According to the International Astronomical Union, Pluto is a "dwarf planet": a large object that orbits the Sun, is large enough to be forced into a sphere by its own gravity, and has cleared its orbit of asteroids and meteors. The category of dwarf planet was created in August 2006 by the IAU for Pluto and similar distant objects, in effect demoting Pluto from the status of a true planet.
However, Pluto still possesses all of the characteristics that made it a planet just five years ago: it is a large, rocky, spherical body that orbits the Sun, albeit very far away (40 times as far from the Sun as Earth is) and very slowly (one year on Pluto lasts 248 years on Earth). What changed was the discovery of other similarly-sized icy bodies, orbiting even farther out than Pluto, such as Eris. Rather than voting to recognize Eris as the tenth planet in the solar system, the IAU instead created a new category of "dwarf planets," to house all of those objects which are too large to be mere asteroids, but too small to be true planets. The third part of their definition - that a planet has to clear its orbit - was their method of accomplishing this, and has been controversial.
- Pluto as a Planet -
When Pluto was first discovered by Clyde Tombaugh in 1930, there was no serious debate that the newly spotted object was a planet: despite its distance, it clearly orbited the Sun, and later telescope observations clearly proved that it was round, like all other planets. Consequently, models of the solar system were revised, and generations of students grew up memorizing a nine-planet solar system.
Scientific study of Pluto never really caught up with the public fascination with the small planet. When the Voyager probes were sent to tour the outer planets in the 1970s, Pluto was bumped off their itinerary in order to make room for a closer study of the gas giants. NASA has finally launched a probe to study Pluto, called New Horizons, which will speed by Pluto in 2017.
It was also clear that Pluto was very different from other planets. It was only half the diameter of the next-smallest planet, Mercury, and substantially smaller than Earth's own Moon. Moreover, since 1992, scientists have known that Pluto's orbit puts it within a massive group of asteroids and comets known as the Kuiper Belt, stretching across a region of space beyond Neptune, perhaps twenty times as wide as the asteroid belt.
The discovery of the Kuiper Belt, and confirmation that Pluto's orbit fell within the normal range for an inner member of the Belt, suddenly meant that Pluto seemed to belong to a larger group of objects - and those objects weren't planets. Some astronomers, such as Neil Tyson of the Hayden Planetarium, were quick to point out that there were historical parallels for this. Early telescopes had confirmed the existence of three apparent planets - Pallas, Ceres, and Vesta - located between Jupiter and Mars. When it was discovered that these were extremely small objects which belonged to a larger population of small rocks (now called the asteroid belt), these three objects were demoted to asteroid status, and effectively were no longer planets. Tyson removed Pluto from the planet display at his Planetarium.
- Eris -
The problems were mere technical curiosities until new, powerful telescopes began to peer deeper into the Kuiper Belt, and found other large objects there. In 2002, a planetoid half as large as Pluto was discovered: Quaoar. This was followed by Sedna, roughly two-thirds the size of Pluto. In 2005, a third was discovered, called Eris. It was Eris which sparked the crisis in Pluto's identity, as it was quickly confirmed that Eris was even larger than Pluto and had its own moon, Dysnomia.
Eris's discovery threw the annual IAU conference in 2006 into disarray. Should astronomers recognize Eris as the tenth known planet? Going by size and by traditional definitions, this made sense: it was larger than Pluto and followed a similar distant orbit, albeit an even larger and slower one. Eris reaches 97 times as far from the Sun as the Earth at the most distant point in its elliptical orbit, and takes over 550 Earth-years to circle the Sun just once. On the other hand, it was clear that Eris and Pluto had little in common with the large rocky planets and gas giants of the rest of the solar system. And the IAU could not be certain that even more Pluto-sized objects would not be located in the Kuiper Belt. (Several others have indeed been found, though none as large as Pluto.)
Because Eris was larger than Pluto, however, the IAU could not refuse to recognize it as a planet without simultaneously casting judgement on Pluto. Nor could it recognize Eris as the tenth planet without then considering whether Quaoar and Sedna were the eleventh and twelfth planets. Astronomers were reluctant to expand the number of planets so rapidly to incorporate the growing number of relatively insignificant Kuiper Belt objects.
- The New IAU Definition of a Planet -
When the IAU met for its annual conference in 2006, an internal committee had already evaluated ways to get around the problem by redefining the word "planet." It was no longer sufficient, the IAU decided, to define planet simply as a spherical object that orbited the Sun. Too many objects fit this definition - and, if it were adopted formally, it would mean that Pallas and the other asteroid belt planetoids would have to be classified as planets again, too.
The response was to create a compromise: a new category called "dwarf planet," into which all Pluto-sized objects could be placed. Dwarf planets, the IAU proclaimed, were all those objects which might be considered planets except that they had not cleared their orbit of asteroids. This meant that objects like Pluto, in the Kuiper Belt, and Pallas, in the asteroid belt, could not be considered as planets. Five dwarf planets currently exist, of which Pluto is the second-largest, and an additional forty objects are currently being studied to confirm whether they fit within this definition.
The reclassification of Pluto did not please all astronomers. Some were quick to point out that asteroids also cross the orbits of most of the inner planets, including Earth itself - so, if the new IAU definition were followed stringently, Earth, Mars and Jupiter could not be planets either. In addition, Pluto's orbital path crosses Neptune's - so if Pluto has not cleared its orbit, according to such critics, Neptune by definition cannot be considered a planet either. So far these technical objections have not resulted in a new, third definition of a planet.
As of 2010, Pluto is therefore considered a "dwarf planet." The technical reason for this classification is that it has not fully cleared its orbit. However, the real reason for this classification is that astronomers have begun to discover similarly sized objects in the outer solar system, and are reluctant to recognize them as planets.