Sol System

The **Sol System** is a gravitationally bound planetary system located in the Orion–Cygnus Arm of the Milky Way and the home system of humanity. Centred around the main-sequence star Sol, the system contains eight planets and a vast number of moons, dwarf planets and smaller bodies. The capital planets of both the United Colonies and the Herschel Nations, Earth and Mars respectively, lie within the Sol System and it is thus a neutral zone.

## Structure

The principal component of the Sol System is Sol, a G2 main-sequence star that contains 99.86% of the system's known mass and dominates it gravitationally. Sol's four largest orbiting bodies, the giant planets, account for 99% of the remaining mass, with Jupiter and Saturn together comprising more than 90%. The remaining objects of the Sol System (including the four terrestrial planets, the dwarf planets, moons, asteroids, and comets) together comprise less than 0.002% of the system's total mass.

## Stars

### Sol

* Diameter: 1.3927 million km
* Mass: 333000 M⊕
* Distance from Parent: 26,000 light-years from galactic centre
* Rotational period: 25 days
* Orbital period: 240 million years
* First landing: N/A

A relatively calm, G-type main-sequence star located 26,000 light-years from the galactic centre, Sol is unique for its amazing energy output despite its relatively young age.

## Inner Sol System

The **inner Sol System** is the region comprising the terrestrial planets and the asteroid belt. Composed mainly of silicates and metals, the objects of the inner Sol System are relatively close to Sol; the radius of this entire region is less than the distance between the orbits of Jupiter and Saturn.

### Inner Planets

The four terrestrial or **inner planets** have dense, rocky compositions, few or no moons, and no ring systems. They are composed largely of refractory minerals such as the silicates—which form their crusts and mantles—and metals such as iron and nickel which form their cores. Three of the four inner planets (Venus, Earth and Mars) have atmospheres substantial enough to generate weather; all have impact craters and tectonic surface features, such as rift valleys and volcanoes.

#### Mercury

* Diameter: 4,879.4 km
* Mass: 0.055 M⊕
* Distance from Parent: 0.4 AU
* Rotational period: 59 days
* Orbital period: 88 days
* First landing: 2060s

Mercury is the closest planet to Sol, at 0.4 AU. The smallest planet in the system, Mercury has no natural satellites. Besides impact craters, its only known geological features are lobed ridges or rupes that were probably produced by a period of contraction early in its history. Mercury's very tenuous atmosphere consists of atoms blasted off its surface by the solar wind. Aside from its vast resources, all of which are owned by Hermes Engineering, Mercury holds nothing of value for the Sol System.

#### Venus

* Diameter: 12,104 km
* Mass: 0.815 M⊕
* Distance from Parent: 0.7 AU
* Rotational period: 243 days
* Orbital period: 225 days
* First landing: 2060s

Venus, the second planet from Sol, is close in size to Earth and, like Earth, has a thick silicate mantle around an iron core, a substantial atmosphere, and evidence of internal geological activity. It is much drier than Earth, and its atmosphere is ninety times as dense. Venus has no natural satellites. It is the hottest planet, with surface temperatures over 400 °C, most likely due to the amount of greenhouse gases in the atmosphere. No definitive evidence of current geological activity has been detected on Venus, but it has no magnetic field that would prevent depletion of its substantial atmosphere, which suggests that its atmosphere is being replenished by volcanic eruptions. Venus is home to the Takhar Prison facility, notoriously the most dangerous and inhospitable prison in the Settled Systems.

#### Earth

* Diameter: 12,742 km
* Mass: 1 M⊕
* Distance from Parent: 1 AU
* Rotational period: 24 hours
* Orbital period: 365 days
* First landing: N/A

Earth is the largest and densest of the inner planets, the only one known to have current geological activity, and the human homeworld. Its liquid hydrosphere is unique among the terrestrial planets of the Sol System. Earth's atmosphere is radically different from those of the other planets, having been altered by the presence of life to contain 21% free oxygen. It has one natural satellite, Luna, the only large satellite of a terrestrial planet in the Sol System. Earth is the capital of the United Colonies and the former capital of the Interstellar Collective. In the 25th century, Earth came under attack in an event called "The Lesson", causing the deaths of billions and significantly reducing Earth's political and socio-economic influence.

