I think a planet should be defined as a body that formed from solar material and originally orbited the sun.
To elaborate on this definition, consider these points:
1. That the planet is relatively close to the ecliptical plane 2. That the planet is consistent with spacing of solar-system formation such as the Titius-Bode Law http://en.wikipedia.org/wiki/Bode's_law 3. That the planet is of sufficent size to have orbited the sun for millions of years 4. That the planet is affected by or affects other planets in the solar system
This would define the asteriod belt, Pluto, and Sedna as not being planets. It would also bring Neptune's planet status into question.
With the definition above, we can confidently describe some of the major events since the formation of the solar system: A. The asteriod belt was formed by a planet that never congealed or was ripped apart by tidal gravity or by a large-foreign body B. Venus somehow spins the wrong direction C. Neptune seems to be out of place. Voyager's discovery that Neptune is not a giant ball of gas has implications; either (a) that Jupiter and Saturn also have very solid surfaces or (b) that Neptune is even more out of place. D. Uranus has an extreme tilt, perhaps from Neptune's unusually close position
Without consideration of the dynamics of solar-system formation, our solar system seems like a hodge-podge of bodies that have very mysterious histories. However, a wholistic view brings meaning and richness to the definition of a planet.
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I think a planet should be defined as a body that formed from solar material and originally orbited the sun.
To elaborate on this definition, consider these points:
1. That the planet is relatively close to the ecliptical plane
2. That the planet is consistent with spacing of solar-system formation such as the Titius-Bode Law http://en.wikipedia.org/wiki/Bode's_law
3. That the planet is of sufficent size to have orbited the sun for millions of years
4. That the planet is affected by or affects other planets in the solar system
This would define the asteriod belt, Pluto, and Sedna as not being planets. It would also bring Neptune's planet status into question.
With the definition above, we can confidently describe some of the major events since the formation of the solar system:
A. The asteriod belt was formed by a planet that never congealed or was ripped apart by tidal gravity or by a large-foreign body
B. Venus somehow spins the wrong direction
C. Neptune seems to be out of place. Voyager's discovery that Neptune is not a giant ball of gas has implications; either (a) that Jupiter and Saturn also have very solid surfaces or (b) that Neptune is even more out of place.
D. Uranus has an extreme tilt, perhaps from Neptune's unusually close position
Without consideration of the dynamics of solar-system formation, our solar system seems like a hodge-podge of bodies that have very mysterious histories. However, a wholistic view brings meaning and richness to the definition of a planet.