New Evidence that Pluto should still be classed as a Planet

University of Central Florida scientist Philip Metzger, who is with the university’s Florida Space Institute, makes the case that Pluto should be reinstated as a planet.

Metzger, who is lead author on the study, reviewed scientific literature from the past 200 years and found only one publication – from 1802 – that used the clearing-orbit requirement to classify planets, and it was based on since-disproven reasoning.

He said moons such as Saturn’s Titan and Jupiter’s Europa have been routinely called planets by planetary scientists since the time of Galileo.

“It’s a sloppy definition,” Metzger says of the IAU’s definition. “They didn’t say what they meant by clearing their orbit. If you take that literally, then there are no planets, because no planet clears its orbit.”

The planetary scientist says that the literature review showed that the real division between planets and other celestial bodies, such as asteroids, occurred in the early 1950s when Gerard Kuiper published a paper that made the distinction based on how they were formed.

However, even this reason is no longer considered a factor that determines if a celestial body is a planet, Metzger says.

Study co-author Kirby Runyon, with Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, says the IAU’s definition was erroneous since the literature review showed that clearing orbit is not a standard that is used for distinguishing asteroids from planets, as the IAU claimed when crafting the 2006 definition of planets.

Icarus – The Reclassification of Asteroids from Planets to Non-Planets


• The literature on asteroid classification is reviewed from 1801 to the present.
• Scientists considered asteroids to be planets until the 1950s.
• Thousands of asteroids sharing orbits was not debarring from planet status.
• They were reclassified as non-planets on the basis of geophysical characteristics.
• Taxonomical terms like planet are determined by the scientific process, not voting.


It is often claimed that asteroids’ sharing of orbits is the reason they were re-classified from planets to non-planets. A critical review of the literature from the 19th Century to the present shows this is factually incorrect. The literature shows the term asteroid was broadly recognized as a subset of planet for 150 years. On-going discovery of asteroids resulted in a de facto stretching of the concept of planet to include the ever-smaller bodies. Scientists found utility in this taxonomic identification as it provided categories needed to argue for the leading hypothesis of planet formation, Laplace’s nebular hypothesis. In the 1950s, developments in planet formation theory found it no longer useful to maintain taxonomic identification between asteroids and planets, Ceres being the primary exception. At approximately the same time, there was a flood of publications on the geophysical nature of asteroids showing them to be geophysically different than the large planets. This is when the terminology in asteroid publications calling them planets abruptly plunged from a high level of usage where it had hovered during the period 1801 – 1957 to a low level that held constant thereafter. This marks the point where the community effectively formed consensus that asteroids should be taxonomically distinct from planets. The evidence demonstrates this consensus formed on the basis of geophysical differences between asteroids and planets, not the sharing of orbits. We suggest attempts to build consensus around planetary taxonomy not rely on the non-scientific process of voting, but rather through precedent set in scientific literature and discourse, by which perspectives evolve with additional observations and information, just as they did in the case of asteroids.

Defining “Planet”

Metzger says that the definition of a planet should be based on its intrinsic properties, rather than ones that can change, such as the dynamics of a planet’s orbit.

“Dynamics are not constant, they are constantly changing,” Metzger says. “So, they are not the fundamental description of a body, they are just the occupation of a body at a current era.”

Instead, Metzger recommends classifying a planet based on if it is large enough that its gravity allows it to become spherical in shape.

“And that’s not just an arbitrary definition,” Metzger says. “It turns out this is an important milestone in the evolution of a planetary body, because apparently when it happens, it initiates active geology in the body.”

Pluto, for instance, has an underground ocean, a multilayer atmosphere, organic compounds, evidence of ancient lakes and multiple moons, he says.

“It’s more dynamic and alive than Mars,” Metzger says. “The only planet that has more complex geology is the Earth.”


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