The radius valley — a deficit of planets between roughly 1.5 and 2 Earth radii — was supposed to mark a clean boundary: rocky planets below, volatile-rich planets above. Photoevaporation or core-powered mass loss strips smaller planets of their atmospheres, leaving bare rocky cores, while larger planets retain their gas envelopes. The picture predicts two populations with a gap between them.
Palethorpe and colleagues measure masses and radii across six multi-planet systems with precisions better than 15% and 5% respectively, using CHEOPS, K2, TESS, and ground-based radial velocities. What they find is not two populations but a continuum. Rocky worlds sit at the valley's lower edge. Above them, intermediate-density planets require volatile envelopes to explain their bulk densities — but the envelopes are not uniform. Some are consistent with thin hydrogen-helium atmospheres. Others require water-rich compositions — “steam worlds” with volatile fractions too large for accreted gas alone. Still others are low-density water-worlds that challenge both the photoevaporation and core-powered mass loss mechanisms.
Several systems show radial-velocity evidence for additional planets that don't transit — companions detectable gravitationally but invisible photometrically, whose compositions remain unconstrained. The observed population is already a continuum; the unobserved population could fill it further.
The clean dichotomy was an artifact of measurement precision. With 50% mass uncertainties, a rocky planet and a water-world look the same. At 15%, they separate — but they separate into a spectrum, not into two bins. The categories were real at the resolution where they were defined. Higher resolution didn't sharpen the boundary; it dissolved it.