Dwarf Planets Pluto Eris Ceres Naming Pluto Pluto's Moons Pluto's Rise and Fall New Horizons 11 Facts about Dwarf Planets
Its title suggests that a Dwarf Planet is a small planet, a round world which orbits the Sun. This is basically true as all objects currently classed as dwarf planets are smaller than any of the regular planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune - they are all also smaller than Earth's moon), but a Dwarf Planet is also different to regular planets because it hasn't "cleared the neighbourhood around its orbit". Each regular planet travels around the Sun. Most of these planets are accompanied by moons which spin around them. However, apart from the odd meteor, there is nothing else in the way of the planet on its route around the Sun. This is because the planet has "cleared" its route, either by absorbing any objects during its formation billions of years ago, or by capturing them to become a moon, or by flinging them away. Dwarf Planets don't have a clear route. Pluto, which was recognised as a planet since 1930 until being reclassified as a Dwarf Planet, is now known to orbit in a region of other objects similar to it. And because Ceres orbits the Sun in the Asteroid Belt, there are a number of asteroids in its path, meaning that it too doesn't have a clear route. But, because it is round and orbits the Sun, the new category of planet means that instead of being continuing to be recognised as an asteroid, Ceres is now a Dwarf Planet!
The early classification of Ceres is similar to the reclassification of Pluto. Of the five current Dwarf Planets, Ceres was the first to be known about. It was discovered on 1st January 1801 by Italian astronomer, Guiseppi Piazzi. So, when everybody else was watching fireworks going off to celebrate the New Year, Guiseppi was standing in his back garden finding Dwarf Planets! Originally, Guiseppi Piazzi thought he had found a comet (must have been those New Year fireworks again), but noticing how it moved, he realised it may be something else. Shortly after its discovery was confirmed, Ceres was actually classified as a planet and became the eighth planet in the Solar System known about at the time (Uranus was discovered earlier in 1751). About 50 years later, it was realised that Ceres was one of a number of similar objects orbiting the Sun in its area. It lost its status as a planet and was reclassified as an Asteroid, or Minor Planet. The area in which it orbits was called the Asteroid Belt. As an Asteroid, It was called 1 Ceres because it was the first object of its kind to be discovered. The Minor Planet Center (MPC), which collects data about all asteroids and comets and calculates their orbits, records Ceres as Minor Planet 1. Just to confuse things, a Minor Planet is not a Dwarf Planet. All asteroids and comets are given MPC numbers.
Orbits of the four Inner Planets plus Ceres and Jupiter
In 2006, the International Astronomical Union met in the Czech Republic to discuss the state of the Solar System. It was getting a mess and needed sorting out! More and more objects were getting discovered in the Solar System - round objects like Pluto - but it was getting difficult to decide what to call them. Should they be planets, asteroids, comets, planetoids, plutons, meatballs.......? A proposal was put forward to call just about all round objects which orbit the Sun "planets". This would have meant that Pluto's moon, Charon, would become a planet (since it orbits the Sun with Pluto, not around Pluto), the newly discovered Eris would be a planet, and Ceres would also become a planet again! However, the scientists at the meeting didn't really like this idea, fearing that the Solar System would contain tens - even hundreds - of planets, most of which are relatively insignificant compared to the eight traditional planets. So, they voted to create the new category of Dwarf Planet, and Ceres instantly became classified as one!