Is Pluto a Planet After All, And Should We Care?

Is Pluto a Planet After All, And Should We Care?
Image Credit: NASA / New Horizons Mission

The image above shows an oblique view of the Norgay and Hillary mountain ranges, and an extensive ice field on Pluto, as photographed by the New Horizons mission.

To many people, it would seem that the recently re-ignited debate over having to decide if Pluto is a planet or something else, is as difficult as it is to find a universally acceptable definition for the concept of time. Many people might also feel that the debate over Pluto’s status is nothing but a minor storm in a small teacup, and/or that the whole thing is actually a non-issue.

However, planetary scientists feel very differently about this. For instance, the difference between being a Kuiper belt object and being a fully-fledged planet (albeit a very small one), has profound implications for current models and theories about how our solar system formed on the one hand, and/or how planetary migrations affect the evolution of multi-planetary systems, on the other.

We need not delve into the intricacies of solar system formation here, but when the International Astronomical Union stripped Pluto of its planet status in 2006, the definition of a planet they used to justify the decision had as many supporters as it had detractors. Briefly, the new definition stated that to qualify as a planet, a object has to a), orbit the Sun, b) it must have assumed a spherical shape due to its own gravity, and c) that it must have cleared all other objects from its orbital path.

Based on these criteria, Pluto passed the test on the first two counts, but failed on the third count on the grounds that it shares its orbital path with one massive (relative to its own mass) moon, and at least four other known Kuiper belt objects. It is precisely this fact that led to a review of the scientific literature about Pluto that stretches back some two hundred years.

According to the authors of a paper entitled “The Reclassification of Asteroids from Planets to Non-Planets”, that was recently published in the journal Icarus, the authors of the paper, Philip Metzger, Mark V. Sykes, Kirby Runyon, and Allan Stern argue that the decision to demote Pluto was based on “flawed” criteria, if not flawed thinking, since no planet is actually able to completely clear out its orbital path.

Based on their review of the literature, it turns out that scientists have been referring to major solar system moons such as Titan and Europa as planets since the early 1600’s. Moreover, a research team from the University of Central Florida could find only one instance of planetary classification that used the “clear its orbital path” criterion. This appeared in a publication from 1802, which was later discredited based on defective reasoning by the author. Additionally, the authors of the Icarus paper point out that there are at least one hundred known instances of modern-era scientists referring to Pluto as a planet simply because it is functionally useful to do so, even though it violates the IAU’s rules on planetary classification.

So, is Pluto a Planet?

According to some scientists, it all depends on what is meant by the term “planet”, since most planetary scientists agree that there are three main types of planet. These are dynamical planets, historical planets, and geophysical planets, and while each type has its own defining characteristics, the umbrella term “planet” applies equally to all of them.

Thus, to planetary scientists, it is important to decide what Pluto is, since it qualifies as both a geophysical and historical planet, but not as a dynamical planet based on the IAU’s definition of a planet. In fact, many astronomers are of the opinion that demoting Pluto to dwarf planet status, or worse, to mere “Trans-Neptunian Object” status based solely on one ill-considered and flawed criterion while ignoring more relevant criteria is ridiculous at best, and indefensible on scientific grounds, at worst.

The counter argument to using geophysical and historical criteria to reclassify Pluto as a planet is that doing so would increase the number of planets in our local system to well over 100. This figure would include all or most of the current major solar system moons, since most have been known as planets for several hundred years, and many are geologically active- some even violently so.


As matters stand now, there seems to be no universally acceptable scientific resolution of this debate in sight. As some astronomers that are involved in this debate state, the biggest issue is the fact that the IAU committee that decided the issue in 2006 was not staffed by planetary scientists, but mostly by cosmologists and experts in other fields of astronomy. In fact, according to some commentators, the committee could not have cared less about Pluto’s place in history, or in the hearts of two generations of astronomers who grew up on the notion that Pluto was the ninth planet.
What do you think, though- is Pluto a planet, after all?

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