The Solar System

What are the six parts of the solar system?


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  1. The Sun - the star at the centre of the Solar System, which contains 99.98% of its total mass. By far the biggest, hottest, brightest and most massive object in the Solar System.
  2. The inner Solar System, containing four terrestrial planets orbiting close to the Sun - Mercury, Venus, Earth and Mars. Terrestrial planets are small, made of rock and metal, have a solid surface, thin atmospheres, no rings and few moons (Earth has one large Moon, Mars has two little ones.) Some asteroids (small objects made of rock and metal) orbit the Sun in this zone too - Mars' two small moons could be captured ex-asteroids.
  3. The asteroid belt, between Mars and Jupiter. Most asteroids orbit here. The largest asteroid, Ceres, is about 13 times smaller than Earth, and big enough for its gravity to pull it into a rounded shape, so it counts as a dwarf planet.
  4. The outer Solar System, containing four spread out giant planets - Jupiter, Saturn, Uranus and Neptune. All four are enormous planets, made of gas and ice, have no solid surface, thick atmospheres, all have rings, and all have many moons. A few asteroids (known as Centaurs) orbit here, as well as icy objects called comets. Comets on highly elliptical orbits sometimes pass into the inner Solar System too, where the Sun's heat warms them and the melted/evaporated ice forms a tail.
  5. The Kuiper Belt, a swarm of icy and rocky material beyond Neptune that was too spread out and moving too slowly to collect together into a big planet. Most Kuiper Belt objects are small chunks of ice and rock - comets and asteroids - but some are big worlds big enough to pull themselves into spheres, and these are dwarf planets too. Pluto is the most famous because it was classified as a planet before we knew about the other KBOs, but Eris is slightly larger and a lot more massive. The outer part of the Kuiper Belt, where Eris orbits, contains many KBOs on highly tilted, eccentric orbits, and is sometimes called the Scattered Disc and made into its own section.
  6. The Oort Cloud. We don't know for sure that this exists, but it's thought to be a huge spherical swarm of comets very, very far from the Sun. Most of the comets in the inner sections come from here.