Solar system

Asteroid groups
Asteroids in the main belt are divided into asteroid groups and families based on their orbital characteristics. Asteroid moons are asteroids that orbit larger asteroids. They arenot as clearly distinguished as planetary moons, sometimes being almost as large as their partners. The asteroid belt also contains main-belt comets which may have been the source of Earth's water.[54]Trojan asteroids are located in either of Jupiter's L4 or L5 points (gravitationally stable regions leading and trailing a planet in its orbit); the term "Trojan" is also used for small bodies inany other planetary or satellite Lagrange point. Hilda asteroids are in a 2:3 resonance with Jupiter; that is, they go around the Sun three times for every two Jupiter orbits.[55]

The inner SolarSystem is also dusted with rogue asteroids, many of which cross the orbits of the inner planets.[56]

The Solar System[a] consists of the Sun and those celestial objects bound to it by gravity, all ofwhich formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. Of the retinue of objects that orbit the Sun, most of the mass is contained within eight relativelysolitary planetswhose orbits are almost circular and lie within a nearly-flat disc called the ecliptic plane. The four smaller inner planets,Mercury, Venus, Earth andMars, also called the terrestrialplanets, are primarily composed of rock and metal. The four outer planets, Jupiter, Saturn, Uranus and Neptune, also called the gas giants, are composed largely of hydrogen and helium and are far moremassive than the terrestrials.

The Solar System is also home to two regions populated by smaller objects. The asteroid belt, which lies between Mars and Jupiter, is similar to the terrestrial planetsas it is composed mainly of rock and metal. Beyond Neptune's orbit lie trans-Neptunian objects composed mostly of ices such as water, ammonia and methane. Within these two regions, five individual...