Letter to the Editor

A break in planet occurrence near the pebble isolation mass should be observable by the Roman microlensing survey

¹ Center for Star and Planet Formation, Globe Institute, Øster Voldgade 5, 1350 Copenhagen, Denmark
² Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA

^★ Corresponding authors: This email address is being protected from spambots. You need JavaScript enabled to view it. ; This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 22 January 2026
Accepted: 30 March 2026

Abstract

Microlensing detections are uniquely well suited to probing the population of planets outside the water ice line, down to planetary masses comparable to the Earth’s. We performed 1D pebble-accretion population synthesis simulations to explore a sample of ice-line planets around stars with masses and metallicities similar to the target population of the Galactic Bulge Time-Domain Survey of the Nancy Grace Roman Space Telescope. We find that the planet distribution in the microlensing sensitivity space deviates from a log-uniform distribution in mass and orbital radius. When planetary core growth comes to a halt as planets reach the pebble isolation mass, M_iso, the combined effects of planetary migration and runaway gas accretion create an occurrence break. Our simulations highlight that, between 1 and 50 AU, the fraction of stars that host isolation-mass planets (1–5 M_iso) is lower by a factor of 20 compared to less massive planets (0.2–1 M_iso). If this break in planetary occurrence rates around the pebble isolation mass is detected in future lensing surveys, it would further validate the core accretion paradigm for giant planet formation.