The term 'dwarf planet' was first coined in 2006. Dwarf planets are smaller than the terrestrial planets in the Solar System and there are five known dwarf planets: Ceres, Eris, Haumea, Makemake and Pluto.
In 2006, following the discovery of celestial bodies similar in size to Pluto, discussions about this prompted the International Astronomical Union to clarify what constitutes a planet. These discussions led to a new classification of a dwarf planet. There are now five known dwarf planets in our Solar System, namely Ceres, Eris, Haumea, Makemake and Pluto. It is thought there may be many more in existence which are yet to be discovered.
There have been as many as 15 planets in our Solar System at any one time, although the current count stands at eight. This is because there has been some disagreement over time about the exact definition of a planet. Pluto was known as the ninth planet following its discovery in 1930 and it completed the Solar System. It was thought to be slightly larger than Mercury. When its moon, Charon, was discovered, Pluto’s true mass was able to be calculated and was actually found to be smaller than Mercury. The discovery of the dwarf planet Eris in the Kuiper Belt, a collection of icy objects beyond the orbit of Neptune, sparked a recent debate.
Eris is the ninth-biggest object known to directly orbit the Sun and was first seen in 2003 at the Palomar Observatory in California by a team of astronomers led by Professor Mike Brown. Although they are credited with the discovery, it was not confirmed until 2005. Eris was found to be slightly larger than Pluto and is the biggest dwarf planet in the Solar System.
As initial indications suggested that Eris was bigger than Pluto, it was a contender to being named the tenth planet of the Solar System. It was during this time that clarification was sought as to what constitutes a planet. As a result of this, Eris was classified as a dwarf planet. Since Eris is bigger than Pluto, Pluto was therefore also downgraded to a dwarf planet.
It was the International Astronomical Union which was tasked with defining what exactly a planet was. It said that to be categorised as a dwarf planet, the object must be in orbit around the Sun, have a mass which allows it to maintain a spherical shape, have not cleared the region around its orbit and not be a moon.
The apparent distinction made when comparing a planet to a dwarf planet is that a planet has a clear orbital path around the Sun. Dwarf planets tend to be grouped together and gather among other similar bodies – as they do in the Kuiper Belt – and cross each other’s orbital paths. In the majority of cases, a dwarf planet is also smaller than Mercury, which is currently the smallest planet in the Solar System.