Physics Question #3570

steve abel, a 51 year old male from Seattle, WA, USA asks on August 24, 2006,

Can you explain the meaning of the phrase: " has cleared the neighbourhood around its orbit.” I have read that Pluto is no longer considered a planet because its orbit overlaps with that of Neptune, behavior which is contrary to the above criteria. Why then isn't Neptune also eliminated from the list of bodies considered to be planets since it has not cleared its neighbourhood of Pluto?

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The answer

Leigh Palmer, Physics professor, Simon Frasser University, Burnaby, BC Canada answered on October 4, 2006

This business is a mess, and it's not science. Look at the image below and decide whether Pluto has "cleared its orbit" or not. NOTE: click on image for larger version.

Orbital diagram showing orbits of Neptune and Pluto. It is clear everywhere by more than 2 Astronomical Units (the mean distance between the Earth and the Sun) from Neptune's orbit (its nearest neighbor), something that can't be said for Earth, Venus, Mercury, or Mars. Words like "overlaps" and "crosses" when applied to the orbits of Neptune and Pluto simply indicate that two dimensional geocentric thinking is going on.

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