A map of the Sol System and the Segmentum Solar after the formation of the Great Rift

The Sol System, also known as the Terran System or the Solar System, is the home star system of Mankind and the principal seat of the Imperium of Man in the galaxy. The system is named after its sun, Sol, and lies at the direct astrocartographic centre of the Segmentum Solar.

The Sol System is part of the Sub-sector Solar and Sector Solar, respectively. Its primary planets include Terra and Mars, the twin capital worlds of the Imperium. There are a total of eight major planets and many dwarf planets in the Sol System, some of which are also orbited by heavily-settled moons, such as Luna and Titan.

The Sol System is home to the governing body of the Imperium of Man, including the High Lords of Terra, the Fabricator-General of Mars, and the Emperor Himself. The Sol System, and the entirety of the Sector Solar, is guarded by Battlefleet Solar, the largest and most potent battlefleet of the Imperial Navy.

The Solar Fastness


A map of the Sol System in 014.M31 just before the start of the Battle of Terra; notice that Titan, the moon of Saturn, is missing, having been moved into the Warp at this time as the Grey Knights Chapter was being raised.

There was a time when the wholesale invasion of the Throneworld of Terra was utterly unthinkable. With the opening of the Great Rift, that time has passed. The Sol System today boasts some of the most formidable fortifications in the galaxy, manned by determined warriors and shielded by technology and faith. Yet still the greatest lynchpin of its defence is the Adeptus Custodes.

Were some courageous or foolhardy invader to approach the Sol System, they would find themselves assailed before they had even cleared the Mandeville Point -- that region of a star system's outer boundaries beyond which it is safe to translate from Warpspace to the real. The system's halo-belt plays host to massive star forts, garrisoned by dedicated regiments of Astra Militarum and empyrically tethered in place. The rest of the outer system is densely laced with thousand-mile-wide fields of void mines, prowling system monitors and huge, vacuum-hardened hunter-Servitors of terrifying aspect.

Were the invader to overcome these hazards, they would find resistance stiffening the deeper they pushed into the system. Heavy naval patrols from the Battlefleet Solar thunder through the darkness, their craggy silhouettes presaging death to any who fall beneath their sights. Monitor-shrines, dock-fortresses, fighter bases and countless weapons platforms dot the darkness, their lumen winking like artificial constellations. The Grey Knights, the Inquisition and the Adeptus Mechanicus all have holdings within the Sol System, boasting suitably ferocious defences. Moreover, since Guilliman's Throneworld Decree, Terra and her sister worlds can raise greater forces in their own defence than ever before.

When coupled with the efforts of the Solar Watch, a group within the Adeptus Custodes who have sworn binding oaths to keep guard over the Sol System, and the presence of the Imperial Fists' mighty mobile battle station Phalanx hanging in geostationary orbit above Terra, it is clear that the defences of the Sol System are all but impenetrable to invading armies. Yet there are subtler threats to the Golden Throne, and it is against these that the Adeptus Custodes must be especially vigilant.

From all across the Imperium come endless streams of pilgrims, merchants, bureaucrats, Adepts, zealots, emissaries, refugees and countless others. Thousands of voidships translate from the Warp every day, entering strictly coordinated approach corridors that lead them to Venus, Mars, Jupiter, Luna and Terra itself. Every world and moon in the Sol System -- barring a few mysterious exceptions -- is ringed with habitats and docking platforms, while all those whose surfaces are sufficiently solid play host to sprawling hab complexes, manufactoria and city-sized fortifications. An unending river of Humanity pours along the system's space lanes and fills its structures to capacity and beyond. It is amongst these masses that rebellion, sedition and heresy can and do foment.

It is, of course, the duty of the Inquisition to root out such threats, and indeed some of the Holy Ordos' most zealous agents spend their entire lives doing just that. However, even these pious Imperial servants are not immune to corruption, whether by nihilistic ideologues, Chaos taint or xenofanatical mesmerism.