##### Luna

* Diameter: 3,475 km
* Mass: 0.0123 M⊕
* Distance from Parent: 384,400 km
* Rotational period: Tidally locked
* Orbital period: 27 days
* First landing: July 1969

Luna is the only major moon of the homeworld of the human species, Earth, and as such was the first world beyond its borders to be colonized by them. One of the only universal focal points in all human culture throughout the entire planet, Luna is under the control of the United Colonies, whose parliment was headquartered in the Koch District of Lovell City. The Moon was destroyed during the Lesson by an antimatter bomb, tearing apart its surface and devastating Earth. Less than a century after the Great Galactic War, the fragments of Luna have been stabilised, but debris continues to impact the Earth and cause environmental destruction.

#### Mars

* Diameter: 6,779 km
* Mass: 0.107 M⊕
* Distance from Parent: 1.5 AU
* Rotational period: 24.6 hours (1 sol)
* Orbital period: 687 days
* First landing: 1993

Mars, located 1.5 AU from Sol, is smaller than Earth and Venus and larger only than Mercury. Before terraforming, it had an atmosphere of mostly carbon dioxide with a surface pressure of 6.1 millibars, roughly 0.6% that of Earth but sufficient to support weather phenomena; in the decades since colonization began, its atmosphere has thickened to around 0.9 atm, allowing liquid water to flow on the surface during the summer. Its surface, peppered with volcanoes, such as Olympus Mons, and rift valleys, such as Valles Marineris, shows geological activity that may have persisted until as recently as 2 million years ago. Its red colour comes from iron oxide in its soil. Mars has two tiny natural satellites (Deimos and Phobos) thought to be either captured asteroids or ejected debris from a massive impact early in Mars's history. Mars was the first interplanetary human colony, the homeworld of the martians, and the capital of the Herschel Nations.

### Asteroid Belt

The asteroid belt occupies the orbit between Mars and Jupiter, between 2.3 and 3.3 AU from Sol. It is thought to be remnants from the Sol System's formation that failed to coalesce because of the gravitational interference of Jupiter. The asteroid belt contains tens of thousands, possibly millions, of objects over one kilometre in diameter. Despite this, the total mass of the asteroid belt is unlikely to be more than a thousandth of that of Earth. The asteroid belt is very sparsely populated, with only three major settlements: Ceres, Vesta and Eros.

#### Ceres

* Diameter: 946 km
* Mass: 0.00016 M⊕
* Distance from Parent: 2.8 AU
* Rotational period: 9 hours
* Orbital period: 4.6 Earth years
* First landing: 1997

## Outer Sol System

The outer region of the Solar System is home to the giant planets and their large moons. The centaurs and many short-period comets also orbit in this region. Due to their greater distance from Sol, the solid objects in the outer Sol System contain a higher proportion of volatiles, such as water, ammonia, and methane than those of the inner Sol System because the lower temperatures allow these compounds to remain solid.

### Outer Planets

The four outer planets, also called giant planets or Jovian planets, collectively make up 99% of the mass known to orbit Sol. Jupiter and Saturn are together more than 400 times the mass of Earth and consist overwhelmingly of the gases hydrogen and helium, hence their designation as gas giants. Uranus and Neptune are far less massive and are composed primarily of ices. All four giant planets have rings, although only Saturn's ring system is easily observed from Earth.

