Pluto's Strange Orbit
The Pluto-Charon system has the strangest orbit of all the planets in the solar system. It has a large eccentricity and a high orbital inclination of 17.1° to the ecliptic. These extreme orbital characteristics suggest that since its formation the Pluto-Charon system may have undergone some considerable orbital evolution.
Shortly after Pluto was first discovered, astronomers realized that unless some special conditions prevailed, Pluto would occasionally undergo close encounters with Neptune, and consequently suffer rapid orbital evolution. In the mid-1960s, however, it was discovered that Pluto is in a special 2-to-3 resonance with Neptune. That is, for every three orbits that Neptune completes about the Sun, Pluto completes two. This resonance ensures that Neptune always overtakes Pluto in its orbit when Pluto is at aphelion, and that the two planets are never closer than about 17 AU. How this orbital arrangement evolved is presently unclear.
The close structural compatibility of Pluto and Triton (i.e., they have the same size, mass, and composition) has led some astronomers to suggest that the two bodies may have formed in the same region of the solar nebula. Subsequently, it is argued, Triton was captured to become a moon of Neptune, while Pluto managed to settle into its present orbit about the Sun. Numerical calculations have shown that small, moon-sized objects that formed with
low inclination, circular orbits beyond Neptune do evolve, within a few hundred million years, to orbits similar to that of Pluto's. This result suggests that Pluto is the lone survivor of a (small) population of moon-sized objects that formed beyond Neptune, its other companions being either captured as satellites around Uranus and Neptune, or being ejected from the Solar System. One important, and as yet unsolved snag with the orbital evolution scenario just outlined, is that Pluto and Charon have different internal structures, implying that they formed in different regions of the solar nebula. It is presently not at all clear how the Pluto-Charon system formed.
Using a specially designed computer, Gerald Sussman and Jack Wisdom of the Massachusetts Institute of Technology, have modeled the long-term orbital motion of Pluto. Sussman and Wisdom set the computer to follow Pluto's orbital motion over a time span equivalent to 845 million years; interestingly they found that Pluto's orbit is chaotic on a time scale of several tens of millions of years.