Pluto has been the center for much discussion and controversy regarding its identity. It has been classified as a planet for 75 years; however, astronomers have re-evaluated Pluto’s characteristics and have decided that it really isn’t a full-fledged, traditional planet. It has now been re-classified as a “dwarf planet”, a spherical celestial body that orbits the sun and is not a satellite (moon). The question is: What exactly is Pluto? What is the basis for changing its classification status? First let’s take a brief look at Pluto’s history.
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The quest to find a planet past Neptune began with two men by the names of Percival Lowell and William Pickering. Determined with the search, Lowell built an observatory in Arizona to help locate the hypothetical planet. In 1929, after Lowell’s death, a man by the name of Clyde Tombaugh was hired at the observatory and he carried on Lowell’s quest.
After 7000 hours of work, Tombaugh had finally found a planet past Neptune, and it was publicly announced on March 13, 1930. The name Pluto, a mythological god of the underworld, was suggested to Tombaugh by a Venetia Burney, and the name stuck.
Pluto has been known as “the ninth and last planet,” but stands apart from the previously discovered planets. In 1978, an astronomer noticed bumps on Pluto that seemed to be moving to different spots in different photographs. The bumps turned out to be Charon, Pluto’s moon. With the discovery of Charon, Pluto’s size could confidently be measured. While Charon is 950 miles across, Pluto is only 1485 miles across (smaller than Earth’s moon), which is very small for a planet. In 2005, two more moons, Nix and Hydra, were discovered orbiting Pluto.
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Its orbital period of 249 years is so stretched out, that Pluto was actually closer to the sun than Neptune from January of 1979 to February of 1999. On numerous occasions, Pluto has passed directly over a star within its orbit, an event known as an occultation. Data suggests that as Pluto gets closer to the sun its surface begins to melt, forming a thin atmosphere. As it orbits away from the sun, the atmosphere vanishes as it freezes back to the surface. For this reason, Pluto is much different than the other eight planets. Rather than having a normal planetary composition, Pluto is similar to the space objects within the Kuiper Belt, a vast region of rocky “ice dwarfs” beyond Neptune. While Pluto's composition is unknown, its low density indicates that it is probably a mixture of 70 percent rock and 30 percent ice.
On August 24, 2006 the International Astronomical Union (IAU) came up with a definition for ‘planet,’ and Pluto didn’t fit the description. After much debate and continued controversy, Pluto has been newly classified as a “dwarf planet.”
According to the new rules set in place by the IAU, a planet must meet three criteria: it must orbit the Sun, be big enough for gravity to squash it into a round ball, and it must have cleared other things out of the way in its orbital neighborhood. Dwarf planets, on the other hand, only have one distinctive trait: to be round. By these rules, Pluto fits more as a dwarf planet than a traditional planet. Pluto’s atmosphere is 90% nitrogen, and 10% other complex molecules such as methane, which is very similar to the composition of Earth, which is also 80% nitrogen.
While Pluto was formed similar to other planets and has similar composition, it differs in that it was formed farther out than its current orbit of the solar system. Its body is equivalent to about 1/3 water, making it a very non-dense celestial body. The re-evaluation process was put in motion when scientists and astronomers noticed the resemblances between Pluto and comets. Both Pluto and comets have been formed beyond Pluto’s orbit within the icy Kuiper Belt. While Pluto’s origin is still unknown, we do know that after its formation it moved into its present orbital path where it was discovered and first classified as a planet.
Despite its demotion, Pluto will continue to be researched and explored. To date, no robotic spacecraft has ever visited the dwarf planet. However, a spacecraft called ‘New Horizons,’ the fastest spacecraft ever launched from Earth, was deployed on January 19, 2006 and is due to arrive on Pluto in 2015. Astronomers and scientists will continue to look at Pluto through telescopes and learn more about the mysterious, frozen world while awaiting the revelations of ‘New Horizons.’
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