The Sol System is the stellar system comprising of a singular star, Sol (or Helios), and the objects that are gravitationally bound to it. Of the millions of objects that orbit Sol, the nine largest are it's planets. The rest are made up of dwarf planets, asteroids, comets, and other natural and artificial structures. The Sol System is the home system of Humanity.
Sol or Helios makes up approximately 99.89% of the total mass of the system. Of the remaining 0.109%, approximately 97.8% (0.106% of the total) is made up by the nine planets. The remainder is made up of moons and other non-planetary bodies.
Inner solar system
The Inner solar system consists of the four smallest planets. Only three natural moons appear in the inner solar system, and there are no natural ring systems. The inner planets are all small, dense worlds made up of metals and silicates. Three of the four planets (Venus, Earth, and Mars) have naturally-occurring atmospheres with weather systems. All have varied geologic and tectonic features.
Mercury (0.4 AU) is the closest planet to Sol and the smallest planet in the system. It is the only planet in the system to not have any moons. Mercury's surface is covered in craters of varying density and depth. It's large iron core makes it one of only two inner planets (the other being Earth) to have a natural electromagnetic field. It is thought that Mercury was once a larger planet, although extremely hot temperatures early in the solar system's formation made most of it's crust and mantle melt away, leaving a relatively thin outer layer and large core. A variety of robotic missions have been sent to the planet, and a manned exploration program has been under consideration since the 2090s.
Venus (0.7 AU) is the second planet from Sol and the second-largest inner planet, possessing nearly 82% of Earth's mass. Like Earth, Venus has a substantial atmosphere and is geologically active. Venus' thick atmosphere (90.79 atm) gives its surface a very hot temperature and surface pressure, making human presence on the surface very difficult. A collection of floating stations exist roughly 50 kilometers above Venus' surface and are intermittently inhabited by the crews of gas carriers, which have been regularly been transporting Venusian carbon dioxide to Mars since the 2080s. Since 2097, Venus has had a single artificial moon, Cupid.
Earth (1 AU) is the third planet from Sol and the largest of the inner planets. Earth is the home world of Humanity and the majority of life in the Sol System. Luna is the only natural moon of Earth, as well as the largest. 55 Asteroids have been maneuvered into High Earth Orbit and orbit around Luna to serve as mining stations.
Mars (1.5 AU) is the fourth planet from Sol and the second-smallest of the inner planets. Originally a cold, desert-like planet, Mars has been undergoing terraforming since the mid-2050s and is expected to be a habitable planet like Earth by the 25th century. Mars was geologically active earlier in it's history, as exhibited by its large rift valleys and volcanoes, however this stopped approximately 3.6 billion years ago when it's iron core cooled. Mars is the only other body in the universe to have developed life in it's history, although all native life went extinct when Mars' geological and electromagnetic activity died. Mars has two natural satellites, Phobos and Deimos, which have a combined population of about 11,000. Mars itself has a population of about 5.4 million.
Main Asteroid Belt
The Main Asteroid Belt (2.1-3.4 AU) consists of millions of rocky and icy bodies between the orbits of Mars and Jupiter. Natural objects in the Main Asteroid Belt range from grains no larger than a few centimeters in diameter to large bodies hundreds or thousands of kilometers in diameter. The 'Belt', as it's colloquially called, is thought to be made up of material from early in the Sol system's formation that failed to combine into a planet due to the gravitational influence of Jupiter. As of 2110, the Belt contains an estimated 260,000 objects over one kilometer in diameter. Despite this, the entire Belt has a combined mass of just 3.6% that of Luna.
Human presence in the Asteroid Belt is limited only to Ceres, which maintains a population of about 800 scientists and engineers.
Ceres (2.7 AU) is the largest body in the Main Asteroid Belt and only dwarf planet within the orbit of Neptune. It has a tiny, rocky crust on top of a large, icy mantle nearly 100 kilometers thick. Large salt deposits on Ceres' surface suggests that the world was once covered in a planetary ocean that was swept away by stellar wind and freezing temperatures. Ceres has maintained a single surface base of about 800 engineers and scientists since the mid 2060s, and is set to expand in the coming decades.
Outer solar system
The Outer planets are the largest planets in the Sol system. Collectively, they have a combine mass of 444.64 times that of Earth, and make up approximately 97% the total mass of the planets. Jupiter and Saturn, the larger of the four as well as the innermost, gas giants that are predominantly made of hydrogen and helium. Uranus and Neptune, the smaller two and outermost, are ice giants that contain relatively high amounts of ices such as ammonia, water, methane. All of the outer planets have ring systems, although Saturn's is the only one visible from Earth.
