A Break In Planet Occurrence Near The Pebble Isolation Mass Should Be Observable By The Roman Microlensing Survey

Microlensing detections are uniquely well-suited to probing the population of planets outside the water iceline, down to planetary masses comparable to the Earth.

Here, we perform 1D pebble-accretion population synthesis simulations to explore a sample of iceline planets around stars with masses and metallicities similar to the target population of the Galactic Bulge Time-domain microlensing 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, Miso, 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 hosting isolation-mass planets (1 to 5 Miso) is lower by a factor 20 compared to less massive planets (0.2 to 1 Miso).

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.

Claudia Danti, Michiel Lambrechts, Hannah Diamond-Lowe

Comments: 9 pages, 7 figures, accepted for publication in A&A
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2603.28400 [astro-ph.EP] (or arXiv:2603.28400v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2603.28400
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Submission history
From: Claudia Danti
[v1] Mon, 30 Mar 2026 13:10:55 UTC (4,887 KB)
https://arxiv.org/abs/2603.28400
Astrobiology,