
A type of exoplanet with a radius between that of Earth and Neptune, often with a hydrogen-rich atmosphere.A sub-Neptune is a type of exoplanet with a radius larger than Earth's but smaller than Neptune's, typically ranging from about 1.7 to 4 times Earth's radius. These planets are common in the galaxy, but their internal structure and atmospheric composition remain poorly understood. Many sub-Neptunes appear to have thick hydrogen-rich atmospheres overlying a rocky or icy core, but some may be water worlds or have exotic interiors.
The first sub-Neptunes were discovered by the Kepler mission in the 2010s, which found a population of planets with radii between 1.5 and 4 Earth radii. The term 'sub-Neptune' became widely used as astronomers realized these planets are distinct from both rocky super-Earths and gas giants like Neptune.
Sub-Neptunes are thought to form in the protoplanetary disk around a young star, accumulating gas and solids. Their hydrogen-rich atmospheres can be studied through transmission spectroscopy, where starlight passing through the atmosphere during a transit reveals absorption lines from molecules like water, methane, or hydrogen. The presence of a hydrogen envelope can also be inferred from the planet's density and radius, as a low-density planet with a large radius suggests a thick gas layer.
Sub-Neptunes are prime targets for atmospheric characterization with JWST, which can detect molecules in their atmospheres and help distinguish between different formation and evolution scenarios.
Some sub-Neptunes may have oceans of liquid water hundreds of kilometers deep, sandwiched between a rocky core and a hydrogen atmosphere, creating conditions unlike anything in our solar system.

























