Neptune

Neptune in its bluish glory.

Astrographical map of Neptune during the Horus Heresy.

Neptune is an ice giant in the Sol system.

Overview and History

Neptune was first observed by Galileo Galilei in 1612 and 1613, though he mistook it for a star because of its unusual orbit and therefore is not credited as its discoverer. In the 19th century, several astronomers from Britain and France noticed that something was messing with the orbit of Uranus and concluded that another planet was probably the cause, but Neptune's existence wasn't definitively established until a German astronomer spotted it with the Berlin Observatory's telescope. France and Britain immediately started arguing over who'd proved its existence first until they finally agreed to compromise and say that they'd both found it.

Neptune is the eighth planet from the sun, the fourth-largest in the system, and has seventeen times the mass of Earth. It is technically classified as an "ice giant" because its atmosphere contains a higher proportion of volatile chemical compounds that are heavier than hydrogen and helium and are classed as ices in planetary science. The first and so far only flyby of Neptune was undertaken by the Voyager 2 space probe in 1989.

Warhammer 40,000

Neptune was first settled during the Age of Terra, with thriving colonies established on its moons. The Age of Strife saw the planet's inhabitants mutated into horrendous monsters that were eventually purged when the nascent Imperium got its act together and started reconquering the solar system. The job of cleansing these mutants fell to the Revenant Legion, who waged a bloody, savage campaign that saw the death of twelve thousand battle-brothers before reinforcements arrived. In the aftermath, the IX Legion replenished its numbers with recruits taken from among the survivors of the purge. In this they were aided by their gene-seed, which was unusually good at turning even the grossest mutants into gorgeous bishonen dudes.

The moons were resettled with Imperial colonists and had grown into a thriving community by the time the Siege of Terra began. One of the moons, Laomedia, was gassed by Chaos cultists, killing the entire population, while the others were ravaged by pirates and traitor Imperial Army ships brought in by Perturabo. After the end of the Siege, Neptune was reclaimed and rebuilt once again, along with the rest of the system. Its moon Triton is now a mining colony that produces something called corvinium. It was recently the site of a battle between the Dread Host of the Adeptus Custodes and a genestealer cult that had taken root in the colony; the Warden in charge of the Custodians personally slew the genestealer patriarch.