astrophysics, Science


A quick reminder, Pluto is not a planet.

It can be hard to accept this. Many people today grew up with 9 planets and it is hard to let one go. But there are reasons why the textbooks needed updating:

Pluto is a rather recent addition to humanity’s knowledge of astral bodies. Its discovery was announced on March 13, 1930 in the Harvard College Observatory when the young astronomer Clyde Tombaugh distinguished a moving body between pictures of the night sky taken between January 23 and January 29 of that year at the Lowell Observatory. This observatory had been tasked with discovering the hypothesized “Planet X” by its founder, Percival Lowell, 14 years earlier. Once discovered, Lowell Observatory members voted on the name for “Planet X” between a list of three: Minerva, Cronos, and Pluto. Pluto, the suggestion of an 11-year-old school girl in Britain, won unanimously.

Originally a planet was a wandering star. Then it was something that moved across the sky. Then it was something that revolved around the Sun. The criterion about when it should be called a planet is something that is changing over time.

- Prof Mark Bailey
Director of Armagh Observatory

While the discovery of a new “planet” made headlines around the globe, this was in a period when little was known about the planets that shared our sun. The name Pluto was proposed under the presumption that something so far from the sun must be cold and dark, reminiscent of the Greek god of the underworld. As more has been learned about the planet, its ranking has become uncertain. The question of Pluto’s classification as planet as been the subject of debate since the late 1970s, when its low mass was determined (It is 1/6 the mass of the Earth’s moon) and other larger bodies were discovered. Some set of rules were needed to determine what was a planet and what was not. The final decision was made on August 24, 2006, when the International Astronomical Union (IAU) codified what qualified a large body in the solar system as a planet. This decision also established the classification of “dwarf planet,” which Pluto falls under. As such, it is actually only the second largest dwarf planet orbiting the sun, after Eris, which is 27% more massive than Pluto.

There are still scientists who object to this decision and argue that, rather than removing Pluto from the classification of Planet, it and other large bodies orbiting the sun should also be planets.

Science works through a process of successive approximation. Our whole view of the solar system, its architecture, was turned upside in the 1990s when we discovered this plethora of Pluto-like planets…Some scientists said, well, that’s too many planets…We have to find some way of limiting the number of planets. I don’t like that way of approaching science, and many don’t.

- Alan Stern
Planetary scientist

Regardless, this makes Pluto an interesting subject of “classification.” Nothing significant on the planet has changed, only our knowledge of it and the solar system it inhabits.


Alan Stern: ‘A Chihuahua is still a dog, and Pluto is still a planet’.” EarthSky. 18 February 2010.

Akwagyiram, Alexis. “Farewell Pluto?BBC News 2 August 2005

Pluto as edited by Wikipedia

Learn More

Brown, Mike. How I Killed Pluto and Why It Had It Coming. 24 January 2012

Houston, Thomas. “Dr. Neil deGrasse Tyson on killing Pluto: ‘All I did was drive the getaway car.’The Verge. 26 March 2012.

Pidpath, Ian. “Pluto: Planet or Impostor?Astronomy: 6–11.

What Is Pluto?NASA.


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