The definition of “planet”

Class, Historical, Physics, Science, Universe

Ever since the IAU gathered in Prague in 2006 and published a new scientific definition of “planet”, there has been debate on how well they did, and whether they were right to “demote” Pluto from planet to the new “dwarf planet” classification. I aim here to critique the IAU’s definition of a planet.

Artist's concept of the New Horizons spacecraft as it approaches Pluto and its largest moon, Charon, in July 2015.

Artist’s concept of the New Horizons spacecraft as it approaches Pluto and its largest moon, Charon, in July 2015. Both Pluto’s low mass and the relative masses of Pluto and Charon led to the IAU’s new definitions of “planet” and “dwarf planet”.

First, here is the final definition the IAU came up with:

Resolution 5A

The IAU therefore resolves that planets and other bodies in our Solar System, except satellites, 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) shape², (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

(3) All other objects³, except satellites, 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 into either dwarf planet and other categories.

³ These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.

Here’s my take on what they published, point-by-point:

  • Right off the bat, the IAU is restricting the definition to objects “in our Solar System”. On the one hand, this caveat is good: it makes sure to not preclude extrasolar objects from being considered planets. That is, if you discovered one of the over 5000 other known planets in the universe, nothing here is suggesting that you can’t call it a planet. On the other hand, nothing here says you can call it a planet either. All of these other objects are explicitly not defined here. Shouldn’t a brand-new definition of “planet” in this age of extrasolar discovery make sure to actually define the word for all—or at least most!—possible scientific uses? I think it should, and thus find the very premise of this definition lacking.
  • A planet or a dwarf planet must be a “celestial body”. Technically, “celestial” means an object in the sky outside of Earth’s atmosphere, precluding Earth from being a planet. This technicality is ignored in practice, but it is still a mistake that should not be in the definition.
  • A planet and dwarf planet must orbit the Sun. Since the definition only covers the Solar System already, this is fine. But it would be easy enough to replace “the Sun” with “a star or star system” here for a broader definition.
  • A planet and dwarf planet must have enough gravity to be basically a sphere. This seems to be an agreeable requirement, and is well-defined within the broader definition.
  • A planet “has cleared the neighbourhood around its orbit”, while a dwarf planet “has not the cleared the neighbourhood around its orbit”. This is the primary distinction between planet and dwarf planet—and its meaning is not at all clear! The read of this definition is given no clues as to what a large space object’s “neighborhood” is, let alone what it would mean to “clear” such a neighborhood. In my opinion, it is completely unacceptable to publish a definition, meant primarily to create a distinction between Pluto and the other original planets, wherein this primary distinction is itself undefined. As the saying goes, “YOU HAD ONE JOB.” In practice, it seems that “cleared the neighbourhood” is interpreted as meaning that any of the object’s moons have their orbital center of mass within the object itself. Thus, Pluto is a “dwarf planet” rather than a “planet”, because the center of mass between Pluto and it’s “moon”, Charon, is between the two objects, rather than inside Pluto. But official definition should lead to the practice; not be left open to a broad interpretation, and it is still not clear to me why other dwarf planets, such as Eres, are not considered to have cleared their neighbourhood.
  • A dwarf planet cannot be a “satellite”. This just means it can’t be orbiting another planet or dwarf planet or moon or whatever, because a moon would still technically fit the “in orbit around the Sun” requirement (by way of orbiting another body that orbits the Sun). It is not clear why the definition for “planet” does not need this rule, which ultimately leads back to the ambiguity of “cleared its neighbourhood”.
  • Everything else is a “Small Solar-System Body”. OK, I’m not sure what the capital letters are for, and it’s a pretty pedantic name, but this is fine I guess.

Overall, I find the definition lacking, particularly in its prime objective of defining exactly why Pluto et al. are not strictly planets (but they are still dwarf planets, which is apparently not just a regular planet with a modifier before it). It makes sense that the IAU didn’t want to open the door to adding potentially dozens of new planets to our Solar System, but it doesn’t make sense that they couldn’t write the official definition in a more robust manner.

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One thought on “The definition of “planet”

  1. I agree with you here. The first thought I had about the definition is how limiting it is. It rules out several of the other planets by not clearly defining what a “neighborhood” is. For all we know, it could include a planet’s moon or the asteroid belt or anything remotely near the planet. Thanks for sharing!

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