Jupiter
Jupiter is a gas giant and the largest planet in the Solar System, dwarfed only by Sol itself. Most people know it for the big fuckoff storm that's raging on it in the present day. Neckbeards know it as the beating heart of the Imperial Navy.
Overview
Jupiter is the fifth planet from the Sun, and also the most massive by a large margin. It's something of a gravitational bully, as it tugs on anything that dares to get even remotely close to its orbit. It's even believed that an early Jupiter is the reason why none of the inner solar system planets are gas giants themselves - it just ate most of the mass that could make them giants in the first place.
Warhammer 40,000
Jupiter is one of the key Forge Worlds and Mining Worlds in the Sol System and, as with all forge-worlds, it is a vast manufacturing center. It is the base of the orbital Jovian shipyards.
History
During the Age of Strife, Jupiter's moons became occupied by Xenos overlords which enslaved the human inhabitants and did typical Age of Strife stuff to them. These moons were not liberated from alien control until the coming of the Emperor in the early days of the Great Crusade. The Human Jovian Void Clans were utilized by the Emperor during the Great Crusade where their shipyards gained fame for producing massive amounts of high-quality warships thanks to technology they guarded even from the Mechanicum.
During the Horus Heresy Jupiter was the center of Rogal Dorn's third defensive sphere under the command of Imperial Fists Captain Efried. In the later stages of the Solar War, the massive array of moons and orbital habitations of Jupiter were captured by Iron Warriors forces led by Perturabo. Instead of reducing every pocket of loyalist resistance or wasting his time during his drive to Terra to occupy the many habitation zones around Jupiter, Perturabo in an unorthodox and un-Perturabo move, instead unleashed fleets of Pirates, traitor Imperial Army, Night Lords, and Emperor's Children to terrorize and subdue the region. Jupiter was subsequently reclaimed by the Imperium during the Great Scouring after the traitor defeat in the Siege of Terra.
Overview
Jupiter is de facto the homeworld of the Imperial Navy. The space docks and workshops that circle Jupiter, like a ring of moons, are home to the millions of technomat and drone Servitors that work under the supervision of Artisan and Rune Priest Tech-priests to build the Imperial Navy's warships.
It is known that as far back as the Horus Heresy, Jupiter held industrial and colonial cohorts with clouds of satellites along with Trojan asteroids containing human colonies, factories along with forges. The Jovian colonies were powered by the radiation that surged from the massive planet and much of the industry fed on the mineral riches that had been yielded from centuries of exploitation, with its full potential having not been fully exhausted. Due to these factors, Jupiter served as Terra's shipyards and its skies were always filled with starships with spacedocks and fabricatories around the various moons that worked tirelessly to construct anything from single manned Raven Interceptors to gargantuan hulls of Emperor-class Battleships.
Each of the Imperium's ship designs is associated with a single shipyard: the Emperor, Tyrant and Dominator are typical of the spaceships produced at the Jovian shipyards.