mini-Neptune

The terms mini-Neptune and sub-Neptune are used for planets between Earth and Neptune in size, that have characteristics more like those of Neptune. There is undoubtedly some variation in how the term is used, depending on which characteristics are under discussion. The terms mini-Neptune and super-Earth may be used based simply upon mass, using some chosen threshold, but if its density has been determined, super-Earth may indicate the density of a rocky planet and mini-Neptune could simply be the same type of rocky solid, but with a gas planet-like deep atmosphere. This can reflect a dichotomy evident in current observational data: there are two peaks in the density function of planet-counts-by-mass that both fall between Earth and Neptune mass (the Fulton gap), so the terms may be used to categorize planets according to which high-density mass-range they fall under. The two peaks suggest there may be two types of planet formation. The term mini-Neptune may also be more-specifically used, e.g., a small ice giant, like Neptune, but smaller. Similarly-used terms are gas dwarf, mini gas giant, and transitional planet.