In 2005, a cosmic object five percent bigger than Pluto was discovered. This new celestial body was named Eris. Scientists believed Eris was the tenth planet of our solar system. This discovery rekindled the idea of what exactly a planet is, and Pluto had to be re-evaluated. Classification is necessary in order to understand the place of an object in the universe. As scientists explore space, they discover more and more celestial objects every day, and in a field evolving at such a rapid pace, classification should be flexible. Planet classification changed over time. The ancient Greeks, observing the night sky, realized that some of the points of light move against the background of fixed stars. They called these lights ‘planet’, the ancient Greek word for ‘wanderer’. For thousands of years, the word planet, a term inexact scientifically, was sufficient to distinguish other celestial objects than stars. From this point of view, Earth was not a planet, as it was not wondering on the sky of the Ancient Greece. This view changed completely with the invention of the telescope. While at the time of its discovery in 1930 Pluto was considered a planet, with the recent discoveries in the Kuiper Belt it only made sense that a re-classification of the celestial objects in the solar system was in place. Therefore, the International Astronomical Union voted to downgrade Pluto from the rank of planet, to that of dwarf planet. This act stirred up a lot of controversy, as Pluto has been the ninth planet for 75 years. Many astronomers still do not accept the official classification of planets and dwarf planets given by the International Astronomical Union. Planet or dwarf planet, the debate over the demotion of Pluto is more a sociological matter than a scientific one. The topic of defining a planet is still being vigorously debated in the world of astronomy (NASA Headquarters).According to the new classification, a planet has to have the following three characteristics: to orbit the sun, to have enough gravity to assume a nearly round shape, and to have cleared its neighborhood of other objects. Unlike planets, dwarf planets did not clear their neighborhoods, and they are also not classified as moons of other planets (NASA Headquarters). Pluto orbits in the Kuiper Belt, and therefore its neighborhood is very busy with other objects. When it comes to controversy, it is quite easy to debate the issue of clearing a celestial object’s neighborhood. Many astronomers assert Jupiter has not completely cleared its orbital path of Trojan asteroids, and therefore it cannot be considered a planet. However, with Jupiter being the largest object in the solar system after the sun, it would be hard to disqualify it to the level of dwarf planet, not to mention that Jupiter has actually cleared its neighborhood, and still clears other planet’s neighborhoods, by swallowing most of the asteroids or comets on their path to other planets, an important detail for the earthlings’ hope for a peaceful life. Since not clearing its orbital path is not a property of a planet, the criterion is actually fulfilled. Other astronomers debate that, if any of the planets would orbit the sun inside the Kuiper Belt, it would not classify as a planet, as it would not be able to clear such a busy neighborhood. They claim that the new planet classification focuses more on what is around a planet, rather than its properties. This idea is also debatable, because Mars and Jupiter are separated by the asteroid belt, and they still cleared their neighborhood long time ago.
Pluto has other peculiar characteristics making it quite different than all the planets in the solar system. Its orbit is tilted 17° out of the plane of the solar system, it moves in an eccentric ellipse, and its orbit crosses the orbit of another planet. Unlike the planets, Pluto is neither rocky, nor gaseous, but made mainly of three types of ice: carbon monoxide, methane and nitrogen. Its moon Charon has a rotation tidally locked by Pluto, and while our planet has done the same thing to its moon, Charon is pretty large as compared to Pluto, and still it has tidally locked Pluto’s rotation (deGrasse 3).
Pluto is therefore a very peculiar celestial body, and its only common characteristics with the planets are that Pluto also orbits the sun, although in an eccentric ellipsoidal orbit, and its shape is round and similar to the planets. While some people might say that not clearing its neighborhood of space debris and having a few unique traits should not mean Pluto does not deserve to be called a planet, it does not matter so much what Pluto is, but rather that it is a member of the solar system, and has its own contributions.
In the meantime, scientists should stay flexible in regard to classification. Classification can and should change based on new knowledge. Humanity is barely making its first baby steps in space exploration, and therefore the amount of surprise waiting out there is so huge, that human classification based on limited knowledge may not mean so much. As far as Pluto is concerned, NASA’s New Horizons mission, a space probe traveling from Earth to the end of the solar system, with an estimate time of arrival in 2015, will provide new insights about the dwarf planet and its surroundings, and who knows how the classification of the celestial objects in the solar system will look like at that time?
References
deGrasse, T. N. (1999).Pluto is not a planet. Natural History Magazine, February 1999. Electronically retrieved from http://www.planetary.org/explore/topics/topten/tyson_pluto_is_not.html
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