Thus the Adeptus Custodes maintain their own presence, and perform their own patrols and monitoring sweeps throughout the Sol System. It is they who supervise the vetting of the countless Imperial citizens who pass through Terra's monolithic spaceports every solar day. It is they who enact surprise sweeps and inspections of the Sol System's defences, following no set pattern or schedule beyond their own intuition to prevent any from predicting and avoiding their vigilance.

The Custodians seed listening devices, spy-Servitors and dictalarcenous subroutines through the hives of the Throneworld and beyond. They gather and analyse every scrap of information they can from even the darkest reaches of the Sol System, feeding ship names, population movements, labourers' shift schedules, demagogues' sermons, Vox intercepts and more into macro-Cogitators the size of Battleships. The data prophecies that emerge from these vast engines aid the Captain-General of the Adeptus Custodes in his command decisions on a daily basis, and help the Ten Thousand to be ever-vigilant.

Notable Planets

  • Mercury (Mining World) - Mercury is a small, Dead World and Mining World that orbits very close to Sol. Despite its unforgiving conditions, shoal facilities operate in Mercury's orbit. During the Horus Heresy, Alpha Legion infiltrators worked with anti-Imperial scavenger clans in the area.
  • Venus Industrial World - Venus is an inner planet of the Sol System, and the second closest to Sol. During the early stages of the Great Crusade, Venus was conquered by the Iron Warriors Space Marine Legion during the Mehr Yasht Campaign. The planet was dominated by War Witches who fielded armies of Litho-Gholem. After the formation of the Imperium, Venus would play host to a wide variety of luxury resort orbitals, all intended to help the elite of the Sol System find respite and relaxation.
  • Terra (Hive World / Throneworld) - Terra is the Throneworld of the Imperium of Man and the original homeworld of Mankind and the Emperor. It is the most sacred and revered place in all the million worlds that comprise the Imperium. Terra is an ecumenopolis, a single cityscape straddling the entire planet, with its vast hive cities covering the entirety of its surface. It is home to hundreds of billions of people and visited annually by tens of billions more seeking to complete religious pilgrimages to the world that their God-Emperor calls home.
    • Luna (Dead World / Civilised World) - Luna, Terra's only natural satellite, is a densely settled Civilised World of the Imperium. Its massive population lives in great cities built deep below the lunar regolith. Luna is most famous for the gene-laboratories where the Emperor of Mankind made the genetic breakthroughs that would ultimately culminate in the creation of the large numbers of Astartes who comprised the Space Marine Legions of the late 30th Millennium. Luna also boasted a large collection of orbital dockyards for starship construction and maintenance, although these were destroyed during the Horus Heresy.
  • Mars (Forge World) - Mars is the greatest Forge World of the Imperium of Man, producing vast numbers of weapons, vehicles and other technology for the Imperial war efforts throughout the galaxy. Mars is also the ancestral home of the Adeptus Mechanicus and their Cult Mechanicus.
    • Phobos (Orbital Fortress) - Phobos is Mars' only remaining natural satellite. A small moon only 11 kilometres in diameter, Phobos' surface is completely covered in weapons-studded fortifications and is used as an orbital fortress that protects the Red Planet and its equatorial Ring of Iron.
  • Ceres - Ceres is a small dwarf planet located within the Sol System's inner asteroid belt. During the Horus Heresy era, Ceres was home to Vox relay and communication stations that helped with communications between the inner and outer reaches of the system. Ceres was also home to various orbital habitat stations who were engaged in the industrial mining of the Asteroid Belt.
