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Pluto

© Copyright 2001, Jim Loy

As I said in my article on Mercury, when I was young Pluto was the second smallest planet. Now it is by far the smallest planet. Estimates of its size were based upon its brightness and supposed reflectivity. Well, the surface of Pluto is a bright white (ice), and Pluto is very small indeed. There are seven moons in the Solar System that are larger than Pluto. Pluto was called a terrestrial planet (as it is not like the gas giants out there), but it is now considered a minor planet, like The Asteroids. It is in a region of space called the Kuiper (or Edgeworth-Kuiper) Belt (beyond Neptune). For a long time, Pluto was the only known object in that region of space. But recently several small objects have been discovered.

Pluto was discovered, in 1930, by Clyde Tombaugh, at Lowell Observatory at Flagstaff AZ. Neptune had been discovered as the result of irregularities in the orbit of Uranus. Neptune's gravity was pulling on Uranus, speeding it up and then slowing it down. Similar irregularities seemed to have occurred in Neptune's orbit, causing astronomers to predict a planet beyond Neptune. The measured irregularities seem to have been the result of inaccuracies of observations, and not due to any planet. There is no planet out there large enough to have caused the measured irregularities. Pluto, in particular, is much too small. Until recently, Pluto appeared as a featureless point of light, when viewed with the largest telescopes. It is extremely difficult to view, with extremely large home telescopes (a 6-inch telescope is not large enough).

Pluto was closest to the sun on 5 Sep. 1989. In fact, it was closer to the sun than Neptune was for about 20 years. For most of its orbit, Pluto is the planet farthest from the sun. Its orbital period (year) is almost 250 years long. Pluto's orbit is very eccentric (0.254), and is highly inclined (about 17 degrees) to the plane (ecliptic) of the orbits of the other planets. When it is closest to the sun, Pluto has a thin nitrogen atmosphere. When it is farther from the sun, the nitrogen probably freezes onto the ground, and then Pluto may have almost no atmosphere.

{short description of image}Pluto has a moon (Charon which was discovered in 1978) which is somewhat smaller than Pluto. On the left is a photo from the Hubble Space Telescope. The discovery of Charon enabled astronomers to determine Pluto's mass. From 1985 to 1990, Charon and Pluto eclipsed each other thousands of times. This enabled astronomers to determine the sizes of Pluto and Charon, with a great deal of accuracy. It also enabled astronomers to deduce the existence of light and dark surface features on both Pluto and Charon.

Here are some data about Pluto:

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