New Reasons why Pluto Should be Considered a Planet After All

In 2006, during their 26th General Assembly, the International Astronomic Union (IAU) passed a resolution to adopt a formal definition for the term “planet”. According to this definition, bodies that orbit the Sun, are spherical, do not orbit other bodies, and have cleared their orbits were designated planets. Pluto, and other such bodies that did not meet all of these requirements, would thereafter be designated as “dwarf planets”.

However, according to a new study led by Philip T. Metzger – a planetary scientists from the Florida Space Institute (at the University of Central Florida) – the IAU’s standard for classifying planets is not supported by the research literature on Pluto, and is therefore invalid. For those people who have maintained that “Pluto is still planet” for the past twelve years, this is certainly good news!

Their study – titled “The Reclassification of Asteroids from Planets to Non-Planets” – was recently published in the scientific journal Icarus. The study was led by Metzer and was co-authored by Mark V. Sykes of the Planetary Science Institute, Kirby Runyon of the Johns Hopkins University Applied Physics Laboratory (JHUAPL), and Alan Stern – the principal investigator of the New Horizons mission from the Southwestern Research Institute (SwRI).

The planets in the Solar System, in accordance with the IAU’s 2006 resolution. Image credit: NASA/JPL/IAU

For the sake of their study, the team reviewed scientific literature from the past 200 years to look for instances where the clearing of orbit was used as a requirement for classifying planets. They found only one publication – from 1802 – that relied on this , and the reasoning behind it has since been disproven. In addition, Saturn’s moon Titan and Jupiter’s moon Europa have been routinely referred to as planets by scientists since the time of Galileo.

As Metzger explained in a recent UCF Today news release:

“The IAU definition would say that the fundamental object of planetary science, the planet, is supposed to be a defined on the basis of a concept that nobody uses in their research. And it would leave out the second-most complex, interesting planet in our solar system.”

According to their study, the literature review showed that the real division between planets and other celestial bodies (such as asteroids and planetoids), occurred in the early 1950s when Gerard Kuiper published a paper that made the distinction based on how they were formed. However, this reason is no longer considered a factor when attempting to determine if a celestial body is a planet.

“We now have a list of well over 100 recent examples of planetary scientists using the word planet in a way that violates the IAU definition, but they are doing it because it’s functionally useful,” said Metzger. “It’s a sloppy 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.”

Pluto and its largest moon, Charon. Credit: NASA/JHUAPL/SwRI

According to Kirby Runyon, the IAU’s definition was erroneous since their review showed that clearing an orbit has not been standard practice when attempting to distinguishing asteroids from planets – as the IAU did for the definition they adopted in 2006. Since this is a false historical claim, he said, it should not have been applied to Pluto.

As an alternative, Metzger and his colleagues claim that the definition of a planet should be based on its intrinsic rather than extrinsic properties (such as the dynamics of its orbit), which are subject to change.  In short, they recommend that classifying a planet should be based on whether or not it is large enough that its gravity allows for it to achieve hydrostatic equilibrium (i.e. becomes spherical). As Metzger explained:

“Dynamics are not constant, they are constantly changing. So, they are not the fundamental description of a body, they are just the occupation of a body at a current era… And that’s not just an arbitrary definition. 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’s active geology and dynamism is what allows for it to have an interior  ocean, a multilayered atmosphere, organic compounds and evidence of ancient lakes and multiple moons. According to Metzger, the only planet that has more complex geology is planet Earth.

A new definition of what is a planet would mean there are at least 110 planets in our Solar System. Image Courtesy of Emily Lakdawalla of the Planetary Society, Data from NASA / JPL, JHUAPL/SwRI, SSI, and UCLA / MPS / DLR / IDA, processed by Gordan Ugarkovic, Ted Stryk, Bjorn Jonsson, Roman Tkachenko, and Emily Lakdawalla.

This is not the first time that Runyon and Sterns have recommended that the classification of planets be based on intrinsic properties. Last year, Runyon (then a final-year PhD student at Johns Hopkins University) was the lead author on a study that was prepared in anticipation of the 48th Lunar and Planetary Science Conference. Titled “A Geophysical Planet Definition“, Runyon and his colleagues (which included Alan Stern) offered the following definition for a planet:

“A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters.”

In short, their definition would apply to any astronomical body that has achieved hydrostatic equilibrium, which not only applies to Pluto but most of the Solar System’s largest moons. All told, this definition would result in a Solar System of 110 planets instead of 8. Ergo, Ceres, Pluto, Eris, Haumea, Makemake, Titan, Europa, Ganymede, et al. – all planets!

Ever since the IAU adopted their formal definition, several alternative definitions have been proposed that emphasize things other than orbital characteristics. If nothing else, this has indicated that the “great planet debate” is far from over. In the future, and with additional exoplanet discoveries, it is entirely possible that a consensus will be reached on how we classify astronomical bodies. Until then, we can expect that this issue will remain a controversial one.

Further Reading: UCF Today, Icarus

18 Replies to “New Reasons why Pluto Should be Considered a Planet After All”

  1. One thing I know for sure: The more we learn about our Solar System, the more difficult it becomes to place all the objects therein into neatly-labeled boxes.

