Slashdot Mirror
How Would You Define a Planet?
It doesn't come easy asks: "The argument over the definition of a planet continues. So far, two definitions are favored but without much consensus so far: base the definition of a planet simply on an object's size. Pluto would be near the lower limit and the newly discovered Kuiper Belt objects could also qualify, giving us 10 or 11 planets so far; or define the single dominant body in its immediate neighborhood as the only qualifying object for planetary status. If no one body dominated (such as the millions of individual asteroids in the asteroid belt) then none would qualify for planetary status. In this case Pluto would be disqualified (Neptune would be the dominant body in Pluto's region of space), and the newly discovered Kuiper Belt objects would also fail to qualify. The International Astronomical Union (IAU) working group charged with pinning down the definition of a planet may vote on the proposals within the next two weeks (or they may decide to start all over again with something new). Maybe Slashdot readers can give them some help. How would you define a planet?"
The problem with disqualifying Pluto because of Neptune being the more dominant body falls apart when you consider the eccentric orbit of Pluto and just how far that takes it from Neptune's "region of space".
What exactly is the definition of a region of space?
How much larger must an object be than its neighbors in order to be considered the dominant object of its neighbors? Twice as large? Four times?
therefore, mercury would NOT be a planet (more like a moon of the sun)
and titan, even though it orbits saturn, WOULD be a planet
i think that makes most the most sense: what an object orbits shouldn't matter, it's composition should be the largest consideration
some other nomenclature can address what it orbits ("a moon of the sun" or "a planet of saturn")
it should be considered either
REGARDLESS of what it orbits
intellectual property law is philosophically incoherent. it is your moral duty to ignore it or sabotage it
Then you have to define "belt" - if you look atthe relative distances of things the object in the kuiper "belt" are more spread out that say the objects (read planets) in the inner solar system of roughly similar size.
Personally I think it is silly to disqualify something because it has "neighbors" or orbits in a "belt".
My $.02:
Any object that revolves around a star, and is not a star, has enough mass to be roughly spherical (say +/- 1% of a perfect sphere) due to its gravity is a PLANET.
Objects that are roughly spherical that revolve around planets are MOONS (regardless of size.) If two objects revolve around each other and their center of gravity lies outside the radius of either partner then it is a binary planet.
Objects that revolve around a star that are not roughly spherical are MINOR OBJECTS. This leads us to a bit of a problem because under my definition there would be no distinction between our friends the comets and asteroids.
I imagine there could be a further classification, based on the shape of the orbit - so we can continue to have "comets" and "asteroids". However I do not like using the orbit shape in any definition.
Objects that revolve around a planet taht are not roughly spherical are SATELLITES.
By the way the reason WHY i do not like using the shape of the orbit or something like the vicinity of other objects in the definition is simple. Those characteristics can be changed. Orbits gradually change over time, especially early in the life cycle of the solar system.
Granted objects are "captured" by planets and stars, and "ejected" in the same way. However, given enough time (and any external influences) don't orbits tend to become circular? So just because comets have highly elliptical orbits now does not mean they will be elliptical forever. The orbital shape is a TRANSITION characteristic. It is not inherent to the object.
Oh well I have rambled on way to long about this....I am ponderig the much larger question - "Why do I care?"