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planet's
2 dictionary results for: planet's
American Heritage Dictionary - Cite This Source - Share This
plan·et
 (plān'ĭt) Pronunciation Key
(click for larger image in new window) n.
[Middle English, from Old French planete, from Late Latin planēta, from Greek planētēs, variant of planēs, planēt-, from planāsthai, to wander; see pelə-2 in Indo-European roots.] Usage Note: Clyde Tombaugh's discovery of Pluto in 1930 added a ninth planet to our solar system, and thereafter students of astronomy were taught the familiar list of nine planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. But in 2006, the International Astronomical Union (IAU) voted to change the definition of planet, requiring that a clestial body must have enough mass to assume a round shape and "clear the neighborhood around its orbit" in order to qualify as a planet. This means that a planet cannot have other objects in or crossing its orbit except smaller objects that have been captured by its gravity, such as those that revolve around it as moons. Because Neptune's large mass has captured Pluto so that the two planets remain in orbits that cross, Pluto has not cleared its own orbit and was therefore demoted from planet status to that of a newly created category, dwarf planet. Like a planet, a dwarf planet orbits the sun, is large enough to assume a nearly round shape, and does not orbit a planet (as our Moon does). But a dwarf planet does not clear the neighborhood around its orbit and may cross the paths of other objects orbiting the sun. Other dwarf planets include Ceres, whose orbit is in the asteroid belt betwen Mars and Jupiter, and Eris, an object in the Kuiper belt beyond Pluto. At the same meeting the IAU created a third category of objects known as small solar system bodies, which includes asteroids (sometimes referred to as "minor planets," compounding the difficulty of the term planet,) comets, objects beyond Netpune's orbit, and other nonspherical bodies. Although officially approved, this new scheme of the solar system remains controversial among astronomers and may well be revised. |
The American Heritage® Dictionary of the English Language, Fourth Edition
Copyright © 2006 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
Cite This Source
Copyright © 2006 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
Cite This Source
The American Heritage Science Dictionary - Cite This Source - Share This
| planet
(plān'ĭt) Pronunciation Key
A large celestial body, smaller than a star but larger than an asteroid, that does not produce its own light but is illuminated by light from the star around which it revolves. In our solar system there are nine known planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Because of Pluto's small size—about two-thirds the diameter of Earth's moon—and its unusual orbit, many astronomers believe it should actually be classed as a Kuiper belt object rather than a planet. A planetlike body with more than about ten times the mass of Jupiter would be considered a brown dwarf rather than a planet. See also extrasolar planet, inner planet, outer planet. planetary adjective |
The American Heritage® Science Dictionary
Copyright © 2002 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
Cite This Source
Copyright © 2002 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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