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Message: <blockquote data-quote="Pbartender" data-source="post: 3025713" data-attributes="member: 7533">As does the proposed definition of a planet.  To use the IAU's own words, "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."That's a pretty fundimental definition there.Yes, it is.  Ceres is different because is "has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape".  Other asteroids don't.  Therefore, Ceres is fundimentally different than other asteroids.They aren't, necessarily.  There's a short of of about a dozen other KBOs that could qualify for planethood jsut as easily as Pluto, if the resolution passes.You're looking at it wrong.  The proposed definition includes a few objects that we used to consider asteroids and KBOs, and would be promoted to planets, if it passed.  But it's all just thories, and that's the point.  What happens if in 20 years we discover that the theories aren't quite right, and planets, asteroids or KBOs didn't form in similar manners or even in the way we thought?  We'd have to reclassify everything, since we based or original classification on something that we weren't certain of to begin with.Yes...  Precisely because you have to include the word "supposed" in that sentence.Yep... and that's a lucky coincidence.Nope...  And in astronomy, supposed origin is not a very useful one.There's your problem.  Categories aren't meant to advance discussions and prompt questions.  They are simply there as an organizational tool. Something that can be used to definitively distinguish one set from another.You can still discuss and ask questions regardless of whether or not a method of classification encourages that or not...  A classification method will never prevent such discussion.Consider the means of disguishing planets from other objects...With regards to planets, the first stipulation writes itself: A planet must orbit a star.After that, you need to set a minimum and maximum size.  Again, the upper limit write itself...  We already have definitions for stars, so if the object qualifies as a star, it's too big and is disqualified from being a planet.  The lower limit is the tricky point.  When you take a good look at planets and asteroids, the one fundemental break point in size is when an object has enough mass to make itself round...  So that's what they used.Finally, there are a lot of moons that could technically fit this description, so you add a moon stipulation...  the common center of gravity.Richard Binzel, a member of the Planet Definition Committee, said, "Our goal was to find a scientific basis for a new definition of planet and we chose gravity as the determining factor. Nature decides whether or not an object is a planet."Also realize that they will be creating sub-categories that support questions based on potential origins..."The IAU draft Resolution also defines a new category of planet for official use: "pluton". Plutons are distinguished from classical planets in that they reside in orbits around the Sun that take longer than 200 years to complete (i.e. they orbit beyond Neptune). Plutons typically have orbits that are highly tilted with respect to the classical planets (technically referred to as a large orbital inclination). Plutons also typically have orbits that are far from being perfectly circular (technically referred to as having a large orbital eccentricity). All of these distinguishing characteristics for plutons are scientifically interesting in that they suggest a different origin from the classical planets."</blockquote>

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