Friday, May 2, 2014


Discovered in 1930, Pluto was classified as a planet for over seventy years until the International Astronomical Union redefined the term “planet” and reclassified Pluto as a dwarf planet. The new definition was sparked by the fact that new bodies in the solar system were found that were similar to Pluto. Once Pluto was demoted, astronomers and citizens had varied reactions to the change. Even though Pluto is no longer considered one of the main planets, NASA launched their first mission to Pluto, New Horizons, in 2006.

Pluto was discovered during the search for “Planet X,” a planet further into space than Neptune that was disturbing Uranus’s orbit. The search began in 2006 by Percival Lowell and William H. Pickering. Once Lowell died in 1916, the search was put on pause until it was given to Clyde Tombaugh, a self-taught astronomer from Kansas. Tombaugh discovered Pluto by chance while looking at images of the sky taken on January 23, 1930 and January 29, 1930 and seeing if objects in the picture had moved. The discovery was officially announced on March 13, 1930 and Pluto was officially named on March 24, 1930. The name Pluto was chosen because Pluto was the god of the underworld in mythology and Pluto was thought to be dark and cold. The name was also chosen since the first two letters of “Pluto” are Percival Lowell’s initials. 

However, in more recent years, several other celestial bodies were found that made astronomers skeptical about how special Pluto was. The other bodies discovered were Ceres, Haumea, Eris, and Makemake, now all considered dwarf planets. These discoveries made the International Astronomical Union (IAU) rethink what a planet was. The original proposal suggested a Solar System that consisted of twelve planets: the original nine plus Ceres, Eris, and Charon. The IAU defined a planet as “a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.” This proposal became controversial among astronomers since it could eventually mean that they would have to classify many more objects as planets. The proposal remained controversial until the IAU meeting in August 2006.

At the 26th General Assembly for the International Astronomical Union, the main topics being debated were the issues about Pluto. In the end, two resolutions pertaining to Pluto were passed: Resolution 5A “Definition of a ‘planet’” and Resolution 6A: “Definition of Pluto-class objects.” The IAU finally defined a planet as 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 neighbourhood around its orbit.” Together, these resolutions demoted Pluto to being a dwarf planet and also made Ceres, Haumea, Eris, and Makemake dwarf planets as well.

Once this decision was made, the media was filled with reactions about the new classification. Astronomers had very mixed reactions, as many believed that the new definition was unclear and not well defined. Other astronomers, however, believed that this definition was consistent with science and that it would be a definition that lasts. Some astronomers were very indifferent about the situation, believing that the outcome did not affect them one way or another. Citizens also had varying reactions. Some science teachers believed that this change was a good thing for their classrooms, since science is always changing. Other people rejected the new definition, saying that they will always believe that Pluto is a planet. Several states passed resolutions in honor of Pluto: New Mexico declaring that Pluto will always be a planet when in the New Mexican skies and both New Mexico and Illinois stating that March 13 is Pluto Planet Day. While the reactions varied, it still remains that Pluto is no longer a planet. 

NASA still wanted to explore this region of outer space, and so in January 2006, they launched New Horizons, the one-way mission to Pluto and the Kuiper Belt. The goals of the mission are to explore and learn more about Pluto and Charon and then travel deeper into the Kuiper Belt to investigate at least one more Kuiper Belt Object. In February 2007, the spacecraft passed Jupiter and it is expected to reach Pluto in July 2015.
Emily Helfer