This paper studies how collisions between close‑in sub‑Neptunes and a swarm of distant, eccentric planetary embryos can strip away atmospheres over millions of years. Sub‑Neptunes are planets a bit larger than Earth that often keep light hydrogen envelopes. The authors show with simulations that high‑speed hits from outer embryos can remove a large part of those atmospheres and so change the measured sizes of these planets.

The team ran many N‑body simulations of systems that start after the protoplanetary gas disk is gone. Each system has three inner sub‑Neptunes (core masses drawn from a distribution with mean about 3 Earth masses and typical orbital radii near 0.15 au) and dozens of small embryos in outer orbits between 1 and 2 au. The embryos are light (about 0.05 Earth masses each) and are set with very high eccentricities (0.7–0.9) so their paths cross the inner planets over time. They ran nine model variants (different embryo numbers and eccentricities) and 20 runs per variant for statistics, using the REBOUND N‑body code.

The collisions they find are fast. Impact speeds range roughly from 2 to 5 times the planets’ escape velocity, depending on embryo eccentricity and the inner planet’s orbital radius. Those speeds produce substantial atmospheric loss in the authors’ post‑processing calculations. On average about 15%–30% of a sub‑Neptune’s atmosphere is lost per collision. That means after a small handful of collisions—roughly 3–6—the gas envelope can shrink to about one‑third of its starting mass. These events typically occur on timescales of several million years after disk dispersal.

Why this matters: the observed distribution of small exoplanet radii shows a clear gap near 2 Earth radii (the “radius gap”), with peaks on either side. The study suggests that later impacts by outer embryos could push some planets from the larger peak into the valley by stripping atmospheres. The authors also note that repeated collisions could help explain the relative lack of sub‑Neptunes with very large atmospheres (the so‑called “radius cliff” above ~3.5 Earth radii, which corresponds to envelopes >10% of planet mass).