The word planet is currently very difficult to define. It has always been a rather fluid notion. When originally coined by the ancient Greeks, a planet was any object that appeared to wander against the field of fixed stars that made up the night sky; hence, "planetes," or "wanderers." This included not only the five "classical" planets, that is, Mercury, Venus, Mars, Jupiter and Saturn, but also the Sun and the Moon. Eventually, when the heliocentric model was accepted over the geocentric, Earth was placed among their number and the Sun was dropped, and, after Galileo dicovered his four satellites of Jupiter, the Moon was also eventually reclassified. A "planet" could then be defined as "any object that orbited the Sun, rather than another object."
The word did not need to be rethought again until 1802, when Heinrich Olbers discovered Pallas, a second "planet" at roughly the same distance from the Sun as the recently discovered "planet" Ceres. Eventually these "planets" numbered in their thousands, and were given their own separate classification, asteroids, letting the concept of planet survive with little modification. Even the discovery of PlutobyClyde Tombaughin1930 had little effect on the idea of what a planet should be, since, despite its size and eccentric orbit, it appeared to fit the concurrent definition.
Then, beginning in 1992, astronomers began to notice large numbers of icy bodies beyond the orbit of Neptune that were similar in composition and size to Pluto. They concluded that they had discovered the long-rumoured Kuiper Belt, the ring of icy debris that is the source of all short period comets including [Halley]]. Pluto's planetary status was thrown into question, with many scientists claiming that it should be demoted to the largest object in the belt and that a "planet" should be redifined along the lines of "an object that orbits the Sun directly and in isolation, rather than as part of a larger group of objects." There has been no little outcry at this suggestion, and the International Astronomical Union has not officially done so as of yet.
However, even with the Pluto controversy excluded, there is no agreed consensus as to what a planet is. The definition of "planet" given in the Wikipedia is, "a body of considerable mass that orbitsastar and that doesn't produce energy through nuclear fusion", which seems fairly unambiguous; however it is frought with uncertainties. Do planets necessarily orbit stars? Many astronomers have claimed to have spotted "rogue planets" drifting in space unattended by any star. "Of considerable mass" is obviously in place to diffrentiate a proper planet from a smaller object, such as an asteroid or a comet, but it is rather vague. What is "considerable mass?" Mercury has avoided any dispute to its planetary status for thousands of years, yet there are moons in the solar system that eclipse it in size. And finally, "does not produce energy from nuclear fusion" certainly separates a planet such as the Earth from a star like our Sun, yet what about brown dwarfs, stars too small to commence fusion in their cores? Many of them are likely to orbit other stars. Does that make them planets?