  • Jupiter (Gas Giant Mining World) - Jupiter is a massive gas giant and the fifth planet of the Sol System. The orbital space docks and workshops that orbit Jupiter are home to massive Imperial Navy shipyards controlled by the Tech-priests of Mars. During the time of the Horus Heresy, the moons of Jupiter were known to possess habitats used by the workers of the shipyards in Jupiter's orbit, all of which were powered by the constant stream of radiation emitted from Jupiter. It was in orbit of Jupiter that the infamous Word Bearers warship, the Furious Abyss, was constructed, before it turned its guns upon its own shipyard in order to maintain the secrecy of its existence.
    • Ananke - A small moon of Jupiter known to have had habitats on its surface by the time of the Horus Heresy.
    • Iocaste - This moon of Jupiter is known to have had habitats on its surface by the era of the Horus Heresy.
    • Europa - Located within the range of Jupiter's Galilean moons, Europa had been geo-engineered into an ocean-moon by the time of the Horus Heresy.
    • Io - The seething orange mass of the moon of Io is home to a uninhabitable volcanic wasteland.
    • Ganymede - Another of the moons of Jupiter, Ganymede is circled by Jupiter's primary orbital shipyards as well as a dozen other smaller moons of the great gas giant.
    • Thule - Thule is a small, irregular asteroid moon of Jupiter that served as the clandestine shipyard for the construction of the Furious Abyss and was destroyed by Renegade members of the Mechanicum during the Horus Heresy.
    • Callisto - Callisto was a Galilean moon of Jupiter known to hold residential orbitals above its surface by the time of the Horus Heresy.
    • Saros Station - Saros Station was a void station that orbited Jupiter during the time of the Great Crusade and Horus Heresy. This space station resembled a vast spindle bathed in a crimson glow which made it appear as if it were a crystal chandelier that had been severed from its mounting and cast free into the void. Unlike the majority of the gas giant's other industrial and colonial space stations, Saros had been a resort platform before the Horus Heresy, where the Jovian elite found respite as well as diversion from the work of the Jovian system's orbital shipyards and manufactoria. It was said that only the Venus orbitals could surpass Saros Station in luxury with its avenues of gold and silver, null-gardens, auditoriums, and the finest opera house outside the Imperial Palace itself.
    • Kadal - Kadal was the name of a Jovian satellite munitions fortress during the time of the Great Crusade and the Horus Heresy. During the Heresy, Kadal was the site of an Alpha Legion-instigated anti-Imperial revolt and was subsequently destroyed during the Solar War.
  • Saturn (Gas Giant) - Saturn is a massive, ringed gas giant and the sixth planet of the Sol System. Unlike the rest of the worlds in the Sol System, Saturn never hosted a large human population, due in part to xenos activity, thus making it the perfect candidate to house the newly-created Inquisition when it was first established at the end of the Horus Heresy. The moons of Saturn serve as the location for a massive complex of military bases and orbital shipyards maintained by the Inquisition. All of these areas are off-limits to anyone who does not serve or have clearance from the Holy Ordos of the Inquisition. Any unidentified vessels nearing Saturn are promptly destroyed by its sophisticated orbital defences.
    • Titan (Inquisition Fortress World / Space Marine Chapter Homeworld) - Titan is the largest moon of the gas giant Saturn and the largest moon in the Sol System. It is wholly controlled by the Grey Knights Chapter of psychically-active Space Marines who serve as the Chamber Militant of the Inquisition's Ordo Malleus. The fortress-monastery of the Grey Knights is based on Titan, and is carved entirely of basalt.
      • Deimos (Forge World) - Deimos was previously the second moon of Mars and served as a small Forge World with smog-ridden manufactoria that produce weapons and armour. When Malcador the Sigillite created the Grey Knights during the Horus Heresy, the moon was displaced from realspace and into the Warp alongside Titan itself. Several standard centuries after the Battle of Terra it reappeared, its orbit relocated to circle Titan by way of unknown arcane technologies. It is now the primary source of the wargear and vehicles used by the Grey Knights.
