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The Webfooted Astronomer - August 2000
Pluto – the Maybe Planet
By Pat Lewis
OUR Sun, we often say, has a retinue of nine planets. Close to its hot radiation are four smallish rocky bodies: Mercury, Venus, Earth and Mars. Further out, after a gap, are four large bodies: Jupiter, Saturn, Uranus and Neptune – composed mainly of gases and ices. And still further out is the ninth—little rocky Pluto.
As far out as Saturn, the planets are easy naked-eye objects, and Uranus and Neptune are easily found by amateur telescopes. Pluto is another matter. A tiny speck among the stars, it is quite hard to observe; I achieved it using a 12 ˝ inch telescope, but a 16-inch scope showed it more clearly.
This little frozen ball of rock has been a center of controversy almost since its discovery. Some claim it really shouldn't be called a planet. What are the criteria by which we may agree that a body is a planet? For some astronomers it is important that it be spherical. Anything more than about 300 miles in diameter will be pulled into roundness by its own gravitation. It should be able to "sweep out" its orbit by attracting to itself small nearby objects. These tests Pluto passes.
Other astronomers believe that to be a planet a body should have formed in orbit by condensation from the solar nebula. Signs of this origin would be a near-circular orbit approximately in the same plane as all the others, and, as with most of them, counterclockwise revolution and rotation as viewed from the north. Pluto's orbit is unique, more elliptical than any other and at 17 degrees to the ecliptic. Its axis of rotation lies almost in the ecliptic. Only one of the other eight, Uranus, is tipped over in such a fashion.
The eccentricity of Pluto's orbit results in an anomaly; for 20 of the 248 years it takes to go once around the Sun, Pluto actually comes inside the orbit of Neptune and becomes temporarily the eighth from the Sun.
But perhaps more puzzling than any other of Pluto's odd characteristics is its small size and makeup. Only half the size of our Moon, it is composed of rock, covered with ice. As the other planets formed, we believe the heat from the Sun drove away volatile elements from the four nearest, leaving heavier elements to form rocks. Then at the distance of Jupiter, four or five times farther from the Sun, the heat was less and the outer planets were able to aggregate huge quantities of gases. But this scenario leaves Pluto unaccounted for. It seems likely that this bit of rock formed somewhere else and moved somehow into its not-quite-planetary orbit.
The International Astronomical Union has authority in matters of naming and classification. Recently they were asked to decide how to classify Pluto. After some debate, they decided that there is no real reason to make a change. Strange as Pluto is, we can still call it a planet.
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