Scientists propose new planet definition that reinstates Pluto

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Unhappy since 2006 with the definition of “planet” imposed by the International Astronomical Union (IAU) that demoted Pluto, planetary scientists, including New Horizons principal investigator Alan Stern, have now proposed a new definition that they think is more appropriate and would reinstate Pluto.

The scientists suggest planets should constitute as “round objects in space that are smaller than stars,” thus excluding white dwarfs, neutron stars, and black holes from the planetary status. “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,” the proposal elaborates, noting that the Earth’s moon would constitute as a planet under the new definition.

Stern and his colleagues note that the IAU’s definition of a planet is too narrow and recognizes planets only as objects that orbit our sun and “requires zone clearing, which no planet in our solar system can satisfy since new small bodies are constantly injected into planet-crossing orbits.”

Make sense to me as well as a lot of people. The definition created in 2006 was never very satisfactory, and I know many planetary scientists who have never accepted it.



  • Steve

    I’d be fine with the current definition that excluded Pluto as long as Pluto is grandfathered in.

    Obviously any new definition would have to take moons into account and exclude them. A body in orbit around a non-stellar object shouldn’t be defined as a planet.

  • LocalFluff

    A planet is round. You know a planet when you see it. If it orbits another planet rather than a star, the planet is called a moon. IAU’s stamp collection definition is fun, but when it fails hard as it has with the planet definition, it is pathetic. The public wonders if they don’t have anything better to do and who feeds them.

    Here’s an illustration to scale of all round objects besides today’s planet definition (whereof Ganymede larger than Mercury). I count 21 (with vesta and Proteus not round enough), that is 29 with the IAU-planets, not too hard to keep track of, is it?

  • Edward

    The 2006 definition was never good, and it was passed without much discussion or refinement, as well as passed after many astronomers had left the conference, making it seem as though it was planned to be passed “in secret in the middle of the night.”

    Neptune, for instance, hasn’t swept Pluto from its neighborhood, thus Neptune should be eliminated as a planet, under the 2006 (mis)definition.

  • ken anthony

    I never cared about a stupid redefinition. Pluto never stopped being a planet to me. Marriage hasn’t changed either.

  • Orion314

    Seriously , I always thought that Plutos demotion from an OBVIOUSLY , historical Planetary status to a “back of the bus” planetoid was nothing less than a poorly concealed “Whitelash”..
    Why? ,
    Consider , if Clyde Tombaugh, Whom I had the honor to meet, was a black man, well. …..the demotion would never have been brought up.

  • Judy

    “including New Horizons principle investigator” Shouldn’t that be “principal”?

  • Judy: Whoops. I make that mistake often. Now fixed. Thank you.

  • Max

    “recognizes planets only as objects that orbit our sun and “requires zone clearing,”

    I just want to point out the fact that the area in our solar system that has the least amount of zone clearing is the space near Jupiter. Not just the astroid belt, but the Trojans that are in front of and behind Jupiter. It appears to me that this theory of “zone clearing” is backwards. The solar system is dirtiest next to the largest vacuum cleaner…
    It occurs to me that a planet definition varies so greatly that you cannot put each body in a one-size-fits-all classification. Just us every sun can be classified by size, brightness, color, unusual properties, etc. planetary bodies could be labeled according to certain criteria. If we make it to another solar system, we should have a classification system that will work in very unusual circumstances that we are familiar with.
    If A planet has Life, they receive a proper name. All others will be labeled according to their size, (GG gas giants)(TP terraform potential, Earth size)( P planetoid, too small to hold an atmosphere)(R rocks, astroids)(C comets, elliptical orbits the cross other Orbits) or given a number that represents the exact distance from the sun at closest approach, with lower case letters labeling the moons of the primary planets. Just a few ideas off the top of my head.
    Now that we have seen Pluto up close, we know that it is unique! Not the dirty snowball the astronomical name callers which don’t like Pluto because it’s too far, and too small, and scribbles outside the orbital lines defined by rigid definitions that rarely occur in the real universe. Pluto should remain a planet under new definitions classifying perhaps dozens of similar circular shaped objects of the elliptical orbiting ice worlds inside the Kuiper belt…

  • Steve Earle

    I keep getting my 6 year old son in trouble at his school….. His teachers and the school books and posters tell him there are 8 planets, but he knows that there are 9 and makes sure to tell them that his Dad said so! ;-)

  • Edward

    A popular revolution against the IAU may be underway, and you and your son may be among the many combatants.

    There was a time when we recognized the difference between a planet, such as Pluto, and an asteroid, such as Vesta or Ceres.

    However, there was also a time when we demoted other planets:
    Ceres, Pallas, Juno and Vesta (in order of discovery) were counted as planets from 1808 until 1845, when smaller asteroids began to be discovered. Astronomically the status of Ceres has changed again. In a proposed Resolution in 2006, it was suggested as one of the 12 planets in our Solar System, but in the end was re-classified as a dwarf planet by the International Astronomical Union.

  • Steve Earle

    Edward said:
    “…A popular revolution against the IAU may be underway, and you and your son may be among the many combatants…”

    I can only hope so Edward. I was amazed along with everyone else at the pictures from the New Horizon Mission and expected that to spark a new vote on Pluto’s status.

    So far that hasn’t happened but I remain cautiously optimistic :-)

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