Solar System

The Solar System is the Sun and all the objects that travel around it. The Sun is orbited by planets, asteroids, comets and other things.

The Solar System is about 4.568 billion years old.^[1] The Sun formed by gravity in a large molecular cloud. It is mainly hydrogen, which it converts into helium through nuclear fusion. The planets are in a flattened orbiting disk. This disk was partly left over from the cloud that formed the Sun, plus other material as the Sun moved through space. Eventually, the gas and dust of the disk came together into planets. It is thought that almost all stars and their planets form this way.

The Sun is a star. It makes up 99.9% of the Solar System's mass.^[2] This means that it has strong gravity. The other objects are pulled into orbit around the Sun. The Sun is mostly made out of hydrogen, and some helium and higher elements. All heavier elements, called metals in astronomy, account for less than 2% of the Sun's mass. Oxygen is about 1% of the Sun's mass. Iron (0.2%) is the most plentiful of the other elements.^[3]

There are eight planets in the Solar System. From closest to farthest from the Sun, they are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The first four planets are called terrestrial planets. They are mostly made of rock and metal, and they are mostly solid. The last four planets are called giant planets. This is because they are much larger than other planets and are mostly made either of gas or ices.

Six of the planets, and the six largest dwarf planets, are orbited by moons. There are more than 200 moons in the Solar System. Mercury and Venus have no moons, and Jupiter and Saturn have the largest number of moons. The largest moon is Ganymede which is a moon of Jupiter. Titan is one of Saturn’s moons. It is the only moon in the Solar System to have an atmosphere, which is mainly composed of nitrogen.

The Solar System also contains other things. There are asteroid belts, mostly between Mars and Jupiter. Further out than Neptune, there is the Kuiper belt and the scattered disc. These areas have dwarf planets, including Pluto, Makemake, Haumea, Ceres and Eris. There are thousands of very small objects in these areas. There are also comets, centaurs, and interplanetary dust.

In Ancient Greece, Aristarchus of Samos proposed the heliocentric model of the Solar System, where the Sun, is at the center of the known universe. He is sometimes known as the "Greek Copernicus".^[4]

1. ↑ Connelly, James N.; et al. (2012). "The absolute chronology and thermal processing of solids in the Solar protoplanetary disk". Science. 338 (6107): 651–655. Bibcode:2012Sci...338..651C. doi:10.1126/science.1226919. ISSN 0036-8075. PMID 23118187. S2CID 21965292.
2. ↑ More precisely, 99.86 of its known mass. The total mass of the bodies in the Oort cloud is not known. Jupiter and Saturn account for 90% of the remaining 0.14%.
3. ↑ Hansen, C.J.; Kawaler, S.A.; Trimble, V. (2004). Stellar interiors: physical principles, structure, and evolution (2nd ed.). Springer. pp. 19–20. ISBN 978-0-387-20089-7.
4. ↑ Draper J.W. 1874. History of the conflict between religion and science. New York: D. Appleton 1874, p172–173. ISBN 978-1-59102-533-7