What’s a Planet … Again

It’s been a while since I griped about how planets are being defined, so I thought I’d chime in again. Nothing recent has popped up in the news or anything, but I feel that even hashed topics can bear a little rehash.


Ceres as seen by HST.
Credit: NASA

The lead up is basically this: we’ve known of planets for millennia. Some were added to the count after telescopes hit the scene — with Ceres briefly being part of the club until it wasn’t — topping out at nine, they ran the gamut from Mercury to Pluto. Then Mike Brown discovered a planet candidate beyond Pluto, eventually named Eris. This led to a vote at the end of the 2006 International Astronomy Union (IAU) that codified a planet definition that demoted Pluto to the new “dwarf planet” classification.

The new classification became this:

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

(1) A 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, 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, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

(3) All other objects, except satellites, orbiting the Sun shall be referred to collectively as “Small Solar System Bodies”.

Generally speaking, the sticking point is with 1c/2c. “Cleared the neighborhood” is vague and pretty tough to quantify. That’s basically the anti-Pluto (, anti-Ceres, anti-iceball planet) clause. Perhaps if it was more specific, then maybe there’d be less controversy. Instead of “cleared the neighborhood”, how about “is unlikely to collide with another body in the system capable of resurfacing or destroying it for [time period definition]”? I’d suggest something like half the age of a solar system for a system following planetary formation — perhaps up to some limit, and (arbitrarily) 500 million years following stellar ignition for young systems still accreting.

Some complain that it doesn’t matter. “Bad Astronomer” Phil Plait has said, “We don’t need a definition of what a planet is: it’s a cultural thing, not a scientific one.” I disagree. Classification matters. It’s what allows differentiation between a run of the mill supernova and a Type 1a supernova, between a brown dwarf and a star, and so forth. To say it doesn’t matter with a Shakespearean “what’s in a name” flippancy seems to be dodging the issue. Yes, the thing doesn’t change because we call it something or other, but the perception of the thing does — and that isn’t just faddish cultural whimsy.

Pluto and Charon via HST Credit: NASA

Pluto and Charon via HST
Credit: NASA

One of the obvious problems with the IAU definition is that it shows a very provincial mindset that was arguably given voice by Neil deGrasse Tyson who, as director of the Hayden planetarium, explained that he wanted to look at commonalities between objects, grouping the terrestrial planets together, the gas giants together, and Pluto with like objects and to get away from simply counting the planets*. To which I ask, “Why?”

The fervor to demote Pluto and the other trans-Neptunian planets wasn’t based on science as much as it was aesthetics and scientific vanity about the structure of our own singular system. The universe, we’ve been discovering, is amazingly creative with where it places planets. With current technology, we have no way of knowing if any exo-planets have “cleared their neighborhood”, but since they aren’t bumping into anything obvious, we’re just accepting that they are planets, good and proper.

Some mentioned the possibility of us discovering dozens of trans-Neptunian planets and how daunting that would be. Imagine, having to remember the names of more than nine eight planets? Well, Jupiter has 67 moons (as of current accounting), and Saturn 62-ish. I don’t see any rush to reclassify them because there are too many to remember.

I prefer a more inclusive definition and propose something to the effect of:

  1. A planet is a celestial body that (a) is in orbit around its star(s), (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) is unlikely to be destroyed or resurfaced due to collision with another celestial object within its stellar system for at least twice the time spanning the ignition of the star and the end of primary accretion and cooling of the non-stellar bodies of the system.
  2. A fraternal planet (or “twin planet” or “binary planet”) is a celestial body meeting the criteria of (1) except it additionally orbits around another planet, their barycenter being external to both.
  3. A satellite planet (or “big moon” or “round moon”) is a celestial body meeting the criteria of (1) except instead of a star it orbits another planet, their primary barycenter located in the interior of the larger body.
  4. A planetesimal (or “proto-planet”) is a celestial body that meets the criteria of (1) except it has insufficient mass to assume hydrostatic equilibrium.
  5. A moon (or “small moon”) is a planetesimal meeting the criteria of (4) except it orbits around a barycenter located in the interior of another non-stellar celestial body.
  6. A small system body (or lilliput) encompasses everything else where gravity is the primary force holding it together (as opposed to electrostatic forces).

I think that pretty much covers it (mostly — there are refinements for comets and the like, but you get the idea). With this updated classification, we still have a well-settled solar system. Very orderly. Most/all of the big collisions are out of the way for the reasonably foreseeable future. As a result, the “big 8” planets would still be planets. Pluto/Charon would be a binary planet system. Ceres would get to be a planet again (as would Make-Make, Eris, etc.). The Moon (and other planets’ large satellites) would be upgraded to satellite planet. AND, in regards to exo-planets, we don’t marginalize a class of planets simply because they aren’t “cool” enough (just cold).

* I haven’t been able to track down the original citation so I can quote it directly. I, like everyone else, it seems, is relying on Wikipedia — so it has to be true.

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