Pluto was discovered in 1930, following an extensive search, by Clyde Tombaugh. The search was initiated by Percival Lowell after it was thought that a planet beyond Neptune might be perturbing Neptune's orbit. Calculations narrowed the range of possible locations on the sky, and a search was carried out. As Fig. 26.1 shows, Pluto doesn't stand out very well against the background of stars. It is detectable as a planet only by its very slow motion with respect to the stars.

For Pluto to have a perturbing effect on other planets, its mass must be greater than that of the Earth. For this reason, since its discovery, Pluto's mass has been overestimated. We now know that its mass is much less than previously thought, and that it has no measurable effect on other planets. In a sense, Pluto's discovery was accidental. It was a result of an extensive search of a particular region in the sky. For this reason, other searches have been carried out for a "tenth planet", none with success.

Pluto's mass is now known reasonably accurately. This is because a moon was discovered orbiting Pluto in 1978. The moon is named Charon, and is shown in Fig. 26.2. By studying its orbital motion we can determine Pluto's mass. Actually, using the Hubble Space Telescope, it has been possible to look at the motions of both Pluto and Charon about their common center of mass. From this it has been deduced that Pluto's mass is 1.3 X 1025 g, and Charon's mass is about 1/12 of that. Pluto's mass is only about 1/500 of the Earth's mass, or one-fifth that of our Moon.

Pluto's size has been estimated from its failure to occult certain stars. (See Section 26.4 for how this technique is used for asteroids.) However, our best measurements now come from optical interferometry techniques. Using this size, and the measured mass, we find that Pluto has a very low density, about 0.5 to 1.0 g/cm3. Charon's density is even lower, only 20% greater than that of liquid water. This suggests that its composition is similar to that of the moons of the giant planets. It has been suggested that Pluto's surface is frozen methane and that its atmosphere is also composed of methane.

Pluto's size, density and orbit raise questions about its status as a planet. Its orbit is the most eccentric of the planets. It even spends part of its orbit closer to the Sun than Neptune. It has been suggested that Pluto may actually be an escaped moon of Neptune. This would explain its small size, low density, and crossing of Neptune's orbit. However, when we trace back the orbits of

(a) HST images of Pluto, showing both hemi-spheres.The insert images are the unprocessed images. (b) Surface map of Pluto, based on the HST images. [STScI/NASA]

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