The Universe Made Simple/The Solar System/Eight Planets

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There are literally millions of objects orbiting the Sun - asteroids, comets, meteors and Kuiper Belt objects - but eight of these bodies are very large and seem to stand out from the rest. These eight enormous spheres are called the planets and are widely recognised to be the most interesting things in the solar system. In order from the Sun, they are:

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

You can use a mnemonic like "Men Very Easily Make Jugs Serve Useful Needs" to help remember the order of the planets from the Sun.

Some of you may be wondering what happened to the ninth planet, Pluto. Pluto's status has now been officially changed from a planet to a "dwarf planet". Astronomers realised that they didn't have an actual definition of the word "planet," even though they'd been using the word for centuries! By 2006 chaos had emerged and nobody could agree whether there were eight, nine or ten planets.

On August 24th of that year the International Astronomical Union (the official astronomy naming community) held a vote to determine what a planet actually is. The first definition they came up with was that a planet is something that orbits the Sun and is massive enough for gravity to pull it into a round shape. This included the nine planets known at the time (Mercury through to Pluto), but would also include several asteroids and Kuiper Belt objects as well. At the time twelve bodies were officially known to be round and orbited the Sun, but that number could have risen to hundreds as more round bodies were found. So the IAU tried again, and decided a planet should have enough mass to "clear the neighbourhood" around its orbit, and this definition won the vote.

Pluto is not a planet, because it orbits in the Kuiper Belt - its orbit is littered with thousands of other small icy bodies. However, the eight other planets have mainly empty orbits except for their moons and a few asteroids or wandering KBOs (known as centaurs.) Astronomers felt that to just get rid of Pluto would be to ignore nearly 80 years of tradition and recognition of Pluto's status as the ninth planet, and so a new category, "dwarf planet" was created. Pluto is a dwarf planet. Dwarf planets are objects not quite big enough to clear their orbits and become planets, but big enough to be pulled into a round shape.

So now our solar system contains eight planets and three dwarfs - Ceres in the asteroid belt, Pluto in the Kuiper belt and Eris in the scattered disk. This page is about them.