What is a planet? (The IAU)

The IAU: 8 Planets, 161 Moons, 292,793 Minor Planets
The rapid growth in the number of Pluto-sized objects quickly led many astronomers to decide that there needed to be a new definition of planet, one that would solve the “problem” of having so many planets. Fortunately, the triennial convention of the International Astronomical Union (IAU for short) was scheduled to be held in 2006 and it would provide the astronomers with the opportunity to resolve the matter.

As all large scientific bodies do when confronted with an intractable problem, they began by creating a committee to investigate. The committee included astronomers, planetologists, historians, and science writers. They explored the idea of planet from a cultural perspective (“That’s a planet because my grandfather said it was”), a planetological perspective (“That’s a planet because it is big enough to be round” [8]), and an astronomical one (“That’s a planet because there isn’t anything else in its orbit”). That committee then begat a second committee that actually wrote the first proposed IAU definition of a planet:

A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.

For most people, that would have been good enough. The simple definition satisfied both the cultural and planetological perspectives. And the American Astronomical Union’s Division of Planetary Sciences endorsed it. But other astronomers looked on it as a disaster [9]. Under the proposed definition, there could have been several hundred planets in this Solar System alone. It would have brought Ceres and possibly Vesta, Pallas, and Juno back into the list of planets, along with at least seventy round objects orbiting out beyond Pluto.

Astronomers also objected to requiring that the planet be in orbit around a star. They had already started exploring for “rogue planets” that had been ejected from other solar systems during planet formation. Though none have yet been seen, some astronomers estimate that there may be billions of such objects in our galaxy alone. Some astronomers also objected to the second requirement because many moons are either captured into or ejected from orbits around other planets; thus, it would present the strange spectacle of a body changing status based on its location.

So the astronomers threw out the planetologists’ definition and proposed an alternate:

The IAU therefore resolves that planets and other bodies in the Solar System be defined into three distinct categories in the following way:

(1) A planet [1] is a celestial body that
(a) is in orbit around the Sun,
(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
(c) has cleared the neighbourhood around its orbit.
(2) A “dwarf planet” is a celestial body that
(a) is in orbit around the Sun,
(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape [2],
(c) has not cleared the neighbourhood around its orbit, and
(d) is not a satellite.
(3) All other objects [3] orbiting the Sun shall be referred to collectively as “Small Solar System Bodies”.

[1] The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
[2] An IAU process will be established to assign borderline objects to the dwarf planet or to another category.
[3] These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.

The new definition removed the catch-all category of “Minor Planet” that had been in use since 1853. It also created the new category of “dwarf planet”; despite the name, the IAU was adamant that the objects in that category were not planets but something else entirely (though they couldn’t say what that “else” was). It also made the rogue planet problem worse as now all large bodies outside of this Solar System were considered to be “non-planets”. But the most onerous requirement was that a planet must have “cleared its neighborhood”[10] .

The idea behind the requirement was simple. A large broom does a better job of sweeping up debris than a small one does. And a large planet will do a better job of getting rid of the left-over chunks of rock and other assorted stellar trash than a small one will. The astronomers therefore decided to use that idea to help define a planet. If a planet is too small to get rid of all of the junk sharing its orbit, then it really wasn’t a planet. That the added constraint could only be determined by an astronomer was merely fortuitous.

Under the new IAU rules, there would be only eight planets in the Solar System.

When the vote was taken on the last day of the conference, it wasn’t even close. The new definition passed by such a large margin that the IAU didn’t bother to count the votes. And a counter-proposal by the planetologists to define “classical planet” as those objects meeting the first set of criteria and admitting the second set to be planets was voted down by such a large margin that once again the IAU didn’t bother to count the votes. It was official; we now knew what planets were, and Pluto wasn’t one of them.

[8]   As we will see later, once an object becomes larger than about 200 km in radius, gravitational forces shape it into a sphere. The larger and more massive the object is, the more closely it approaches a perfect sphere.

[9] Geologists also objected to the proposal, but for a more prosaic reason: the IAU proposed calling objects like Pluto “plutons”. But the word “pluton” has been used for 300 years in geology to mean “a large, buried solidified mass of magma”. The geologists asked that the term be changed to something less likely to cause confusion.

[10] It is ironic that the idea of clearing the neighborhood had been proposed by two planetologists as a way of differentiating between different sizes of planets. In their scheme, Pluto was a “subdwarf planet” and the Earth was a “dwarf planet”.

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7 thoughts on “What is a planet? (The IAU)

  1. Puck says:

    On face, this is an entry about the evolution of the definition of a planet.
    Having spent most of my adult life (such as it’s been) in academia, though, I’d say this is really a post about giant, flaming rows, unless you physical science types are a lot more civilized than biologists (and I know enough to be skeptical of that).
    Any chances of including some dirt? This is the sort of thing that makes me howl while I’m reading the wizards of Unseen University.
    Of course, you may have a more dignified purpose in mind than chronicling Scientists Behaving Badly, and if that’s the case I’ll continue to munch my popcorn in silence. And I’ll try not to leave any on the floor.

    • venera4 says:

      We aren’t civilized (remind me to tell you about Newton’s death feud with Hooke sometime), but we are cautious. Since many of the folks involved in this debacle are still alive, I’d rather avoid the lawsuits and pretend that we are all just interested in the truth here…

      If you’d like a view from the inside, Mike Brown’s How I Killed Pluto and Why It Had It Coming is pretty good.

      • Puck says:

        I’d read about Newton vs. Hooke and laughed in the past, but as told in Gleick’s biography I referenced, it wasn’t the least bit funny.

  2. Are you going to include discussion of binary planets? I know the IAU has been careful to make sure Earth/Moon doesn’t qualify under their definition, but they are not being consistent with their binary star definition. I’m all for reclassifying us.

  3. […] Union (a body that was set up to keep us from arguing over names) famously decided that not only was Pluto not a planet, it wasn’t even related to “real” planets. Interestingly, they did this even though their own subcommittee said that Pluto should remain a […]

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