Neptunian exoplanet

Neptunian exoplanets are similar in size to the ice giants Neptune and Uranus in the Solar System. Neptunian exoplanets may have a mixture of interiors though all would be rocky with heavier metals at their cores. Neptunian planets typically have hydrogen- and helium-dominated atmospheres.

There are several recognized subtypes of Neptunian exoplanets:

  • Hot Neptune – orbiting close to its star
  • Helium planet – a hypothetical type that can form from hot Neptune via hydrogen evaporation, leaving a helium-dominated atmosphere
  • Mini-Neptune – considerably smaller than Neptune but still having a thick hydrogen-helium envelope, relatively common
  • Super-puff – inflated planets with unusually low mean density
  • Hycean planet – a hypothetical transitional type to ocean planets with a thin, hydrogen-helium-dominated atmosphere overlaying a liquid water ocean
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