Pluto: Is it a planet again?

In recent weeks, Twitter and Facebook have been abuzz with news that Pluto is now a planet again. It would seem that many people have great affection for the little ball of ice and rock orbiting the Sun out past Neptune. However, is it truly a planet? 

Pluto, discovered in 1930 by Clyde W. Tombaugh, has been the subject of intense scrutiny for many decades. Many argued that perhaps it should not be a planet. This controversy came to a head in 2005, when scientists discovered a scattered disc object (later named Eris) which, according to hubblesite.org, has a mass 27% greater than Pluto. The discovery of Eris, as well as other large icy bodies, called Pluto’s status as a planet into question, and in 2006, the International Astronomical Union (IAU) was forced to define what makes a planet a planet. According to the IAU, an object in space is a planet if it is in orbit around the Sun, has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and would have to have “cleared the neighborhood”. Clearing the neighborhood, according to the IAU, means that the object in question has gravitational dominance in its area, meaning there are no other objects of comparable size that have influence on its gravity. Since Pluto only orbits around the Sun and has hydrostatic equilibrium, the IAU demoted Pluto to a dwarf planet. 

Pluto’s position as a dwarf planet has recently been challenged. On September 18, 2014, the Harvard-Smithsonian Center for Astrophysics hosted a debate among three expert scientists regarding Pluto’s status as a planet. The argument for Pluto being a planet, according to Harvard’s website, was that the definition of a planet as it currently stands in the IAU applied only to “…planets in the solar system…” and not “exoplanets orbiting other stars.” The argument asked if planets orbiting other stars were not planets, and if dwarf planets are really nothing but small planets. Essentially, the argument for Pluto being a planet is that the definition of a planet as it stands is far too limiting. According to Harvard’s website, “Dr. Gareth Williams, associate director of the Minor Planet Center, presented the IAU's viewpoint,” while “…Dr. Dimitar Sasselov, director of the Harvard Origins of Life Initiative, presented the exoplanet scientist's viewpoint” and “Science historian Dr. Owen Gingerich, who chaired the IAU planet definition committee, presented the historical viewpoint.” Gingerich argued that a planet is a “‘…culturally defined word that changes over time…’” and that Pluto should therefore be considered a planet. Williams argued that because the IAU have defined what a planet is, Pluto should remain a dwarf planet. Sasselov argued that a planet is “‘…the smallest spherical lump of matter that formed around stars or stellar remnants,’” and that Pluto, by this definition, should be a planet. After all debates were heard, and each scientist made their claims to the best of their ability, the audience voted on who had the best argument. The results were counted, and by audience vote, Sasselov’s definition of a planet won the day, and that Pluto should be considered a planet. However, is it truly considered a planet again? 

It is not in the eyes of the greater scientific community, at least not yet. An audience vote in a hall does not a consensus make. The IAU has yet to agree with this vote. However, this conversation has sparked great debate among the scientific community. The Providence Journal has reported that Alan Stern, principal investigator of NASA’s New Horizons space probe, told CNN, “If you put Earth where Pluto is, it would be excluded! Any definition of planethood that excludes Earth, in any circumstance, is deeply flawed.” This New Horizons probe, according to NASA, is designed to “…help us understand worlds at the edge of our solar system by making the first reconnaissance of Pluto and Charon…”. The probe will be entering Pluto’s orbit, by NASA’s expectations, in July of 2015. While Pluto may not yet officially be a planet, there are many strong cases for it to be one, and New Horizons may just bring in the physical evidence scientists need to reinstate it. Those interested in seeing the debate can easily access a video through YouTube.