A sub-Neptune, also known as a mini-Neptune, is a type of exoplanet smaller in radius than Neptune, but larger than the small planet radius gap. Based on their low bulk densities, these planets likely possess a low density envelope surrounding a rocky core. Despite being one of the most numerous types of exoplanets discovered as of 2026, no sub-Neptune is known to exist in the Solar System. The exact nature of sub-Neptunes is uncertain. It is debated whether they are "gas dwarfs" with a gas envelope of hydrogen and helium over a rocky core, "water worlds" with large amounts of volatiles such as water, or smaller versions of ice giants with both gas and volatiles. In order to discern between these scenarios, several sub-Neptunes such as GJ 1214 b and K2-18 b have been observed by telescopes such as the Hubble Space Telescope and the James Webb Space Telescope in order to infer their compositions and formation histories.
Terminology The terms "sub-Neptune" and "mini-Neptune" are used inconsistently in literature. Many research papers use either term to refer to the small planet population located above the radius gap, possessing a low density envelope, in contrast to super-Earths, which are smaller and lack an envelope. Other terms used to refer to this planet population include "gas dwarf", which now usually specifically means water-poor sub-Neptunes, and "small Neptune". "Sub-Neptune" is occasionally used to collectively refer to any planet between 1–4 Earth radii, including super-Earths and mini-Neptunes. This population is also known as "small planets", or "Kepler planets". "Mini-Neptune" is sometimes used to refer to specific subsets of sub-Neptunes. The term has been used to describe sub-Neptunes with a massive hydrogen atmosphere and no defined surface, or water-rich sub-Neptunes.
Occurrence
Sub-Neptunes occur much more frequently than giant planets in the inner planetary system. A 2013 study using data from the Kepler space telescope found that planets between 2–4 Earth radii orbiting within 245 days can be found around 31% of sun-like stars, while less than ten percent of such stars host larger planets orbiting within 418 days. Subsequent works show that their occurrence increases with orbital period until around 12–13 days, where the occurrence rate flattens out. Microlensing surveys, which are sensitive to planets located at large orbital separations, indicate that long period sub-Neptunes and super-Earths may also be common. Short period sub-Neptunes are common around G-type and K-type stars, and their occurrence peaks around early M-type stars. Their occurrence decreases around stars hotter than the Sun, as well as around cooler stars. Sub-Neptunes are rare around mid-to-late M dwarfs less massive than around 0.4 solar masses, with an occurrence rate of 0.148 ± 0.045 planets orbiting within 30 days per star, while super-Earth-size planets remain much more common. Sub-Neptune occurrence weakly correlates with the host star's metallicity, unlike larger giant planets. In addition to a broad suppression in small planet occurrence, stellar companions with separations of less than around 100 au suppress the occurrence rate of sub-Neptunes.
Characteristics
Orbit
Sub-Neptunes are frequently found with other sub-Neptunes (or super-Earths). They tend to have similar sizes as other sub-Neptunes in the system. Such systems are known as "peas in a pod" systems. Most sub-Neptunes are not in mean motion resonance with their neighbours. However, there is an excess of sub-Neptunes near first-order resonances, especially just wide of it. Transiting sub-Neptunes near mean motion resonances, such as those orbiting Kepler-223 and HD 110067, exhibit strong transit timing variations, which allows measurements of their masses. The sub-Neptune planet population's average eccentricity is lower than larger planets. The transition between the two populations occurs at a radius of around 3.5 Earth radii.
Size
The radius range of the main sub-Neptune population lies between the small planet radius gap, at around 1.7–1.8 Earth radii, and the radius cliff, the sudden drop in planet abundance beyond approximately 3–4 Earth radii. This definition is not rigid; some sub-Neptune-sized planets with masses exceeding Neptune's have also been referred to as sub-Neptunes, as well as planets larger but less massive than Neptune. Young sub-Neptunes, such as the transiting sub-Neptunes orbiting Kepler-51 and V1298 Tauri, can be significantly larger than their older counterparts, some being as big as gas giants even though they are much less massive. As they age and radiate heat away, they undergo Kelvin–Helmholtz contraction, shrinking in radius until they reach typical sub-Neptune sizes.
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