Lots of talk recently about what a planet is. I'm mostly bored by the discussion, because it's an argument on semantics for most people (except for the scientists who need to classify things). The one aspect of it that amuses me is this contradiction that I saw in a bunch of places (
newsday CNN ABC News):
Pluto is automatically disqualified because its oblong orbit overlaps with Neptune's.
This based on the recent IAU resolution 5 (below). So here's my question: based on the above logic, wouldn't Neptune also be disqualified since Pluto overlaps with its orbit? I suppose they (the media) phrased it this way because they figured (1)(c) below is too confusing for most people to understand. What does "has not cleared the neighbourhood around its orbit" mean, anyway? I think the IAU knows what it meant. I'm curious whether Neptune had anything to do with them not calling Pluto a planet. I think not (seems to me that Pluto's moon Charon, or other nearby objects might have something to do with it).
RESOLUTION 5A
The IAU therefore resolves that planets and other bodies in our Solar System be defined into three distinct categories in the following way:
(1) A planet1 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) shape2, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
(3) All other objects3 orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".
(see the rest of the resolutions
here)
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