A hot Neptune or Hoptune is a type of giant planet with a mass similar to that of Uranus or Neptune orbiting close to its star, normally within less than 1 AU. The first hot Neptune to be discovered with certainty was Gliese 436 b in 2007, an exoplanet about 33 light years away. Recent observations have revealed a larger potential population of hot Neptunes in the Milky Way than was previously thought. Hot Neptunes may have formed either in situ or ex situ.
Because of their close proximity to their parent stars, hot Neptunes have a much greater rate and chance of transiting their star as seen from a farther outlying point, than planets of the same mass in larger orbits. This increases the chances of discovering them by transit-based observation methods.
Transiting hot Neptunes include Gliese 436 b and HAT-P-11b. The exoplanet Dulcinea (or HD 160691 c) discovered in 2004 might also be a hot Neptune. Another may be Kepler-56b, which has a mass somewhat larger than Neptune's and orbits its star at 0.1 AU, closer than Mercury orbits the Sun.
If these planets formed ex situ, i.e., by migrating to their current locations while growing, they may contain large quantities of frozen volatiles and amorphous ices. Otherwise, if they formed in situ, their inventory of heavy elements should be made entirely of refractory materials. Yet, regardless of the mode of formation, hot Neptunes should contain large fractions (by mass) of gases, primarily hydrogen and helium, which also account for most of their volume.
Image created by Pablo Carlos Budassi in 2023 (pablocarlosbudassi.com)