Jupiter (5.2 AU) is the fifth planet from Sol and the largest planet in the system, at nearly 316 Earth masses. It is comprised of main hydrogen and helium, along with trace amounts of methane and ammonia. Jupiter's strong internal heat allows for complex and dynamic weather systems such as it's Great Red Spot, a swirling storm that has been around for at least 400 years. Jupiter has 87 moons, ranging in size from tiny moonlets less than a kilometer across, to the massive Galilean moons of Io, Europa, Ganymede, and Callisto. The first manned mission to Jupiter, Zeus-1, is set to reach Jupiter in 2113.
Saturn (9.5 AU) is the sixth planet from Sol and the second-largest of the outer planets (as well as second-largest in the system). Like Jupiter, Saturn is predominantly comprised of hydrogen and helium, with traces of methane and ammonia. Although Saturn has approximately 60% of Jupiter's volume, it has only 29.9% of it's mass, making it the least dense planet in the Sol system. Saturn's ring system is the largest and most prominent of the planets, with it being easily visible from the surface of Earth. Saturn has 93 moons, with only 8-9 being consider 'major moons'. One of these moons, Titan, has a substantial nitrogen atmosphere and seas of liquid methane. Another moon, Enceladus, has a substantial subsurface water ocean. A manned mission to Saturn has not yet been attempted, although the UNSA has set 2125 as the proposed launch year.
Uranus (19.2 AU) is the seventh planet from Sol and the third-largest of the outer planets. Uranus is the first of the so-called 'ice giants', whose atmospheres contain relatively high concentrations of ices (elements with freezing temperatures above -173 °C) like ammonia and water. Uranus is unique in that it orbits Sol on its side, possessing an axial tilt of nearly 98° relative to its orbit. Uranus generates very little internal heat, and as such its atmosphere produces little in the way of dynamic weather systems. Uranus has 30 moons, with the five largest (Titania, Oberon, Umbriel, Ariel, and Miranda) being considered major.
Neptune (30.1 AU) is the eighth planet from Sol and the smallest of the outer planet. Like Uranus, Neptune is an ice giant, containing high amounts of ammonia, water, and methane in its atmosphere. Although Neptune is slightly smaller than Uranus, it is more massive by approximately 2.6 Earth masses. Neptune has 17 moons, although only the largest, Triton, being considered major. Triton is unique in that it is the only major moon in a retrograde orbit around it's planet.
Kuiper Belt & Oort Cloud
The Kuiper Belt (30-50 AU) is a giant ring of debris, similar to the Main Asteroid Belt, beyond the orbit of Neptune. Unlike the Main Asteroid Belt, the Kuiper Belt mainly consists of small, icy bodies with relatively little metals and silicates. The Kuiper Belt is made up of an estimated 500,000 bodies larger than 1 kilometer. Despite their abundance, the Kuiper Belt is thought to have a combined mass of less than 10% that of Luna. Many Kuiper Belt objects have multiple natural satellites, and have very inclined and eccentric orbits.
The Oort Cloud (0.95-1.46 ly) is a spherical cloud of trillions of small bodies that orbit the sun in extremely-long periods. The Oort Cloud is thought to be the source of most long-period comets, and as such is thought to be made up of said comets that were sent outward by the gravitational influence of the planets.
The Kuiper Belt is the home of every dwarf planet in the Sol System, apart from Ceres. The eight outer dwarf planets are all generally similar in orbital characteristics and physical composition, with each being small, icy bodies with highly inclined orbits and long orbital periods.
Despite this, each seems to have something which sets it apart from the others. Pluto (39.4 AU) is the largest of the outer dwarf planets, and the only one with a measurable atmosphere. It is also known to have trace amounts of many organic molecules. Eris (67.7 AU) is the second-largest of the outer dwarf planets, and has a substantial amount of methane on its surface. Nerrivik (479.6 AU) and Degei (45.5 AU) are, like Pluto, covered in organic molecules and hydrocarbons, which results in them being some of the 'reddest' objects in the Sol System. Gonggong (67.3 AU), Tobet (43.6 AU), and Shiryō (39.2 AU) possess relatively dark surfaces and are geologically active, with both possessing substantial cryovolcanic systems. Quetzalcoatl (43.2 AU) has a rotation period of just under 4 hours, which results in it having a very oblate shape. This rotational is thought to have been caused by the collision that created it's natural satellites.
Tartarus (594 AU) is the ninth planet from Sol. It is the largest of the non-gaseous planets, possessing a mass just under 10 times that of Earth. Tartarus is the most-recently discovered planet, with its' existence being confirmed for only about 45 years as of 2110. Tartarus' surface is similar to that of many Kuiper Belt & Scattered Disc objects, almost totally covered in extremely thick ice sheets and dusted with Tholins. Tartarus' ring system is also unique. At nearly 100 kilometers thick, it is theorized that the rings are relatively young and the result of a recent collision or breakup of one of its' moons. Tartarus has 12 confirmed moons, with only the two largest (Thanatos and Typhon) being considered major.