The Identity Crisis of a Planet: Pluto’s Discovery and Reclassification

Written by Kat Jivkova

Pluto is presently known as a dwarf planet, after decades of research, constant debate and discovery. Located in the Kuiper Belt, its features include red snow, blue skies, incredibly tall mountains and heart-shaped glaciers. It is considered the most interesting planet in our solar system, as the principle investigator of the Southwest Research Institute has stated: ‘It’s clear to me that the solar system saved the best for last!’ But why was Pluto’s discovery so significant in the first place?

It’s 1905 and astronomer Percival Lowell notices that the orbit of Uranus seems to be disturbed by a gravitational force of some kind. Initially, this abnormal orbit pattern is pinned on the eighth planet from the sun, Neptune. However, Lowell begins to hypothesise in the Lowell Observatory of Arizona that the culprit behind this is in fact a further away, mysterious, Planet X. Calculating the location of this ninth planet, he searches for over a decade for what may have caused the “wobbles” of Uranus’ orbit. He dies in 1916 without discovering what this enigmatic solar body could be, passing the mission on to young researcher, Clyde Tombaugh. Tombaugh takes on the tedious task of monitoring photographic plates in order to determine whether any objects in the sky had shifted. After months of searching, he finally finds Lowell’s planet on 18 February 1930.

After Tombaugh’s discovery, Pluto quickly became a public icon as the first planet discovered by an American. News articles about this new edition to the solar system filled newspapers on March 13 and 14, 1930, marking the end of the search for Planet X. Pluto was given its name after the god of the underworld, which certainly encapsulated a planet so far away from the sun, while its first two letters were also coincidentally the initials of Percival Lowell. The celebrations soon came to a halt one month later as the astronomy community noticed the rate at which Pluto was shrinking. The New York Times reported:

Doubts that the new member of the solar system discovered beyond Neptune is the ninth major planet predicted by Percival Lowell have been raised by computations of its orbit made at the Lowell Observatory.

Astronomers began to suggest alternative descriptions for Pluto as ‘a unique asteroid or an extraordinary comet-like object’ as opposed to a planet responsible for Uranus’ orbital patterns. To makes matters worse, astronomers realised upon the re-examination of Lowell’s calculations that Uranus’ orbit did not need a Planet X at all. Pluto’s discovery was a fluke. Almost four decades later, it had shrivelled up to one five-hundredth of Earth’s mass, while the discovery of other icy objects in its zone further discredited Pluto’s classification as a planet. The discoveries of its moons, on the other hand, enabled astronomers to detect the existence of a region of space at the edge of our solar system, the Kuiper Belt. This is a densely populated zone in which icy objects known as Kuiper Belt Objects (KBOs) follow “elliptical” orbit patterns around the Sun.

Questions began to rise among scientists as to whether Pluto behaved more similar to a KBO than a planet, causing the International Astronomical Union (IAU) to establish a Planetary Definition Committee that would standardise a planet’s definition. The Committee defined “planet” in two ways: (1) as a round object that orbits a star; and (2) it dominated its orbital path – Pluto did not fit the latter criterion, since it is surrounded by KBOs. On 24 August 2006, the 420 IAU members who attended the conference thus reclassified Pluto as a “dwarf planet”, which is defined as ‘a body orbiting the Sun that is round but is not the dominant body in its orbit.’ The solar system once more consisted of eight planets.

Pluto’s reclassification triggered public outcry. Hundreds of school students protested in defence of Pluto, branding the IAU as “planet killers”, while others sent emails condemning the Museum of Natural History’s decision to remove Pluto from its solar system exhibition. Meanwhile, New Mexico’s state legislature felt so strongly about Pluto’s status as a planet that a bill was passed in March 2007 stating Pluto was to be recognised as a planet within the state boundaries. One professor of planetary science who had been involved in the reclassification argued that ‘The word planet and the idea of planets can be emotional because they’re something we learn as children.’ Indeed, the solar system is one of the very first things we are taught in science, which would explain why Pluto’s “demotion” may have sparked such strong emotions among the general public. Meanwhile, its reclassification also resulted in the invention of a new word, “plutoed”, which means to downgrade, and this was awarded the Word of the Year in 2006 by the American Dialect Society.

Ultimately, the story of Pluto reflects the role of scientific institutions in the construction of science. The dwarf planet’s fate was sealed by the IAU. It was the IAU who redefined what a planet was, and who “plutoed” Pluto with its arguably inflexible Aristotelian classification system. The reclassification also reflected the importance of Pluto as both a scientific and cultural object. Planetary science became the subject of the professional and lay media simultaneously, which made Pluto’s demotion even more so emotional. However, its divergence from “planet-hood” should not be viewed in a negative light. If anything, Pluto’s interesting characteristics transcend that of a regular planet, and have enabled scientists to identify other icy bodies within the outer solar system. As astronomer David Jewitt has stated, ‘We have discovered minus one planet and plus one Kuiper belt. It seems like fair trade to me.’


Broughton, Suzanne H, Gale M Sinatra, and E. Michael Nussbaum. “‘Pluto Has Been a Planet My Whole Life!’ Emotions, Attitudes, and Conceptual Change in Elementary Students’ Learning About Pluto’s Reclassification.” Research in science education (Australasian Science Education Research Association) 43, no. 2 (2011): 529–550.

 Chevlen, Dorie. “On the Birthday of Pluto’s Discovery, Science Takes a Look Back on the Dwarf Planet’s Long, Strange History.” Science (American Association for the Advancement of Science) (2017).

Weintraub, David A. “Is Pluto a Planet?” In Is Pluto a Planet?, 179–184. Princeton: Princeton University Press, 2015.

Messeri, Lisa R. “The Problem with Pluto: Conflicting Cosmologies and the Classification of Planets.” Social studies of science 40, no. 2 (2010): 187–214.

HISTORY. ‘Pluto Discovered.’ [Online]. [Accessed on 27 March].

NASA. ‘Pluto.’ [Online]. [Accessed on 27 March].

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