Neptunianmass

Neptunianmass is a planetary-mass category used in planetary science and exoplanet studies to describe planets whose mass is close to that of Neptune. In the solar system, Neptune has a mass of about 17 Earth masses, and exoplanets in the same range are commonly labeled Neptunian or Neptune-mass planets. The typical mass range for this class is roughly 10 to 20 Earth masses, though definitions vary by author.

Neptunian-mass planets are often referred to as ice giants or mini-Neptunes. They generally have thick volatile envelopes composed of hydrogen, helium, and ices such as water, ammonia, and methane, surrounding a possible rocky or metallic core. They do not have a solid surface in the usual sense, and their radii can vary: roughly 2.5 to 4 Earth radii for higher-density cases, and larger radii for lower-density, gas-rich compositions. The exact mass-radius relationship is degenerate, meaning different internal structures can yield similar observed masses and sizes.

Many Neptunian-mass exoplanets are found in short-period orbits, earning the label "hot Neptunes." Others lie farther from their stars, similar to Neptune’s distance in the solar system. Formation theories suggest they form beyond the frost line and may migrate inward.

Notable examples of Neptune-mass exoplanets include GJ 436 b and HAT-P-11 b, among others. The term is a descriptive label rather than a rigid classification, with overlapping ranges with both sub-Neptunes and small gas giants.