Because Pluto is neither terrestrial nor jovian in its makeup, and because of its similarity to the ice moons of the outer planets, some researchers suspect that Pluto is not a "true" planet at all. This view is bolstered by Pluto's eccentric, inclined orbit, which is quite unlike the orbits of the other known planets.

Soon after Pluto was discovered, its orbital association with Neptune suggested to some theorists that a catastrophic encounter of some sort might have ejected Pluto from its original orbit around Neptune and perhaps even simultaneously knocked Triton onto its present retrograde path. Although this was an attractive theory at the time, a plausible sequence of events that could account in detail for the present orbits of those bodies proved elusive. Furthermore, since 1978 the picture has been greatly complicated by the presence of Charon. It was much easier to suppose that Pluto was an escaped moon before we learned that it had a moon of its own. As a result, few astronomers still regard this theory as a likely explanation of Pluto's origin.

Pluto may be just what it seems—a planet that formed in its current orbit, possibly even with its own moon right from the outset. Because we know so little about the environment in the outer solar system, we cannot yet rule out the possibility that planets beyond Neptune should simply look like Pluto. However, there is mounting evidence that this is not the case either. Astronomers now know of numerous large chunks of ice circulating in interplanetary space beyond the orbits of the jovian planets (see Chapter 14), and many researchers believe that there may have been thousands of Pluto-sized objects initially present in the outer solar system. Indeed, as we will see in Chapter 15, such objects are the natural building blocks of planets, according to current theories of solar system formation.

Almost all these Pluto-sized objects have since been "kicked" out to larger distances from the Sun following gravitational interactions with Uranus and Neptune. The capture of a few of them by the giant planets could explain some of the strange moons of the outer worlds, especially Triton, and if there were enough moon-sized chunks originally orbiting beyond Neptune, it is quite plausible that Pluto could have captured Charon following a collision (or near-miss) between the two. Pluto remained behind after the other bodies were captured or ejected, perhaps because its orbital resonance with Neptune prevented it from ever experiencing a close encounter with that (or any other) planet.

At present, our scant knowledge of the compositions of the two bodies does not allow us to confirm or disprove either the "coformation" or the "capture" theory of the Pluto—Charon system. Unfortunately, this uncertainty may persist for some time—there is no present or proposed space mission that might suddenly and radically improve our understanding of these distant worlds.