Mini-Neptune
Mini-Neptunes are exoplanets larger than Earth but smaller and less massive than Neptune (typically ~2–4 R⊕, ~5–20 M⊕), commonly with thick gaseous envelopes over rocky/icy cores. They are the most abundant planet class detected to date and span a range of formation pathways — from in-situ accretion of nebular hydrogen to ex-situ accretion of volatile-rich ices beyond the water-ice-line followed by inward planetary-migration.
The “radius cliff” at ~3 R⊕ is a population-level feature potentially shaped by atmospheric escape and formation pathway. According to src-jwst-toi-1130b-atmosphere-2026-05, the jwst characterization of toi-1130b (μ = 5.5 amu, high metallicity, low He I) supports volatile-rich ex-situ formation as one contributor to mini-Neptune diversity, implying the population is not a single formation class.
Compare to the hycean world subclass (k2-18b), which interprets some sub-Neptunes as hydrogen-atmosphere ocean planets — a contested reading in the broader sub-Neptune retrieval debate.