Is Pluto still a planet?
By: Christian H. Oslapas
Tags: NASA, Pluto, Space, Planets, Dwarf Planet

Written by Christian H. Oslapas | 31 July 2020

The Solar System including the Kuiper Belt Objects. Image credit: NASA’s The Space Place, via http://spaceplace.nasa.gov/ice-dwarf/ 0

Discovered in the 1930’s, Pluto is considered to be America’s favorite planet. As the search continues in the Solar System, many celestial bodies are found to be comparable to Pluto. So the decision needs to be made: to accept other large bodies in the Solar System as planets or to reclassify Pluto as a dwarf planet. People love Pluto, so it can’t be demoted right?

The discovery of Pluto was announced with excitement on the 13th of March, 1930. Pluto was discovered by Clyde Tombaugh at the Lowell Observatory. This observatory is still in operation and is located in Flagstaff, AZ. Pluto’s name comes from the Roman god of the underworld and was initially suggested by an 11-year-old named Venetia Burney from Oxford, England. The team at the Lowell Observatory wanted to honor the founder of the observatory Percival Lowell by naming the planet after him. They agreed to naming it Pluto because the first two letters of Pluto are the initials of Percival Lowell.1 In his book The Pluto Files, Astrophysicist Neil deGrasse Tyson explains why Pluto is America’s favorite planet. He shares that since its discovery occurred in America, many Americans have admired Pluto and felt a special connection to the planet. He also explains that since Pluto is the planet that is farthest from Earth in the Solar System, it has always held a certain foreign mystique and curiosity.2

In 1951, astronomer Gerard Kuiper predicted that a belt of icy objects exists beyond the orbit of Neptune. It wasn’t until 41 years later in 1992 that a large icy belt was discovered. Like the Asteroid Belt between Mars and Jupiter, this newly discovered icy belt is composed of many sub-planet sized objects. The Kuiper Belt, named after the astronomer who predicted its existence, contains over 35,000 objects. Among many of the large objects in this belt is Pluto. One of the big differences between the Kuiper Belt and Asteroid Belt is its size. In comparison, the Kuiper Belt is about 20 times wider and about 100 times more massive. An Astronomical Unit or AU is the average distance between the Earth and the Sun. This distance is about 93 million miles and astronomical units are commonly used in astronomical distances within the Solar System because of its simplicity in comparison to using millions/billions of miles. Mars for example is quite close to Earth and is only about 1.5 AU from the Sun. The beginning of the Kuiper Belt starts at about 30 AU and extends to about 50 AU. 3

Since discovering the Kuiper Belt in 1992, many astronomers have searched the Kuiper Belt region looking for new objects within the belt to discover. On December 28, 2004, Haumea was discovered in the Kuiper Belt. This object is roughly the size of Pluto but is greatly distorted and oblong because of its fast spin. Haumea has two moons and has several similarities to known planets. On the 5th of January 2005, a planetary body about the same size as Pluto was discovered and named Eris. While Pluto is about 40 AU from the Sun, Eris is about 68 AU from the Sun. Eris has its own moon and has a lot of similarities to Pluto. On March 31st of 2005, Makemake was discovered to be about 46 AU from the Sun and has its own Moon. 4

Haumea, Eris, and Makemake are three examples of objects found in the Solar System that are roughly the same size as Pluto and have their own Moons. If Pluto is a planet, then shouldn’t these three objects be upgraded to planetary status? In 2006 the International Astronomical Union (IAU) decided to upgrade the definition of a planet. This new definition required a planet to orbit the Sun, have enough gravitational forces to become nearly spherical, cannot be a moon/satellite and cleared its neighborhood around its orbit. Pluto, Haumea, Eris, and Makemake meet all of these planetary requirements except being able to clear its neighborhood. A planet clears its neighborhood by gravitationally interacting with objects nearby. Objects in the neighborhood of a planet will eventually accrete with the planet or become its moons. Since Pluto does not meet the 2006 IAU definition of a planet, it has been demoted to dwarf planet status. Additionally, Haumea, Eris, and Makemake and many other Solar System bodies have been promoted to dwarf planet status. 5 The image below shows some of the largest to smallest bodies in the Solar System. 6

Solar System Year Book (NASA) 6

The demotion of Pluto to dwarf planet status does not mean that Pluto has changed. Pluto has not gotten smaller or significantly reduced its mass. What happened is that five dwarf planets in the Solar System were discovered including Ceres, Pluto, Eris, Haumea, and Makemake. The decision had to be made to either title these celestial bodies planets or dwarf planets. There has been great backlash from people about the demotion of Pluto’s planetary status. It should be remembered though, that Pluto is actually smaller than Earth’s Moon. The Moon has a diameter of 2159 miles while Pluto only has a diameter of only 1476 miles. While five dwarf planets have been located in the Solar System, it is possible that many more could be discovered. Although many astronomers agree that Pluto is no longer a planet, Pluto can still be your favorite dwarf planet. 6

References

0 NASA. (2020, July 30). Solar System [Photograph found in Space Place NASA]. Retrieved August 2, 2020, from http://spaceplace.nasa.gov/ice-dwarf/

1 Choi, C. (2017, November 14). Dwarf Planet Pluto: Facts About the Icy Former Planet. Retrieved August 02, 2020, from https://www.space.com/43-pluto-the-ninth-planet-that-was-a-dwarf.html

2 Tyson, N. D. (2014). The Pluto files: The rise and fall of America’s favorite planet. New York: W.W. Norton & Company.

3 Theplanets.org. (2020). Kuiper Belt Facts: Interesting Facts about the Kuiper Belt. Retrieved August 02, 2020, from https://theplanets.org/kuiper-belt/

4 Davis, P. (2017, November 30). Kuiper Belt: In Depth. Retrieved August 02, 2020, from https://solarsystem.nasa.gov/resources/399/kuiper-belt-in-depth/

5 Howell, E. (2018, April 07). What Is a Planet? Retrieved August 02, 2020, from https://www.space.com/25986-planet-definition.html

6 NASA. (2015, July). Solar System Yearbook. Retrieved August 2, 2020, from https://i.redd.it/8ra30hjcf4kx.png

7 Bartusiak, M. (2018). Dispatches From Planet 3 : Thirty-Two (Brief) Tales on the Solar System, the Milky Way, and Beyond. Yale University Press.