Many people have tried to come up with a definition of a planet, but so far no-one has managed to find a statement that definitely describes all planets. One of the recent attempts at a definition is by Gibor Basri of the University of California. In his paper he says:
"A planet is a spherical object never capable of core fusion, which is formed in orbit around an object in which core fusion occurs at some time."
In other words, a planet is spherical, will never start nuclear fusion, and forms around a star.
The only problem with this definition is that no-one is really sure whether all planets are formed around stars. Astronomers have recently detected large Jupiter-like planets wandering through space, not in orbit around any star. This may mean that they were formed around a star and for some reason were knocked out of orbit, or that they were formed in the middle of space in some other process.
Our Solar System
There are nine planets in our Solar System. In order, from the Sun outwards, they are:
The first four are called the terrestrial planets. They are made of rock, like the Earth, with iron rich cores. The next four are called jovian planets, meaning ‘Jupiter-like’ and they are made mainly of gas. Pluto is rocky like the terrestrial planets, but it is so small that many astronomers claim that it is not a planet at all, but is part of a belt of rocky bodies called the Kuiper Belt. In between the terrestrial and jovian planets, there is an asteroid belt containing thousands of rocky asteroids, ranging in size from a few cm to a few km.
Formation of the Solar System
The Solar System formed from a giant rotating cloud of gas and dust. The centre of the cloud collapsed under its own gravity to form the Sun, surrounded by the remaining matter. The rotation of the cloud caused this left over gas and dust to form into a giant disc surrounding the young star. The planets formed from the dust in the disc. Scientists are unsure exactly how this occurred but somehow the dust grains less than 1 micron in diameter accreted to form planets thousands of kilometres across.
There are many clues that the Solar System formed from a giant disc: all of the planets orbit the Sun in the same direction and in one plane – the ecliptic. Almost all of the planets rotate in the same direction as they orbit, and the Sun also spins in this direction. We can see other solar systems being formed in the Orion Nebula.
Click on the links below to find out more about our Solar System and other bodies:
Authors: Carolyn Brinkworth and Claire Thomas
Last updated: July 2001