Before 2006, the categorisation of objects in the solar system was simple. Because there were no official rules defining the status of celestial bodies, objects were described according to these rules of thumb:
1. The sun is at the centre of the solar system.
2. Any large object which orbits the sun is a planet.
3. Anything which orbits a planet is a moon.
4. If 1, 2 and 3 don’t apply, then the object is something else - probably an asteroid, comet or meteoroid.
The problem with these rules is that the word 'large' is rather open to personal interpretation. Nevertheless, in the 20th century, the scientific community was generally happy that there were nine planets, beginning with Pluto. There was the exciting possibility of more planets beyond the orbit of Pluto, but there was no expectation that this would cause any difficulties with the prevailing classification.
Doubts about the status of Pluto surfaced in the late 1970s, since it had been revealed that the planet was significantly smaller than first calculated. In a short space of time, Pluto had gone from being a little larger than mercury, to being less than half the diameter - smaller than some of the solar system’s larger moons. However, this still meant Pluto was significantly larger than all the minor bodies which had been discovered up until then, and so its status as a true planet was not in jeopardy.
As technology improved towards the end of the century, astronomers began to discover more objects out in the far reaches of the solar system. It was already known that Pluto’s orbit was close to, or overlapped, a region known as the Kuiper Belt, which was populated with many small bodies. What wasn’t known is that the Kuiper Belt contains a number of objects comparable in size to Pluto. By 2005, scientists had discovered three objects which were proving difficult to classify: Eris, Sedna and Quaoar.
If Pluto was a planet, then it was likely that at least some of these new objects would have to be called planets as well. If predictions of a multitude of these discoveries came true, then the list of planets was likely to get out of hand. So began the debate on what should officially constitute a planet... and what everything else that wasn’t a planet should be called. The general assembly of the International Astronomical Union in 2006, reached a consensus on the future of Pluto, Eris and all the rest of the new discoveries.
In future, objects were to be classified according to three properties:
1. Is the object in orbit around the sun, and not around another body?
2. Does the object have enough mass that it forces itself into a nearly spherical shape under its own gravitation?
3. Has the object cleared its orbit of all other objects?
Objects fulfilling all three criteria would be classified as planets. Those fulfilling the first two, but not the third, would be classified as dwarf planets. Anything that didn't fit with the first criterion would, as in the past, be classified as a moon regardless of its size. Anything else that wasn’t a moon, planet or dwarf planet would be classified as a ‘small solar system body’. Pluto was immediately downgraded to dwarf planet status, and other objects were examined to decide where they would fit in the new regime.
It is the third property that has been key in the new definitions of objects orbiting the sun. Studies of small items sharing orbits with the planets showed that the eight planets had cleared all other debris out of their way. This is generally achieved by the smaller objects colliding with the larger, or the gravitational field of the larger throwing the smaller out of the orbit. Some of the smaller objects, including Pluto, have not achieved this orbital clearance, and the measurement of exactly how clear the orbit is now gives scientist and exact cut-off for what constitutes a planet or a dwarf-planet.
There is still some room for confusion in the interpretation of the second rule. Firstly, an objects tendency to for a sphere is dependent on what it's made of. In particular, ice bodies turn spherical more easily than rocky ones, meaning that some small ice objects might be classified as dwarf planets, while larger rockier ones are not. Additionally, some objects, particularly those in the asteroid belt, were once spherical, but have been knocked out of shape by collisions in recent history (cosmologically speaking). They have yet to reachieve their spherical shape, but should do so millions of years into the future.
In September 2006, Eris was officially classified as a dwarf planet, along with Ceres, the largest object in the asteroid belt. Interestingly, Ceres had already been considered a planet for a few decades after its discovery in the early 19th century, until it was downgraded to an asteroid due to its size. Dwarf planets Makemake and Haumea were designated in 2008. Because the precise forms of Sedna and Quaoar have not yet been resolved, they remain on the ‘unknown’ list for the time being. Astronomers have already discovered another 42 objects beyond Neptune which have a good chance of gaining dwarf planet classification and statistical estimations suggest the final total could be around 200.
One other object in the asteroid belt could be a dwarf planet - the asteroid Vesta. NASA's Dawn probe is due to reach the asteroid in mid-2011, and should provide confirmation of its status. NASA has also sent a probe towards the more distant candidates. The New Horizons mission will reach Pluto in 2015 and, if successful, will explore the Kuiper Belt and its dwarf planets for a further 10 years.