#### Jupiter

* Diameter: 139,820 km
* Mass: 317.8 M⊕
* Distance from Parent: 5.2 AU
* Rotational period: 10 hours
* Orbital period: 12 years
* First landing: 2028

Jupiter is the largest planet in the Sol System, and is approximately 2.5 times as massive as all of the other planets in the Sol System combined. Due to the planet's radiation fields, there are few orbital stations, with colonies focussed on the four Galilean moons. Rule of the four moons is split between four different factions: Ganymede is governed by the United Colonies; Callisto is owned by the United Jupiter Company, which also owns mining rights to the planet itself; Io is home to a Transport Union research facility and space traffic station; and Europa is governed by the Herschel Nations.

##### Ganymede

* Diameter: 5,268.2 km
* Mass: 0.025 M⊕
* Distance from Parent: 1,070,000 km
* Rotational period: Tidally locked
* Orbital period: 7.1 days
* First landing: 2034

Ganymede, one of the Galilean moons, is the largest and most massive of the Sol System's moons. The ninth-largest object of the Sol System, it is the largest without a substantial atmosphere. It has a diameter of 5,268 km, making it 26 percent larger than the planet Mercury by volume, although it is only 45 percent as massive. It is the only moon known to have a magnetic field, although it is significantly overwhelmed by that of Jupiter.

##### Callisto

* Diameter: 4,820.6 km
* Mass: 0.018 M⊕
* Distance from Parent: 1,883,000 km
* Rotational period: Tidally locked
* Orbital period: 17 days
* First landing: 2028

The first Jovian colony due to its subsurface ocean and its orbit outside of Jupiter's main radiation belt, Callisto is a cold world used as a base to colonize Europa and Ganymede. As part of the power sharing agreement over the Jovian system, Callisto is owned and governed by the United Jupiter Company, which also owns the planet itself.

##### Io

* Diameter: 3,643.2 km
* Mass: 0.015 M⊕
* Distance from Parent: 422,000 km
* Rotational period: Tidally locked
* Orbital period: 42 hours
* First landing: 2035

A hostile world embedded deep in Jupiter's radiation field, Io was the last of the Galilean moons to be settled by humanity. With no subsurface ocean and an inhospitable surface, Io was long ignored by the Interstellar Collective until it became the Home Systems headquarters of the Transport Union.

##### Europa

* Diameter: 3,121.6 km
* Mass: 0.0083021 M⊕
* Distance from Parent: 671,000 km
* Rotational period: Tidally locked
* Orbital period: 85 hours
* First landing: 2029

The smallest of the Galilean moons, Europa was long considered the best candidate for colonization in the outer Sol System because of its lower radiation exposure compared to Callisto and its subsurface ocean. Finally reached in 2029, Europa was the first extraterrestrial body discovered to harbour life; a complex ecosystem of small vertebrates and gargantuan invertebrates dwells in the subsurface Aegean Ocean, sustained by the little sunlight that is able to penetrate Europa's icy surface.

#### Saturn

* Diameter: 116,460 km
* Mass: 95.16 M⊕
* Distance from Parent: 9.5 AU
* Rotational period: 10.7 hours
* Orbital period: 29 years
* First landing: 2033

Saturn , distinguished by its extensive ring system, has several similarities to Jupiter, such as its atmospheric composition and magnetosphere. Although Saturn has 60% of Jupiter's volume, it is less than a third as massive. The rings of Saturn are made up of small ice and rock particles. Saturn has 83 confirmed satellites; two of these, Titan and Enceladus, show signs of geological activity, with primitive lifeforms found on both. Titan, the second-largest moon in the Sol System, is bigger than Mercury and the only satellite in the Solar System to have a substantial atmosphere. The planet itself and many of its moons are owned by Cronos Incorporated, a board member of the Free Economic Zone, while Titan is controlled by the United Colonies.

##### Titan

* Diameter: 5,149.5 km
* Mass: 0.0225M⊕
* Distance from Parent: 1.2 million km
* Rotational period: Tidally locked
* Orbital period: 16 days
* First landing: 2033

The largest moon of Saturn - being larger than the planet Mercury - and the only moon in the Sol System with a substanial atmosphere, Titan was the fourth extraterrestrial colony settled by humanity after Luna, Mars and Callisto. A major world of the United Colonies and the sole moon of Saturn not owned by Cronos Incorporated, Titan is an important source of hydrocarbons for all of the U.C.'s worlds.

