Science paper slams IAU planet definition

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Worlds without end! A paper published August 29 in the science journal Icarus has hurled serious criticisms of the definition of planets imposed on the world by International Astronomical Union in 2006 that also robbed Pluto of planetary status.

“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,” said Dr. Kirby Runyon, from the Johns Hopkins University Applied Physics Laboratory. “We showed that this is a false historical claim. It is therefore fallacious to apply the same reasoning to Pluto.”

According to the team, 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. So, they are not the fundamental description of a body, they are just the occupation of a body at a current era,” Dr. Metzger said. “We recommend classifying a planet based on if it is large enough that its gravity allows it to become spherical in shape.”

I must also note that the IAU’s definition had ignored the recommendations of its own committee on coming up with a new planetary definition and was voted on at the very end of a conference when almost everyone had left.

In other words, the IAU’s actions in 2006 were purely political, were bad science, and should be dumped as quickly as possible. And now the scientists are saying this, in peer-reviewed papers.



  • Kirk

    Question: If the definition is changed to restore Pluto’s status as a planet, how many other minor planets would come along with it?

    It used to be that in kindergarten we would cut out construction paper representation of the nine planets to glue to poster board and then learn “My Very Educated Mother Just Served Us Nine Pies.” How many “planets” (however they are defined) should everyone be expected to know well enough to name in order and give a vague description of?

  • wayne

    (I guess that would depend on the definition of “minor planet?”)

    If you were looking at an astronomy text circa the 1890’s, our “solar system” would be listed as:
    “Mercury, Venus, Earth, Mars, Vesta, Juno, Ceres, Pallas, Jupiter, Saturn, Uranus, Neptune.”

  • Alex Andrite

    Follow the money.

  • Orion314

    N.D.G Tyson took public credit for having Pluto demoted. a true astro-racist.

  • Localfluff

    Yeah, you’re right(!)
    There can only exist as many planets as a school kid can remember. About as many as there are continents and oceans on Earth. Thank God we’re not geocentric anymore! That’s a sound scientific definition. Now, how many stars can there exist? Is there any school kid around to ask? And shouldn’t Earth have more Moons since any kid can remember more than one? The IAU should make up some more moons.

  • mkent

    If the definition is changed to restore Pluto’s status as a planet, how many other minor planets would come along with it?

    If they go with using hydrostatic equilibrium as a proxy for roundness, there would be twelve so far: Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Charon, and Eris.

  • Max

    From the article;

    “In 2006, the International Astronomical Union (IAU) established a definition of a planet that required it to clear its orbit.”

    If this is true, then Jupiter needs to be unclassified as a planet. It has plenty of material still floating around in it’s general vicinity. Astroid belt, Trojans that have not been cleared from its orbital path. In fact out of the entire solar system, Jupiter has the most debris…

  • Edward

    My understanding is that it was the discovery of Eris and NASA’s description of it as the tenth planet that panicked the IAU into defining planets in such an erroneous way. The IAU, too, was concerned that school kids would stop learning the names of planets if there were too many of them, and they feared that there could be hundreds of Earth-sized and thousands of other Eris-sized bodies in the Kuiper Belt.

    But then again, there are already too many planets, but most of them orbit other stars. It is worth noting that the IAU definition of planet prevents extrasolar planets from being planets.

    A “planet” is a celestial body that (a) is in orbit around the Sun, …

    There is no official definition for extrasolar planets, but if we can detect it around another star then it must be a planet. Right?

  • Kirk

    @Localfluff, you misunderstood me if you though I was advocating that educational concerns should drive scientific definitions when I was asking the converse of how such a scientific decision should drive what is taught in the earliest levels of education. If the list were extended to 10 or 12, then that might be taught, but were it in the dozens, then only a subset would be. Is there a name better than “important planets” which would single out a smaller group such as the traditional nine?

  • Localfluff

    Why not focus general education the classical planets, the ones visible to the naked eye? (with Uranus being a borderline case, there’re no perfect definitions of such natural phenomena.) Those will have historic and cultural meaning too. Historically the Moon and the Sun were “planets”, but not Earth.

    Ideally a new word should be made up that replaces “world”, which can be either a planet or a moon or a vagabond planet. Something that focuses on the object itself, mainly being large enough to be round. Even so, I think Vesta is a “world” since it was formed round and later deformed by an impact. The IAU should never have raised the question! “Minor planets” worked just fine for over a century. Different definitions make sense for different purposes in astronomy and space exploration.

    What about “orbis”? Too similar to “orbit” maybe. “Mundus”?
    And give Uranus a Latin name, for Christ’s sake! Why is there a Greek among the Romans? Sure, Cicero and Caesar spoke Greek, they were descendants of Trojan soldiers who escaped the defeat. But anyway. I love the mess of astronomical nomenclature! It’s a recorded history of mistakes. Never have so few been so wrong about so much. That’s the traces of true science.

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