Sub-Neptune

A sub-Neptune sits in the size gap between rocky super-Earths and the full ice giants: bigger than Earth, smaller than Neptune. Most are thought to be a rock or ice core swaddled in a deep atmosphere of hydrogen and helium, which makes them far puffier and lighter than a bare rock of the same width. There is likely no solid surface, just gas that thickens with depth.

Sub-Neptunes are one of the galaxy's most common kinds of planet, yet the Sun has none, so everything we know about them comes from other stars. The line between a large super-Earth and a small sub-Neptune is exactly where the interesting question sits: at what size does a rocky world start holding a suffocating gas blanket? gravityfinder sorts each world into a class by its measured radius.