By the end of the nineteenth century, observations of the orbits of Uranus and Neptune suggested that Neptune's influence was not sufficient to account for all the irregularities in Uranus's motion. Furthermore, it seemed that Neptune itself might be affected by some other unknown body. Following their success in the discovery of Neptune, astronomers hoped to pinpoint the location of this new planet using similar techniques. One of the most ardent searchers was Percival Lowell, a capable, persistent observer and one of the best-known astronomers of his day. (Recall that he was also the leading proponent of the theory that the "canals" on Mars were constructed by an intelligent race of Martians—see Interlude 10-1.)

Basing his investigation primarily on the motion of Uranus (Neptune's orbit was still relatively poorly determined at the time), and using techniques similar to those developed earlier for Neptune by Adams and Leverrier, Lowell set about calculating where the supposed ninth planet should be. He searched for it, without success, during the decade preceding his death in 1916. Not until 14 years later did American astronomer Clyde Tombaugh, working with improved equipment and photographic techniques at the Lowell Observatory, finally succeed in finding Lowell's ninth planet, only 6° away from Lowell's predicted position. The new planet was named Pluto for the Roman god of the dead who presided over eternal darkness (and also because its first two letters and its astrological symbol are Lowell's initials). Its discovery was announced on March 13, 1930, Percival Lowell's birthday.

On the face of it, the discovery of Pluto looked like another spectacular success for celestial mechanics. Unfortunately, it now appears that the supposed irregularities in the motions of Uranus and Neptune did not exist and that the mass of Pluto, not measured accurately until the 1980s, is far too small to have caused them anyway. The discovery of Pluto owed much more to simple luck than to elegant mathematics!

Some orbital and physical data for Pluto are presented in the Pluto Data box. Unlike the paths of the other outer planets, Pluto's orbit is quite elongated, with an eccentricity of 0.25. It is also inclined at 17.2° to the plane of the ecliptic. Here already we have some indication that Pluto is unlike its jovian neighbors. Because of its substantial orbital eccentricity, Pluto's distance from the Sun varies considerably. At perihelion, it lies 29.7 A.U. (4.4 billion km) from the Sun, inside the orbit of Neptune. At aphelion, the distance is 49.3 A.U. (7.4 billion km), well outside Neptune's orbit. Pluto last passed perihelion in 1989, and it will remain inside Neptune's orbit until February, 1999. Its sidereal period is 248.0 years, so the next perihelion passage will not occur until the middle of the 23rd century.

Pluto's orbital period is apparently exactly 1.5 times that of Neptune—the two planets are locked into a 3:2 resonance (two orbits of Pluto for every three of Neptune) as they orbit the Sun. As a result, even though their orbits appear to cross, Pluto and Neptune are in no danger of colliding with each other. Because of the orbital resonance and Pluto's tilted orbit plane, the distance between the two planets at closest approach is actually about 17 A.U. (compare with Pluto's closest approach to Uranus of just 11 A.U.). The orbits of Neptune and Pluto are sketched in Figure 13.20.

Figure 13.20 The orbits of Neptune and Pluto cross, although Pluto's orbital inclination and a 3:2 resonance prevent the planets from actually coming close to each other. Between 1979 and 1999, Pluto was inside Neptune's orbit, making Neptune the most distant planet from the Sun.

At nearly 40 A.U. from the Sun, Pluto is often hard to distinguish from the background stars. As the two photographs of Figure 13.21 indicate, the planet is actually considerably fainter than many stars in the sky. Like Neptune, it is never visible to the naked eye.

Figure 13.21 These two photographs, taken one night apart, show motion of the planet Pluto (arrow) against a field of much more distant stars. Most of Pluto's apparent motion in these two frames is actually due to the orbital motion of Earth rather than that of Pluto.