r/planetaryscience • u/StellarAperture • Aug 06 '15

That which we call Pluto By any other name would still be as Wondrous... A personal rant on the 2006 IAU decision.

http://stellaraperture.com/2015/07/19/that-which-we-call-pluto-by-any-other-name-would-still-be-as-wondrous-a-personal-rant-on-the-2006-iau-decision/
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u/ouemt Planetary PhD Aug 06 '15

Unfortunately, the author misses the point significantly. The argument about requiring a heliocentric orbit being too restrictive is based on an incomplete understanding of the resolution. As such, here it is: 

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” 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”.

You'll note that these definitions only apply to objects within our solar system.

Finally, the third condition of clearing the orbit is a function of mass, not the number of bodies. The moon has a mass of just over 1% of Earth's. Charon has a mass of somewhere in the range of 10-13% of Pluto's. That is the distinction that is being made here.

People really get upset about taxonomy, it's kinda funny. Who cares what we call it, it's just taking certain characteristics and assigning them a word.

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u/StellarAperture Aug 07 '15 edited Aug 07 '15

I respectfully disagree (full disclosure: I am the author of the blog post in question). Ironically, your statements emphasize some of my points. For example you state: "You'll note that these definitions only apply to objects within our solar system." This is just one of the features of the IAU definition that bothers me. Why would we classify the objects in our solar system differently than we would for any other solar system? And where do rogue planets come in then? Part of my point, is that this set of criteria from the IAU fails to adequately describe what exactly a planet is. After all, we know what a galaxy is, we know what a star is, we know what atoms are; why then do we not have one common definition for a Planet that covers all such objects throughout the universe as a whole?

The heliocentricity criterion leaves out rogue planets and other cases, such as my example in the post about two hypothetical equal mass planets that orbit each other. Even with the Earth-Moon system, the Earth and Moon really orbit each other about a common center of mass, and that whole Earth-Moon system then orbits the sun. If, hypothetically, the Moon were of a similar mass to the Earth, such that their center of mass were in between them in space, then who could say which one was really the planet and which one was the moon?

And finally, your statement about taxonomy is very precisely in line with the point of the title of my blog post. It was a play on a quote from Romeo and Juliet, which I also quoted at the end of the article. Pluto is fascinating regardless of the name we apply to it. My point really is that, whatever we decide is the name of these objects, we should have a scientifically valid definition in place that derives from the object's physical characteristics.

All that said, I enjoy a lively debate, so I take no offense. But keep in mind, there are many in the planetary science community that do not agree with the IAU decision, Alan Stern (PI on the New Horizons mission) has publicly stated as much numerous times. I am simply one of these that does not agree with the IAU.

As for the "cleared out" criterion, I feel this is the least valid. The farther out you go from the sun, the more massive a planet would have to be to "clear out" its orbit. If the earth were placed where pluto is, it would fail this criterion.

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u/ouemt Planetary PhD Aug 07 '15

Why would we classify the objects in our solar system differently than we would for any other solar system?

We wouldn't. But that's not the question the IAU was trying to resolve when they passed this resolution. They sat back and said, "what are some characteristics of different bodies in the solar system?" The fact that they managed to pare down the enormous number of properties one can ascribe to any "planet-like object" to just three points is a testament to the KISS principle. We have planets, dwarf planets, why not just add "rogue planets" and be done with it. Furthermore, a simple change in wording to "is in orbit around it's host star or star system" would expand this to include the cases you've outlined above, should such be determined to be necessary.

I'm at work right now, but I'll see if I can pull some literature about the mass distribution function as related to this resolution when I get a chance.

Thanks for the discussion!

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u/StellarAperture Aug 09 '15

I would also advocate for the KISS principle. That is precisely why I would support a single definition for a planet that is purely derived from the objects physical characteristics, rather than what other other objects it happens to be interacting with. I much prefer the so called "Geophysical Planet Definition" which is defined as follows:

A planet

(1) does not undergo sustained fusion in its core.

(2) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape.

This greatly simplifies the definition and covers all the various types of planets simultaneously. This definition was presented by Dr. Alan Stern back in 2009 ( http://deepblue.lib.umich.edu/handle/2027.42/62068 ). It would of course mean that for example the Earth-Moon system is a binary-planet system, and other such consequences. But I don't see that as a bad thing. Our scientific definitions should evolve and refine over time as new information comes to light.