Derek Shirlaw | Astronomy | Planets
There are 8 planets in our Solar System, however, Scientists have spotted over 250 more orbitting other stars in our galaxy, the Milky Way.
Our planet, Earth, is the only one we know of that supports life.
But what makes a planet, a planet?
A planet is described by the International Astronimcal Union (IAU) as a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, not massive enough to cause thermonuclear fusion in its inner core, and has cleared its neighbouring region of planetesimals (small bits of rock and dust less than about 1km across in size).
This definition means that Pluto is no longer a planet, but a dwarf planet, along with Ceres and Eris. The eight planets of our solar sytem are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Neptune and Uranus.
We often split the planets of the solar sytem up further. The inner four are rocky 'terrestrials', whilst the outer four are 'gas giants'. The dwarf planets can be either 'terrestrial' or 'ice dwarfs'.
