Elevated Eccentricities In The Radius Valley Hint At Water-Rich Mini-Neptunes

While recent planet-formation models broadly reproduce the observed population of super-Earths and mini-Neptunes, as well as the bimodal radius distribution (the “radius valley”), it remains unclear whether all these planets share a common rocky composition (a single popoulation of planets) or instead comprise two distinct populations — rocky planets and icy planets (two populations of planets).

The inferred eccentricity-radius relation, which shows a modest peak near the radius valley, provides a useful diagnostic for distinguishing between these scenarios. Here we use N-body simulations to examine how the radii and eccentricities of close-in planets depend on the masses and orbital configurations of their progenitor protoplanets.

We find that final planetary eccentricities scale with the system initial Safronov number. In two-population systems, energy equipartition between rocky and relatively more massive icy protoplanets creates a strong eccentricity contrast between the two groups, which appears as a peak near the radius valley.

This signature does not appear if planetary systems are composed exclusively of rocky planets (with or without H-rich atmospheres), as assumed in photoevaporation and core-powered mass loss models. Because the eccentricity-radius relation traces a dichotomy in the underlying protoplanet mass distribution — most plausibly arising from formation at different disk locations — our results suggest that a significant fraction of mini-Neptunes are water-worlds.

The observed radius and eccentricity distributions may reflect a mixture of systems that host exclusively rocky planets, systems dominated by icy planets, and systems with both rocky and icy planets.

Sho Shibata, Andre Izidoro

Comments: Accepted for ApJ. 19 pages, 18 figures
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2602.23250 [astro-ph.EP] (or arXiv:2602.23250v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2602.23250
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Related DOI:
https://doi.org/10.3847/1538-4357/ae4a28
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Submission history
From: Sho Shibata
[v1] Thu, 26 Feb 2026 17:25:37 UTC (4,714 KB)
https://arxiv.org/abs/2602.23250
Astrobiology,