The Missing Mass Crisis: Free-Floating Planets Expose a Core Flaw in Planet Formation Theory
Lee's preprint highlights a critical mass deficit in planet-forming disks when accounting for abundant free-floating planets, exposing gaps in formation theory that standard disk observations cannot resolve.
A new preprint by Eve Lee examines how recent microlensing surveys implying up to 21 free-floating planets per star create an insurmountable mass budget shortfall for protoplanetary disks. This theoretical analysis, drawing on observational constraints rather than new data collection, reveals that even T Tauri disks lack sufficient solids to form the observed population of bound and unbound planets, even under maximal conversion efficiency. Younger Class 0/I disks fare marginally better but still fall short when realistic pebble or planetesimal accretion efficiencies are applied. This gap is not merely numerical; it points to a deeper unresolved issue in planet formation models that affects both solar system reconstruction and exoplanet demographics. Mainstream coverage often overlooks how a bottom-heavy free-floating planet mass function would require either unexpectedly massive initial disks around high-mass stars or a sharp revision to formation pathways. Related work, such as ALMA surveys of disk masses by Pascucci et al. (2016, ApJ), consistently shows median disk masses too low to match these demands, while microlensing results from Mróz et al. (2017, Nature) underpin the high free-floating planet frequency. If verified, this could imply preferential formation of free-floaters in rare massive disks, aligning with the observed decline in bound planet occurrence around higher-mass stars. The study remains a preprint without peer review, limited by its reliance on extrapolated mass functions without direct mass measurements of individual objects.
HELIX: The mass budget mismatch implies planet formation models must either invoke hidden reservoirs of solids or accept that free-floating planets dominate mass consumption in atypical disks, reshaping interpretations of both solar system and exoplanet data.
Sources (3)
- [1]Primary Source(https://arxiv.org/abs/2605.26216)
- [2]Related Source(https://ui.adsabs.harvard.edu/abs/2016ApJ...831..125P)
- [3]Related Source(https://www.nature.com/articles/nature23276)