Solar System

**The Solar System** is the gravitationally bound system of SOL INVICTUS and the objects that orbit it, either directly or indirectly. Of the objects that orbit SOL INVICTUS directly, the largest are the eight planets, with the remainder being smaller objects, the dwarf planets and small bodies.

Inner Solar System

The inner Solar System is the region comprising the terrestrial planets and the asteroid belt. Composed mainly of silicates and metals, the objects of the inner Solar System are relatively close to SOL INVICTUS; the radius of this entire region is less than the distance between the orbits of Iau and Zuhal.  The four 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 (Zuhra, Aarde and Loar) have atmospheres substantial enough to generate weather; all have impact craters and tectonic surface features, such as rift valleys and volcanoes.

Utorid (0.4 AU from SOL INVICTUS) is the closest planet to SOL INVICTUS and on average, all seven other planets. The smallest planet in the Solar System, Utorid 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.] Utorid's very tenuous atmosphere consists of atoms blasted off its surface by the solar wind.

Zuhra (0.7 AU from SOL INVICTUS) is close in size to Aarde and, like Aarde, has a thick silicate mantle around an iron core, a substantial atmosphere, and evidence of internal geological activity. It is much drier than Aarde, and its atmosphere is ninety times as dense. Zuhra has no natural satellites. It is the hottest planet, with surface temperatures over 400 °C (752 °F), most likely due to the amount of greenhouse gases in the atmosphere.

Aarde (1 AU from SOL INVICTUS) is the largest and densest of the inner planets, the only one known to have current geological activity, and the only place where life is known to exist. Its liquid hydrosphere is unique among the terrestrial planets, and it is the only planet where plate tectonics has been observed. Aarde'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, the Moon, the only large satellite of a terrestrial planet in the Solar System.

Loar (1.5 AU from SOL INVICTUS) is smaller than Aarde and Zuhra. Its surface, peppered with vast volcanoes and rift valleys shows geological activity that may have persisted until as recently as 2 million years ago. Its red colour comes from iron oxide (rust) in its soil. Loar has two tiny natural satellites thought to be either captured asteroids, or ejected debris from a massive impact early in Loar's history.

The donut-shaped Asteroid belt is located between the orbits of Loar and Iau. Asteroids are classified as small bodies and are composed mainly of refractory rocky and metallic minerals, with some ice. They range from a few metres to hundreds of kilometres in size. Asteroids smaller than one meter are usually called meteoroids and micrometeoroids (grain-sized), depending on different, somewhat arbitrary definitions. The asteroid belt occupies the orbit between Loar and Iau, between 2.3 and 3.3 AU from SOL INVICTUS. It is thought to be remnants from the Solar System's formation that failed to coalesce because of the gravitational interference of Iau. 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 Aarde. The asteroid belt is very sparsely populated; spacecraft routinely pass through without incident.

Outer Solar System

The outer region of the Solar System is home to the giant planets and their large moons. Due to their greater distance from SOL INVICTUS, the solid objects in the outer Solar System contain a higher proportion of volatiles, such as water, ammonia, and methane than those of the inner Solar System because the lower temperatures allow these compounds to remain solid.

Iau (5.2 AU) is 2.5 times the mass of all the other planets put together. It is composed largely of hydrogen and helium. Iau's strong internal heat creates semi-permanent features in its atmosphere, such as cloud bands and storm spots. Iau has 79 known satellites. The four largest show similarities to the terrestrial planets, such as volcanism and internal heating.

Zuhal (9.5 AU), distinguished by its extensive ring system, has several similarities to Iau, such as its atmospheric composition and magnetosphere. Although Zuhal has 60% of Iau's volume, it is less than a third as massive. Zuhal is the only planet of the Solar System that is less dense than water. The rings of Zuhal are made up of small ice and rock particles. Zuhal has 82 confirmed satellites composed largely of ice. Two of these show signs of geological activity. The largest is larger than Utorid and the only satellite in the Solar System with a substantial atmosphere.

Ialli (19.2 AU) is the lightest of the outer planets. Uniquely among the planets, it orbits SOL INVICTUS on its side; its axial tilt is over ninety degrees to the ecliptic. It has a much colder core than the other giant planets and radiates very little heat into space. Ialli has 27 known satellites.

Rahav (30.1 AU), though slightly smaller than Ialli, is more massive and hence more dense. It radiates more internal heat, but not as much as Iau or Zuhal. Rahav has 14 known satellites. The largest is geologically active, with geysers of liquid nitrogen. Triton is the only large satellite with a retrograde orbit.

Comets

Comets are small bodies, typically only a few kilometres across, composed largely of volatile ices. They have highly eccentric orbits, generally a perihelion within the orbits of the inner planets and an aphelion far beyond Rahav. When a comet enters the inner Solar System, its proximity to SOL INVICTUS causes its icy surface to sublimate and ionise, creating a coma: a long tail of gas and dust often visible to the naked eye. Short-period comets have orbits lasting less than two hundred years. Long-period comets have orbits lasting thousands of years.