The Planet Cliff

Planets grow by accreting solid material — pebbles in the protoplanetary disk that drift inward and are captured by a growing core’s gravitational influence. As the core grows, it eventually reaches the pebble isolation mass: the mass at which it carves a gap in the disk that blocks the inward drift of pebbles. At isolation, the core stops growing by pebble accretion. Its subsequent fate is binary: either it migrates inward (joining the close-in planet population) or it rapidly accretes gas (becoming a giant planet). Both outcomes remove the core from the mass range near isolation.

Danti, Lambrechts, and Diamond-Lowe (arXiv:2603.28400, March 2026) show through population synthesis that this binary fate produces a 20-fold drop in planet occurrence at intermediate masses — a cliff, not a slope. Between 1 and 50 AU, planets in the mass range 1-5 times the pebble isolation mass are 20 times rarer than planets just below or above this range. The isolation mass acts as a population drain: cores passing through this mass range are removed efficiently by migration or gas accretion, leaving a desert.

The sharpness of the transition is the finding. A gradual depletion would suggest that some cores linger at isolation mass without migrating or accreting gas. A 20-fold drop means essentially no cores survive in this range — the removal mechanisms are nearly complete. The transition is observable: the Nancy Grace Roman Space Telescope’s microlensing survey should resolve the cliff directly, distinguishing it from smoother demographic models.

The structural observation: a single physical threshold — pebble isolation — creates a population desert through the rapidity of the processes it triggers. The desert is not caused by the absence of a formation pathway but by the efficiency of the escape routes. Planets at this mass are not forbidden; they are unstable, transiting through the mass range faster than the survey can catch them. The cliff is a kinetic phenomenon — a bottleneck in mass space where residence time is short — not a thermodynamic one.