Pluto used to be a planet, and is not any more. Now, it is a planetoid, or a dwarf planet, depending on whose definition you agree with. It is still also commonly listed as a planet by bodies such as the United States. This paper will look at why Pluto was first named a planet, why a need for a new definition arose, the debate itself, and the consequences of that decision.
Pluto was discovered in 1930, and classified as the ninth and final planet from the sun. It is a significant distance further from the sun than other planets - its orbit is at 30 to 45 Astronomical Units (AU) away from the sun. In perspective this means that it takes light from the sun four hours to reach it at its closest point.
Originally, it was thought that Pluto was the only object of its size at that distance caught in orbit around the sun. It is very small. at around 2,400km in diameter, it is around one-fifth the size of the Earth, and half the size of Mercury. It was later found to have five satellites, with the largest moon (Charon) being discovered in 1978.
However, a problem arose as or telescopes became more powerful at around that time. Other objects that were around Pluto's size were found in significant numbers at the end of its orbit and beyond it, in an area now known as the Kuiper belt, which runs from Neptune's orbit out to 55AU, 6 further than Pluto's maximum distance from the sun. Once it was established that there were many objects in these orbits, it became clear that there was no specific reason why Pluto wold be the only object of its size within the Kuiper belt, fulfilling all the same planetary characteristics that Pluto had.
We now estimate that there are around 70,000 100km+ diameter objects that have roughly the same with the same composition as Pluto in the Kuiper Belt (Cain, 2012). this leads us to a subsnttial problem. If we are to accept that Pluto is a planet, then it means that we will also need to extend this problem. Even if we set Pluto as the minimum size, it is likely to lead to new planets having to be added. On the other hand, if Pluto is not included as a planet because of its size or orbit, it means that the definition of planet will have to be reworked.
Eventually, in an object called 2005 FY9 was discovered by astronomer Mike Brown. It is only slightly smaller than Pluto, and made the problem more urgent. Later the same year, Brown and his team discovered an object named 2003 UB313, which is now known as Eris (since it is also a dwarf planet by the same criteria as Pluto). Eris is approximately 25% heavier than Pluto.
At this point, we must either introduce Eris as a tenth planet within the solar system, or remove both it and Pluto from the list of planets. At this point, controversy arose. At the XXVIth General Assembly of the International Astronomical Union (IAU),in 2006, the body decided to vote upon the matter. they had several choices, including one that would have increased the number of planets to 12, including Ceres, which is currently classed as an asteroid. There were also proposals for 9 and 8. Each of these had definitions which were relatively arbitrary, and in the end the vote was cast for a total of 8 planets, not including Pluto.
the new definition requires three things in order to be a planet. These are that it needs to be in orbit around the Sun, have enough gravity to pull itself into a spherical shape, and to have "cleared the neighbourhood" of its orbit. Since Pluto does not do this (there are many other objects in its relatively wide orbit, and it comes very close to Neptune during one point of its cycle), it does not count as a planet. To put this in perspective, Pluto is 0.07 times the mass of the other objects in its orbit, while the Earth is 1.7 million times the mass of the other objects.
Pluto was thus downgraded to a 'dwarf planet', the first time the term was used. A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite. However, there is still controversy about this, as there is no clear definition for 'cleared its orbit'.