  2. Typo on the image with the 110 planets:

    ” new definition of what is a planet would mean there are at least 110 planets in our Solay System.”

    Didn’t see a way to email the author.

  3. Go back to the basics. I like Sesame Street. One of these things is not like the others.

    Planets like within 7.5 degrees of the earth’s ecliptic plane. (Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, Plu–, uh no)

    Planets are at increasing intervals from the Sun. (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Plu–Uh, no)

    Planets have a roughly circular orbit around the Sun (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Plu–Uh, no)

    Planets do not cross the orbit of another planet (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Plu–Uh, no)

    Pluto is 0 for 4.

  4. Why should an object have to orbit on Earth’s ecliptic plane to be a planet? Isn’t such a requirement a violation of the Copernican principle, according to which Earth is not in any way special or the center of anything, just one of many planets.

    All planets have elliptical rather than circular orbits. The only difference among them is the degree ellipticity. Many giant exoplanets have orbits around their stars that are far more elliptical than Pluto’s is around our Sun.

    Is there any scientific principle governing the interval between planets in both our solar system and others? Planetary spacing in many exoplanet systems is far different than it is in ours. Whether or not Pluto fits a possible pattern of intervals seen in other solar system planets tells us nothing about Pluto itself. This is yet one more argument that gives primacy to an object’s location over its intrinsic properties.

    Several exoplanet systems have been discovered in which two giant exoplanets cross one another’s orbits. If orbit crossing precludes an object from being a planet, what are these giant objects?

    The argument that “one of these things is not like the others” does not even hold true regarding Pluto’s composition, which in many ways is similar to that of Earth and the terrestrial planets. Pluto is 70 percent rock; is geologically differentiated into core, mantle, and crust, has an atmosphere that interacts with its surface, and has active geology. Earth actually has more in common with Pluto in terms of intrinsic properties than it does with Jupiter, which has no known solid surface and has a composition like that of the Sun, mostly of hydrogen and helium.

  5. Matt, thank you for writing a fair and balanced article that acknowledges this fascinating ongoing debate. I can’t wait until we start discovering small planets like Pluto in exoplanet systems. It will be interesting to see where in the systems they are located and what intrinsic properties they have.

    My previous comment was a response to the comment by david.paige.

  6. Yeah, I’m having trouble on this new comment system too. Can’t tell what’s a top-level comment and what is a reply to another post.

  7. I agree with Laurele as I had done over 5 years ago here on UT – Nice to see your comments after so many years Laurele 🙂

    To me it is obvious the intrinsic properties will eventually win the day as exoplanet research expands..

    However.. 110 planets.. Yikes – What will Kindegarteners have as a poem to remember that? More like War and Peace than a Haiku… I would advocate “Major planets” and “Minor Planets.” The Major planets being the ones with the same intrinsic properties that are clearly the largest body in their own orbital path – That way the school kids (and the rest of us) can have a way to discuss and classify the main primary planets in a solar system…

    1. Ps. With the Major Planet tweak to intrinsic definition suggestion, I guess Ceres might have to be added to the Mnemonics but seems a small price to have a more friendly, recognisable classification system..

  8. Thank you weeasle! I’ve been writing on this topic since 2006! There is a problem with using the terms major and minor planet because , as scientist David Weintraub notes in his book “Is Pluto a Planet?” the term “minor planet” has been used for 150 years as a synonym for objects not large enough to be rounded by their own gravity–asteroids and comets. Using these terms creates a false dichotomy in which an object is either a major or minor planet, resulting in spherical objects in hydrostatic equilibrium being put in the same class as asteroids and comets. A better system would be that recommended by Alan Boyle in his book “The Case for Pluto,” which is having multiple subcategories under the umbrella of planet. These would include terrestrials, gas giants, ice giants, dwarf planets, etc. Dwarf planets would refer to the class of planets that are not gravitationally dominant in their orbits. This class would include spherical TNOs.

    There is reason why kids should be memorize a list of planet names or even be taught a mnemonic. That mode of teaching goes back to the pre-space age days, when little else was known about the planets other than their names. We don’t ask kids to memorize the names of all the rivers and mountains on Earth or of all the elements in the Periodic Table. A better way to teach the solar system is to educate kids (and adults) about the different subtypes of planets and their major characteristics.

    1. Oops, my second paragraph should read, “There is no reason kids should have to memorize a list of planet names.” It would be good to have an edit function for comments!

      1. Thanks Laurele for the detailed reply. I did not know that about the minor planet definition.. (I just knew about Ceres and Vesta being minor planets)..

        I see your point about not expecting Kids to memorize all the planet names.. I guess its human nature to want to cling to what is familiar..

      2. To this, I would like to quote what Runyon said during our last conversation about the possibility of there being 110 planets:

        ““Fifty states is a lot to memorize, 88 constellations is a lot to memorize. How many stars are in the sky? Why do we need a memorable number? How does that play into the definition? If you understand the periodic table to be organized based on the number of protons, you don’t need to memorize all the atomic elements. There’s no logic to the IAU definition when they throw around the argument that there are too many planets in the Solar System.”

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