##### Enceladus

* Diameter: 504.2 km
* Mass: 1.8×10-5 M⊕
* Distance from Parent: 238,000 km
* Rotational period: 1.4 days
* Orbital period: 33 hours
* First landing: 2033

A cold moon that is otherwise unremarkable, Enceladus, Saturn's sixth-largest moon, attracted significant attention in the early 21st century when it became the first extraterrestrial body proven to have a subsurface ocean beneath its icy crust, later discovered to be home to a complex ecosystem similar to that of Europa. Almost completely abandoned by Cronos Incorporated after a mining accident, Enceladus is now a stronghold of Twin Moon pirates.

## Trans-Neptunian Region

### Scattered Disc

#### Bifrost

* Diameter: 1000 km
* Mass: 342000 M⊕
* Distance from Parent: 75 AU
* Rotational period: Unknown
* Orbital period: Unknown
* First landing: 2047

Discovered in 1998, Bifrost was the first naturally-occurring wormhole discovered by humanity. Located at the edge of the **Sol System**, its mass is greater than Sol but its gravity is weaker than Mars' moon Phobos; these properties, combined with its lensing effect despite its low gravity, were enough for the WIS to operate the clandestine *Crusader* Program to study it throughout the 21st and 22nd centuries. These studies concluded in 2191 when its matter, known as Hammond-Zhōu Quintessence, was isolated and reproduced, allowing for the development of roemer drives and faster-than-light travel. Bifrost's origins are as mysterious as its properties, with some believing that it is a relic left behind by the travellers.

## Farthest Regions

### Detached Objects

#### Sedna

* Diameter: 995 km
* Mass: 1.7-6.1×1021 kg
* Distance from Parent: 84 AU
* Rotational period: 18 hours
* Orbital period: 11,408 years
* First landing: 2130s

A dwarf planet known for its exceptionally elongated orbit, and takes approximately 11,400 years to return to its closest approach to Sol at a distant 76 AU. Known both for its orbit and reddish surface, Sedna was originally a mining colony but is now owned by the Sedna Salvage & Shipping corporation.

## Galactic Context

### Celestial Neighbourhood

The Sol System is surrounded by the Local Interstellar Cloud, itself within a 300 light-year region called the Local Bubble; the star systems within this Bubble were the first to be colonized and are thus known as the Home Systems. Within ten light-years of the Sol System there are relatively few stars, the closest being the triple star system Alpha Centauri, which is about 4.4 light-years away; Alpha Centauri A and B are a closely tied pair of Sun-like stars, whereas the closest star to Earth, the small red dwarf Proxima Centauri, orbits the pair at a distance of 0.2 light-year. Humanity's first extrasolar colony was settled on Centaurus, a super-Earth orbitting Proxima Centauri.

## Trivia

* A total of eight bodies in the Sol System are home to some form of native life. In position from Sol, they are:
	+ Venus, home to bacterial lifeforms that metabolize sulfur
	+ Earth, home of humanity and billions of other forms of complex life
	+ Ganymede's subsurface ocean is home to simple invertebrates
	+ Europa's subsurface ocean is home to a complex ecosystem of bioluminescent invertebrates and vertebrates
	+ Callisto, like Ganymede, has a subsurface ocean with primitive life
	+ Titan, Saturn's largest moon, is home to microorganisms that metabolize methane and nitrogen, as well as larger invertebrates in its lakes
	+ Enceladus, sixth-largest moon of Saturn, has a subsurface ocean filled with hydrothermal vents and sulfide-processing bacteria
	+ Triton, Neptune's largest moon, has a subsurface ocean filled with hydrothermal vents and sulfide-processing bacteria