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So how come calling Pluto a planet is so goofy? page 3
Pluto: Not demoted, just reassigned, by Greg Black, of the University of Virginia

 Pluto, imagined here with its moon Charon, was demoted recently to dwarf planet. But there is more to the Plutonic 'neighborhood' than meets the eye.
RON MILLER/ATOMIC ART
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Date published: 5/16/2010

continued

Secondly, a planet must be large enough to be round or nearly so. Small objects, such as most comets and asteroids, don't have enough gravity to pull them into a round shape and are thus disqualified from planet status, with a few hundred miles being roughly the minimum size needed to become roundish. Some argue this is not a test since nothing, not even the large planets, are perfectly round, so there still may be some gray area in deciding how round is round enough.

The third criterion of the final definition represents the sense that a planet should be the largest object in its part of the solar system. This is stated as "clearing the neighborhood around its orbit" in reference to the formation of the planet and its accumulation or ejection of all other material near it. There are still small bodies orbiting near the large planets, especially near Jupiter, but those are so much smaller that essentially the large planets dominate.

How does Pluto fit into this recipe? It does orbit the sun and is large enough to be roundish. However, it fails the third requirement: It has not cleared its neighborhood and is just one of many objects in that region of the solar system. It is not even the largest of those. The IAU defined a new class for this situation, "dwarf planet." A dwarf planet orbits the sun, is nearly round, but has not cleared its orbit. Depending on who is counting, there are five to 15 dwarf planets currently known. The largest three are Eris, Pluto, and Ceres. Objects not large enough to assume a rounded shape are simply grouped as small solar system bodies.

WE WERE USED TO PLUTO


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POPULAR PLUTO FAILED THE FINAL TEST

Gregory Black is a planetary scientist at the University of Virginia.