Hot Jupiter radius inflation

Why are some gas giants close to their stars bigger than expected? Extra heat or slower cooling could keep them swollen.[1]

Some giant planets close to their stars remain wider than cooling models predict for their mass, age and composition. What keeps their interiors warm enough to maintain that size?

Deep heating supplies new energy below the outer layers. Delayed cooling instead reduces the rate at which existing internal heat escapes. These processes can coexist, but they need not produce the same evolution: a planet that receives sufficiently deep energy can grow again as its star brightens, while merely preserving old heat does not by itself reverse contraction.[1] [2]

Starlight matters, but observations have not pinned down how it changes the deep interior.[1] [2]

The 2026 analysis of 328 planets finds statistical connections between radius, orbital period and stellar temperature after accounting for other measured variables. Comparisons with theoretical scalings narrow candidate pathways; they do not measure heat deposition or supply complete causal histories. Distinguishing mechanisms still requires tests controlling for received starlight, age, composition and selection effects, and more than one mechanism may operate.[1] [2]

A planet’s size reveals how it gains and loses heat.[1]

A large radius can encode ongoing heating or a history of unusually slow cooling. Distinguishing those accounts changes what we infer about interiors from observed sizes. Comparing planets around stars at different stages of brightening offers another test of atmosphere–interior energy exchange, beyond fitting one radius at one time.[1]