We all know what a planet is, right? We live on one and are surrounded by seven others in our Solar System. Defining a planet is actually more difficult than it seems. The Ancient Greeks believed that Earth was a fixed object and the planets were wandering stars that orbited it. The only planets that ancient civilizations could see with the naked eye are Mercury, Venus, Mars, Jupiter, and Saturn. Back then, the Sun and Moon were also considered planets – an idea that held dominance for centuries until Copernicus formulated his heliocentric model. With the heliocentric model, which put the Sun at the center of the Solar System, the Sun and Moon were no longer classified as planets. The Moon was classified as the first satellite.
In the late 1700’s, Sir William Herschel discovered Uranus, which became the seventh planet in our Solar System. Neptune was discovered in 1846 and Pluto was sighted in 1930. For over seventy years, we had nine planets. This changed in 2006 when astronomers discovered an object larger than Pluto out beyond Neptune. The object, which was named Eris, stirred things up and made the definition of a planet a lot more complicated. In summer of 2006, the International Astronomical Union (IAU) met to decide on the definition of a planet. After much debate, they decided that a 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, and (c) has cleared the neighborhood around its orbit.”
According to this definition, Pluto was reclassified as a dwarf planet because it has failed to clear the neighborhood around its orbit. After Pluto was demoted, we were left with eight planets in our Solar System. The IAU’s definition was not universally accepted though. Many astronomers believe the definition is unclear and arbitrary. A number of people – including Alan Stern who is in charge of NASA’s New Horizons mission – are petitioning for a change of the definition. There are many people who want to see Pluto regain its status as a planet. The classification issue will be readdressed in fall of 2009 when the IAU meets again. As history has shown us, the definition of a planet is continually changing. Many people believe that the current definition is far from perfect and want to see it revised. As astronomers learn more about the celestial objects in our Solar System and in others, we will most likely develop a more accurate definition.
Astronomy Cast has an episode on Pluto’s planetary identity crisis.