There's been a few news articles I've stumbled across lately that addresses the question of planethood. The first article addresses a debate between two "rock stars" in the fields of astrodynamics and planetary science regarding the status of Pluto. Good arguments were made by both, but I really do fall on one side, as indicated in a few paragraphs.
The second article talks about the history of the definition of a "planet" through time. I was very interested to learn that the word planet is derived from the Greek term asters planetai, meaning "wandering stars". Then the article gets into the two primary positions on what a planet is, namely one derived from "dynamic" considerations, and one derived from "geophysical" conditions.
Currently, the IAU made the ruling squarely on the dynamic side, ruling that a planet isn't a planet unless it meets the following criteria:
1. It orbits around the sun.
2. It has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (i.e. nearly round) shape.
3. It has cleared the neighborhood around its orbit of other massive bodies.
There's a few problems with this. First, in #1 above, they used the unfortunate "the" prior to the word "sun", which means that planets around other stars don't fit this definition. Second, in #2, the amount of "roundness" required of a planet is not quantified. And third, in #3, by this definition even Earth isn't a planet since our very own moon is actually quite massive. Regarding this, by "cleared" it is interpreted that this means either by "sucking" up offending bodies, capturing them into stable orbits, or by kicking them out to other orbits.
By this interpretation, Earth fits, but the interpretation of the word "cleared" has given planetary scientists argumentative fits in much the same way as theologians have struggled over John 1:1 from the Bible where it reads: "In the beginning was the Word, and the Word was with God, and the Word was God." What I mean by this is, "Huh?!" How exactly do you interpret something like that?
The "geophysical" definition is really just #2 from the above list, augmented by two distinct features. I'll repeat for clarity:
1. It has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (i.e. nearly round) shape.
2. It is not so massive that it causes internal nuclear fusion (i.e. it's not a star).
3. If gravitationally paired with other massive objects that fit #1 and #2, it is the largest of all the bodies.
Well, okay, so #3 isn't anywhere in the literature, but it's one that I added because it clarifies things beautifully. This is my personal, preferred definition for "planet". In the case of Earth, both Earth-proper and the moon fit both criterias #1 and #2, but since Earth is the largest of the two masses, it is considered the "planet" and the moon is considered a "moon" (naturally). This also allows for the fact that Jupiter has four such very large bodies that orbit it that we can continue to classify as "moons"; as would be so for Pluto, with it's relatively large "moon" of Charon. (Did you catch that, by my definition, Pluto is once again a planet!)
Now, note that my "geophysical" definition doesn't address what the objects may be orbiting. Not only, then, does this provide for objects orbiting other stars to be considered planets, but it also allows for bodies that may actually not be orbiting any star to be considered planets (what are most often called "rogue" planets).
Of course, my simplistic definitions have plenty of holes, too. I think that #1 of the "geophysical" definition should be augmented with a specific number that would define just how round an object would need to be to be considered round enough. Pick a number and suddenly you have a very crisp method for instantly categorizing objects.
And, of course, I think people just need to relax on this whole topic. To me, it's okay to have more than 9 planets. It's okay to swap definitions for objects if things change. Seriously, our universe is a dynamic place, with things moving and changing and crashing together and coming apart all the time! If Pluto were to suddenly be captured into the orbit of a larger body, then, wow! we suddenly lost a planet ... that doesn't make me lose sleep. Neither does the discovery of new objects way out beyond Neptune that are bigger than Pluto. Really, as I've said before, my heart is big enough for all the planets we can find in our little corner of the galaxy.
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