A Hot Neptune, or Hoptune, is a type of giant planet with a mass similar to that of Uranus or Neptune, but orbiting much closer to its host star, normally within less than 1 AU. The first hot Neptune to be discovered with certainty was Gliese 436 b in 2007, an extrasolar planet around 33 light years away from Earth. Recent observations have revealed a larger potential population of hot Neptunes in the Milky Way than was previously thought.
Because of their close proximity to their parent star, 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. Gliese 436 b (also known as GJ 436b) was the first hot Neptune to be discovered with certainty in 2007. The exoplanet Mu Arae c (or HD 160691 c) discovered in 2004 might also be a hot Neptune, but it has not been determined definitively. 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.
The first theoretical study of how hot Neptunes could form was carried out in 2004. 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 accounts for most of their volume.