Sub-Neptune

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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.

Read on Wikipedia · Image Keith Miller (Caltech/IPAC/SELab), Public domain