    • Mimas (Inquisition Prison World) - Mimas is the closest major moon to Saturn, marred by an immense impact scar that covers a quarter of the surface. This crater houses the Inquisitorial Prison Complex which contains the worst criminals hunted by the Inquisition, who are kept in isolated cells with psychic wards woven into the walls and guarded by Gun-Servitors and an Inquisitorial Storm Trooper regiment.
    • Encaladus - Encaladus houses a citadel of the Inquisition and is a vast palace from which Lord Inquisitors of the Ordo Malleus hold court, and also the world where many of these Lords maintain their personal estates. The primary citadel used by the Inquisition is known as the Encaladus Fortress. In the libraries of that fortress, there are countless volumes of forbidden lore on the subject of Chaos and daemons. The Encaladus Fortress also incorporates the Admiralty Spire, where the empire of Saturn's ancient rulers signed their treaty with the Emperor during the Great Crusade to join the Imperium of Man.
    • Tethys - Tethys is the site of the hidden archive of the Librarium Daemonicum, access to which is restricted to Lord Inquisitors. Tethys is home to a number of daemons trapped beneath its surface. One of the set trials for all aspiring Paladins of the Grey Knights is to survive on its surface without their Power Armour.
    • Iapetus (Imperial Navy Fortress) - Iapetus is the furthermost major moon in orbit of Saturn, and home to a vast naval fortress and series of dockyards that serves the various strike fleets of the Grey Knights Chapter, along with the starships requisitioned from Battlefleet Solar by the Inquisition. As it is under Inquisitorial command, Iapetus serves as the only reliable means of getting into and out of the rings of Saturn. Iapetus was a stronghold of the Ringers, the culture that occupied parts of the Saturnian system before the coming of the Emperor and the Imperium during the Great Crusade.
  • Uranus (Gas Giant) - Uranus is a massive gas giant and the seventh world of the Sol System. Though never habitable, it is home to orbiting human colonies, most notably the Azurites, a peaceful community of artisans who made their livings through scavenging. The Azurite population was largely wiped out during the Great Crusade during the so-called Unheard War when their population became infected by a Warp-spawned disease known as The Screaming which began to mutate them into daemons. Though the Imperial Fists Legion was despatched to provide aid, the Azurites were annihilated by the Astartes' need to prevent the plague from spreading across the Sol System. The moons of Uranus are now used mostly for mining by the Imperium. By the time of the Horus Heresy, Uranus' moons and close orbitals had been repopulated by new human colonists.
    • Ariel - One of Uranus' major moons, Ariel is home to Imperial mining operations.
  • Neptune (Gas Giant) - Neptune is a massive gas giant, the eighth and furthest major planet from Sol. Neptune was first colonized by humanity during the era known as the Age of Terra. By the time the Emperor reconquered Neptune for the Imperium during the Great Crusade, its inhabitants had been transformed into mutants by the isolation from Terra and the harsh environment, which caused them to be destroyed by the forces of the Emperor.
  • Pluto - Pluto is a small dwarf planet located in the outer rim of the Sol System. During the Great Crusade era, Pluto was home to relay stations that were responsible for warning Terra of any imminent invasion. During the Horus Heresy, the world and its accompanying moons was the site of a major Alpha Legion attack in what became the first major engagement of the Solar War.
    • Charon - Charon is the largest natural satellite of Pluto.
    • Styx - Styx is a natural satellite of Pluto.
    • Nix - Nix is a natural satellite of Pluto.
    • Kerberos - Kerberos is a natural satellite of Pluto and is used as a heavily armed orbital fortress.
    • Hydra - Hydra is a natural satellite of Pluto and houses an Astropathic-relay station.
  • Sedna - Sedna was a planetoid-sized trans-Neptunian object of xenos origin located within the Sol System's inner Oort Cloud. During the early Great Crusade eight Space Marine Legions fought against an unremembered xenos species on the dark surface of Sedna, finally leaving the entire Sol System in human hands for the first time since the start of the Age of Strife.

