Sol

> "*The Solar System is so humongously big*"
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> —Ilya Bryzgalov

The Solar System is the system of the Sun and the objects that orbit it. The main objects in the system are the eight planets: four terrestrial planets (Mercury, Venus, Earth and Mars); two gas giants (Jupiter and Saturn); and two ice giants (Uranus and Neptune). The Solar System also has nine dwarf planets, located in the asteroid belt and Kuiper belt. All four terrestrial planets belong to the inner Solar System, primarily made up of iron and silicates. The four giant planets belong to the outer Solar System do not have a definite surface, and are mainly composed of hydrogen and helium gas. More than 99% of the Solar System's mass is in the Sun and nearly 90% of the remaining mass are in the gas giants.

The solar system's asteroid belts includes a vast number of small objects, including asteroids, comets, meteoroids and dust clouds. These are mainly located between the orbits of Mars and Jupiter (1.5–4.5 AU), and the Kuiper belt just outside Neptune's orbit (30–50 AU). Multiple dwarf planets exist within these belts - including Ceres, Pluto, and Eris

The Solar System is constantly flooded by the Sun's charged particles, the Solar wind, forming the heliosphere. Pushed against by the surrounding interstellar medium of the Local Cloud, the Solar wind starts slowing at 75 to 90 AU being the boundary between Sol and interstellar space. The outermost region of the Solar System is the Oort cloud, the source for long-period comets. The closest star to the Solar System, Proxima Centauri, is 4.25 ly away. The Solar System orbits the Galactic Center of the Milky Way galaxy, as part of its Orion Spur, at a distance of 26,000 ly.

The Solar System developed 4.6 billion years ago, when a molecular cloud was compressed, forming the Sun and a protoplanetary disc. Over time, the disk gradually coalesced into planetary bodies. Hundreds of these bodies may have existed at a time, but most were either destroyed, subsumed into larger bodies, or survived as dwarf planets and stellar debris.

## Sun

The Sun is the Solar System's star and by far its most massive component. Its large mass, which comprises 99.86% of all the mass in the Solar System, produces temperatures in its core high enough to sustain nuclear fusion of hydrogen into helium. This releases an enormous amount of energy, mostly radiated into space as heat and light. The sun is currently in its main sequence phase of life, and is expected to remain so for a further 5 billion years

## Inner System

### Mercury

Mercury is the closest planet to the Sun and smallest planet in the Solar System. Its geology is dominated by impact craters and cooled magma basins, the remnants of early volcanic activity. Mercury has a tenuous atmosphere made primarily of particles captured from the solar wind. Regions of Mercury are currently claimed by various spacefaring nations which seek to exploit its rich mineral crust. However, Mercurian outposts are currently still small scientific research stations, with colonies not expected to become self-sustaining for the foreseeable future

### Venus

Venus is the second planet from the sun. It is similar in size to Earth and has a similar internal geology. The planet displays evidence of volcanic activity, and internal geological activity. Its atmosphere is the thickest in the solar system, over 90 times as dense as Earth's. The Venusian atmosphere is made up primarily of greenhouse gases, and is consequently the hottest planet in the solar system at over 400. It is believed the atmosphere is continually being replenished by volcanic activity, on account of its lack of magnetic field. Various nations currently maintain research stations and military outposts in orbit around the planet - its harsh surface conditions making surface colonization unfeasible

### Earth

*(See: Earth)* Earth is the third planet from Sol and is the only place known to harbor native life. Its liquid hydrosphere and active geology make it unique among the planets. Its atmosphere consists of oxygen and nitrogen, a byproduct of photosynthetic activity. It has a strong magnetosphere, able to shield the planet from harmful cosmic rays. Earth is currently divided into over 180 political polities

#### Luna

Luna is Earth's only natural satellite, and is the only major moon in the inner solar system. The moon is tidally locked to Earth, and it has a substantial effect on the planet's climate. The lunar surface is covered in regolith and marked by mountains, impact craters and dark maria ("seas"), which are plains of cooled magma. It is currently politically dominated by the Lunar Federation, the first independent nation outside of Earth

### Mars

*(See: Mars)* Mars is the fourth and final inner planet. It is smaller than Earth and Venus, and has a significantly lower surface gravity. It has an atmosphere of mostly carbon dioxide 0.6% as thick of that of Earth. However, Mars still supports weather, including wind. Its surface shows a history of geological activity, with volcanoes, rift valleys and river basins that suggest Mars previously had liquid water and a potentially hospitable climate. Its red color comes from iron oxide in its soil, while the polar regions are made up of ice caps consisting largely of water. Mars is currently claimed by the Martian Confederacy

#### Deimos & Phobos

The two moons of Mars are small, irregular bodies thought to be captured asteroids. Phobos, the innermost and larger moon, is heavily cratered, with deep grooves on its surface. Deimos orbits further out with a noticeably smoother surface. Both have been developed by Mars, and contain the majority of Confederacy orbital assets

### The Belt

The Belt is a region of space defined by is relatively dense concentration of asteroids, located primarily between the orbits of Mars and Jupiter. Despite the majority of bodies being between a few centimeters and a few meters in size, the Belt has several large bodies - Ceres, Vesta, Pallas, and Hygiea - which are hundreds of kilometers in size and contain half the total mass of the Belt

## Outer System

### Jupiter

#### Io

The innermost and third-largest of the four Galilean moons of the planet Jupiter. Slightly larger than Earth’s moon, Io is the fourth largest moon in the solar system. It is also the most geologically active body, with over 400 active volcanoes. Its thin atmosphere is made up primarily of sulfur dioxide. The moon is also deep within Jupiter's magnetosphere and radiation belt, making it extremely inhospitable.

#### Europa

The smallest of its four Galilean moons, but sixth-largest in the Solar System. It is primarily made of silicate rock with a water-ice crust, and an iron–nickel core.

#### Ganymede

The largest of Jupiter's moons, and the largest moon in the solar system. Ganymede is composed of approximately equal amounts of silicate rock and water, with a liquid iron core and subsurface ocean.

#### Callisto

The second largest Jovian moon, and is tidally locked with Jupiter. Callisto is about half ice with a metal and rock core.