The Solar System, also referred to as Sol after the star at its center, is a planetary system comprised of the yellow star Sol and the objects that orbit it, either directly or indirectly. Of the objects that orbit Sol directly, the largest eight are the planets that form the planetary system around it, while the remainder are significantly smaller objects, such as dwarf planets and small bodies such as comets and asteroids. Of the objects that orbit indirectly, two (Ganymede and Titan) are larger than the smallest planet (Mercury).
The Solar System is home to mankind, which originated on Earth, the third planet in the system. The Solar System has been the site of many conflicts between Xenomorphs, Yautja and humans, particularly on Earth itself.
The Solar System formed 4.6 billion years ago from the gravitational collapse of a giant interstellar molecular cloud. The vast majority of the system's mass is in the star Sol, with most of the remaining mass contained in Jupiter. The four smaller inner planets, Mercury, Venus, Earth and Mars, are terrestrial planets, being primarily composed of rock and metal. The four outer planets are giant planets, being substantially more massive than the terrestrials. The two largest, Jupiter and Saturn, are gas giants, being composed mainly of hydrogen and helium; the two outermost planets, Uranus and Neptune, are ice giants, being composed largely of substances with relatively high melting points compared with hydrogen and helium, called ices, such as water, ammonia and methane. All planets have almost circular orbits that lie within a nearly flat disc called the ecliptic.
The Solar System also contains smaller objects. The asteroid belt, which lies between Mars and Jupiter, mostly contains objects composed, like the terrestrial planets, of rock and metal. Beyond Neptune's orbit lie the Kuiper belt and scattered disc, populations of trans-Neptunian objects composed mostly of ices, and beyond them a newly discovered population of sednoids. Within these populations are several dozen to possibly tens of thousands of objects large enough to have been rounded by their own gravity. Such objects are categorized as dwarf planets. Identified dwarf planets include the asteroid Ceres and the trans-Neptunian objects Pluto and Eris. In addition to these two regions, various other small-body populations, including comets, centaurs and interplanetary dust, freely travel between regions. Six of the planets, at least three of the dwarf planets, and many of the smaller bodies are orbited by natural satellites, usually termed "moons" after Earth's Moon. Each of the outer planets is encircled by planetary rings of dust and other small objects.
The solar wind, plasma flowing outwards from Sol, creates a bubble in the interstellar medium known as the heliosphere. The heliopause is the point at which pressure from the solar wind is equal to the opposing pressure of interstellar wind; it extends out to the edge of the scattered disc. The Oort cloud, which is believed to be the source for long-period comets, may also exist at a distance roughly a thousand times further than the heliosphere. The Solar System is located in the Orion Arm, 26,000 light-years from the center of the Milky Way.
- For many years, Pluto was considered the ninth planet in the Solar System. However, after 1992, its status as a planet fell into question following the discovery of several objects of similar size in the Kuiper belt, including Eris in 2005, which is 27% more massive than Pluto. Eris' discovery led the International Astronomical Union to define the term "planet" formally for the first time the following year. The definition excluded Pluto and reclassified it as a member of the new "dwarf planet" category (and specifically as a plutoid). Some astronomers think that Pluto, as well as the other dwarf planets, should be considered planets.