09-15-2006, 11:37 AM
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Confirmed IT Professional
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Join Date: Nov 2005
Location: Hollywood, CA
Posts: 3,744
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Quote:
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Originally Posted by ADL Colin
So Jupiter is not a planet cuz it's gaseous? Man, don't get them started.
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Let me directly quote from my blog post at my science blog...
Quote:
So what?s all the fuss about?
Since it?s discovery by Clyde Tombaugh at Lowell Observatory in 1930, Pluto has been incorrectly categorized as a planet. With recent observations and studies Pluto has shown itself to not fit well with its counterparts. Although Pluto does share some characteristics which define a planet (like being nonluminous and orbiting a star) it was classified incorrectly in the first place for several reasons?
1. Size. Pluto?s mass is 25 times smaller than Mercury?s and only 9 times larger than that of Ceres, the largest body in the asteroid belt. Some recent large Kuiper Belt Objects (notably Sedna) are likely larger than Ceres, if smaller than Pluto (and the recently discovered ?Xena? or 2003 UB313 may be larger than Pluto). However, any lower cutoff in size for planets is arbitrary, and putting it above or below Pluto?s size and mass is a question of individual preference.
2. Environment. Our solar system can be roughly divided into two kinds of regions: those where major planets orbit, which are mostly free of small bodies, and the regions where there are no planets and where many small bodies orbit (the examples of the latter are the asteroid belt and the Kuiper belt). While this division is not perfect (comets and some asteroids do cross planetary orbits, but these are few and their orbits are unstable), it does reflect an important fact that a planet?s gravity strongly influnces its surroundings, while asteroids affect each other mainly by direct collisions.
According to the IAU the definition of a planet follows:
(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 equalibrium (nearly round) shape, and
(c) has cleared the neighbourhood around its orbit.
Now lets take a look at the definition of a dwarf planet according to the IAU:
(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 equalibrium (nearly round) shape,
(c) has not cleared the neighbourhood around its orbit, and
(d) is not a satellite.
In light of the recent changes made by the International Astronomical Union to the definition of a planet, I have to agree with their re-classifications and removal of Pluto from the list of official planets.
Read more?
Official response: IAU: The status of Pluto: A clarification
Official definition: PDF: Resolution 5, Definition of a Planet in the Solar System
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