MiniNeptunes

A mini-Neptune is a term used in exoplanet science to describe planets with radii between about 1.6 and 4 Earth radii and relatively low densities, indicating substantial volatile content. They are believed to have thick envelopes of hydrogen and helium, or water-rich layers, surrounding a rocky or icy core.

Compared with Neptune, mini-Neptunes are smaller and less massive, and they represent a transition between rocky super-Earths and ice/gas giants. Their exact composition varies; some may have predominantly H/He envelopes, while others may be water-worlds with high water fractions and thick atmospheres. The mean density is generally lower than that of terrestrial planets.

Formation and evolution: Likely form by accreting gas from the protoplanetary disk after assembling a solid core. They often reside close to their stars or in resonant chains; over time, atmospheric loss from photoevaporation or core-powered mass loss can reduce their envelopes, potentially leaving behind a rocky core that resembles a super-Earth.

Detection and occurrence: Most mini-Neptunes have been found by transit surveys such as Kepler, K2, and TESS. Radii are derived from transit depth, while masses are obtained from radial-velocity measurements or transit-timing variations. They appear to be common around a wide range of stellar types, particularly in short-period orbits.

In research contexts, mini-Neptunes are important for understanding planet formation and atmospheric evolution, and they define a large portion of the observed planetary radius distribution. They are unlikely to be habitable due to their thick atmospheres and irradiation, though their atmospheres remain a focus of atmospheric characterization with current and future telescopes.