Exoplanet radius valley

Small planets close to their stars often fall into two size groups. What creates the gap?[1] [2]

Small planets near their stars cluster into two size groups with a relative shortage in between. Did gas loss separate initially similar planets, or did they form with different rock and water inventories? High-energy starlight can power atmospheric escape, called photoevaporation; a cooling planet can also supply energy for core-powered loss. Mixed rocky and water-rich progenitors offer another explanation. The question is how much each contributes, rather than whether either mechanism can make a gap in a model.[1] [2]

Losing gas may matter, but different starting compositions may matter too.[1] [2]

The March 2026 stellar-population analysis favors core-powered loss in its selected Kepler sample, but age groups mix different stellar populations. February simulations link a peak in orbital eccentricity to mixed progenitors under idealized mass and architecture assumptions; unseen companions also affect orbital interpretation. These are different tests, not a unique causal mixture established for all systems.[1] [2]

A planet’s size can reflect both its interior and a surrounding gas layer. The gap offers clues to how planets acquire and lose material, so two planets of different sizes need not have been born with equally different interiors.[1] [2]

A size histogram can make distinct histories look alike. Combining sizes with stellar ages and orbital shapes turns it into a test of where planets assembled and when their envelopes changed, rather than assuming that every planet in the larger size group has the same composition.[1] [2]