Solar system
The solar system is the best-known planetary system, consisting of the Sun, eight major planets and a large number of minor bodies.
Major planets
Mercury
Mercury is the innermost planet of the solar system. It is a small, rocky world without an atmosphere, and temperatures varying between +400°C on the day side and -150°C on the night side. It was long believed to be tidally locked to the Sun, with one hemisphere permanently facing the Sun and one in eternal night. This has turned out to be incorrect; rather, the planet revolves three times during two orbits around the Sun.
Venus
- Main article: Venus
Venus is the second planet of the solar system. The planet is slightly smaller than Earth and permanently covered in clouds. In the 19th and early 20th century, the fancy that it was a hot and wet but habitable world covered in jungles and coral seas was popular, but we now know that it is a very inhospitable place with a very dense carbon dioxide atmosphere, clouds of sulfuric acid, and surface temperatures of +475°C.
Earth
- Main article: Earth
Earth is the third planet of the solar system and the only celestial body currently known to harbour life. While there are marginal possibilities of microbial life on Mars and Europa, it is pretty certain that advanced, complex lifeforms exist on Earth alone of all worlds of the solar system.
The single Moon of Earth is one of the largest of the solar system. Unlike Earth, the Moon is a barren, lifeless rocky world without an atmosphere.
Mars
- Main article: Mars
Mars is the fourth planet of the solar system. Mars is a small, cold and dry rocky planet with a thin carbon dioxide atmosphere. In the past, it was fancied to be much more hospitable than it actually is (compare Venus). If any kind of life exists on Mars, it is microbial; at any rate, no life has been found yet.
Jupiter
Jupiter is the fifth and largest planet of the solar system, being more massive than all other planets combined. It is a gas giant without a solid or liquid surface - its hydrogen atmosphere just becomes more and more dense with depth until it gradually blends into a liquid-like state.
Jupiter has many satellites. The four largest are Io, Europa, Ganymede and Callisto. It is likely that Europa has an ocean of liquid water beneath its icy crust, kept liquid by tidal friction, in which lifeforms of some kind could exist.
Saturn
Saturn is the sixth planet of the solar system, similar to Jupiter but smaller. Its largest moon, Titan, is the only place in the solar system beyond Earth which has beaches; however, with the "sand" being ice, the "water" being liquid methane, and temperatures of -180°C, there is no trace of beach life.
Uranus
Uranus is the seventh planet of the solar system, quite similar to Jupiter and Saturn but considerably smaller.
Neptune
Neptune is the eighth planet of the solar system, which is very similar to Uranus.
Minor bodies
Asteroids
Asteroids are small rocky objects. Most orbit between the orbits of Mars and Jupiter. While it was once believed that they are the shards of a shattered planet once orbiting between Mars and Jupiter, they are now held to be leftovers of the planet formation process.
The Kuiper Belt
The Kuiper Belt is an assortment of small icy bodies orbiting the Sun beyond Neptune. One of the largest, Pluto, was discovered in 1930 and held to be the ninth major planet. Recently, more bodies of the same kind, some even larger than Pluto, have been found.
The Oort Cloud
The Oort Cloud is a spherical zone even farther out than the Kuiper Belt in which small icy bodies orbit the Sun.
Comets
A comet is an icy body on an eccentric orbit which leads it into the inner solar system. Their origin lies in the Kuiper Belt or the Oort Cloud.
