the valley. If it is not, it keeps a thin envelope above. The valley is the boundary between these fates, and its position is set by the timing of the window, not by a balance between loss and supply.

That makes the valley an age-dependent feature. Young clusters should show a valley at a different radius and with a different depth than old ones, because the transition is still happening. The valley should also be sharper in coeval populations and blurrier in mixed-age samples.

The prediction is checkable with existing survey data by binning in age, which is the kind of test that a steady-state model cannot pass by tuning because it predicts no age dependence at all.

04What this is not

05Predictions

• P1The valley radius should differ measurably between hundred-million-year and billion-year populations.
• P2The valley should deepen with age as the transition completes.
• P3The valley's slope should track the star's high-energy output history rather than a single efficiency parameter.

06What would refute it

A valley at the same radius and depth across ages, at fixed detection sensitivity, would restore the steady-state picture.

07What it would change

It would make the valley a chronometer for atmospheric loss and change how planet populations are compared across clusters.