Notable Celestial Objects

  • Asteroid Belt - The Asteroid Belt lies between Mars and Jupiter and separates the Inner and Outer Solar System. Ceres is the largest settlement of the Asteroid Belt. During the early 31st Millennium the Asteroid Belt was populated by various mining operations and orbital habitat stations, some of which were controlled by Belter Coalitions.
  • Kuiper Belt - Another asteroid belt, the Kuiper Belt is nearly 20 times wider and 200 times larger than the inner Asteroid Belt. The Kuiper Belt reaches from the orbit of Neptune and out towards the Sol System's Oort Cloud. Several celestial bodies reside within the Kuiper Belt, including Pluto and its moons, and the dwarf planets Haumea and Makemake.
  • Shrine Comet - The Shrine Comet is an artificial construct built by unknown persons during the Dark Age of Technology that orbits the Sol System every thousand Terran years, tracking an irregular elliptical route around the Sun. In millennia past, when its trajectory was stable and more accurately predicted, it possessed a different name, but what it was had been lost in the murky fog of time. During one such appearance, the Unification Wars of Terra were coming to an end and the Emperor decided to intercept the Comet and modify it as a shrine commemorating His victory. Known as the Shrine of Unity, it was a potent symbol of Imperial strength and unified purpose. During that time, the zealous XVII Legion (the Word Bearers) secretly built a shrine deifying the Emperor on the comet. Following their chastisement by the Emperor at Monarchia, the Primarch Lorgar charged Captain Sor Talgron with secretly tearing down the shrine, before anyone found out, so as not to risk further embarrassment. But in reality, this mission was only a cover story to conceal the Word Bearers' true intentions -- to sabotage the Imperial Palace in preparation for an eventual Chaos invasion.
  • Artefact 9-Kappa-Mu - Artefact 9-Kappa-Mu is a mysterious relic of the Sol System that orbits the Sun between Terra and Venus. A geodesic sphere of black metal nearly 30 kilometres in diameter, the artefact is marked by honeycombed passages and catacombs. The artefact's origin is completely unknown, and may even predate the Dark Age of Technology.
  • Centaurs - The Centaurs are small, dwarf planets that usually feature unstable orbits due to previous encounters with larger planetary bodies. Centaurs mostly exist in the outer reaches of the Sol System, although due to their unstable orbits they sometimes cross into the Inner Solar System.
  • Comets - Comets are small, icy bodies that usually have highly eccentric elliptical orbits, entering the Inner Solar System and then leaving for many hundreds or thousands of standard years.
  • Oort Cloud - The Oort Cloud is a massive, spherical cloud of predominantly icy planetesimals that surrounds the entire Sol System.


  • Codex: Adeptus Custodes (8th Edition), pp. 28-29
  • Codex Adeptus Mechanicus - Cult Mechanicus (7th Edition), Temples of the Omnissiah
  • Realm of Chaos: Slaves to Darkness (1988), pg. 241
  • The Horus Heresy: Book One - Betrayal (Forge World Series) by Alan Bligh, pp. 18, 80-81
  • The Horus Heresy: Book Three - Extermination (Forge World Series) by Alan Bligh, pp. 74-75, 105
  • The Horus Heresy: Book Four - Conquest (Forge World Series) by Alan Bligh, pg. 80
  • Galaxy in Flames (Novel) by Ben Counter
  • The Flight of the Eisenstein (Novel) by James Swallow
  • The Purge (Novella) by Anthony Reynolds, Ch. 4, pg. 27
  • Battle for the Abyss (Novel) by Ben Counter
  • Nemesis (Novel) by James Swallow, pp. 153, 163
  • The Outcast Dead (Novel) by Graham McNeill, Ch. 17
  • Praetorian of Dorn (Novel) by Jong French
  • The Hunt for Vulkan (Novel) by David Annandale
  • The Beheading (Novel) by Guy Haley
  • Grey Knights (Novel) by Ben Counter
  • Hammer of Daemons (Novel) by Ben Counter
  • Vaults of Terra: The Carrion Throne (Novel) by Chris Wraight
  • Watchers of the Throne: The Emperor's Legion (Novel) by Chris Wraight
  • The Horus Heresy Weekender 2019 Live Blog
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