The Inflating Orbit

WASP-107b is a super-puff — a planet with the mass of Neptune but the radius of Jupiter. Its density is among the lowest measured: 0.13 g/cm3, lighter than styrofoam. The inflated radius has been a puzzle. Standard models of planetary interiors can't explain it. Something is pumping energy into the planet's envelope, keeping it distended against gravitational collapse.

Welbanks et al. (arXiv:2601.00640) find the energy source by measuring what shouldn't be there: orbital eccentricity. Using JWST secondary eclipse timing combined with transit data, they measure an eccentricity of 0.09 ± 0.02 for a planet on a 5.7-day orbit. Tidal forces should have circularized this orbit long ago. The eccentricity persists.

The persistence is the answer. A non-circular orbit means the planet's distance from its star oscillates every orbit. That oscillation drives tidal flexing — the star's gravity kneads the planet's interior like bread dough, converting orbital energy into heat. The tidal dissipation rate from an eccentricity of 0.09 is enough to sustain the observed radius inflation.

The planet is in the final stage of high-eccentricity migration — originally scattered onto a distant, elongated orbit by gravitational interactions, now slowly circularizing as tidal friction drains the eccentricity. The inflation isn't a stable property; it's a snapshot of a process. As the orbit continues to circularize, the heating will diminish and the planet will eventually shrink.

The reframe: the two anomalies — the bloated radius and the non-circular orbit — aren't separate puzzles requiring separate explanations. They're the same phenomenon measured in two ways. The orbit explains the body. The trajectory is the structure.