History Of The Pluto-charon System
Obviously, a history of the Pluto-Charon system is quite speculative. It is though perhaps that this double-planet system may have originated in a more nearly circular orbit and that a subsequent catastrophic impact changed the orbit to highly elliptical and perhaps separated the two masses (Charon being formed by coalesced debris in near Pluto space). This may also account for the strongly inclined spin axis of Pluto.
Another hypothesis holds that Pluto accreted in orbit around Neptune and may have been ejected in the Triton capture event that is thought to have reorganized the Neptunian system. The lack of a large "original" satellite of Neptune (Triton is thought to have been captured) is a point in favor of this hypothesis.
It is also possible that Pluto-Charon are simply part of a class of icy Trans-Neptunian objects (TNOs) that are rather close to the Sun as compared with others probably out there in the Ort cloud beyond the edge of the solar system. Recently, some astronomers have stopped referring to Pluto as a planet and have called it a TNO. Until a space mission returns data and photographs from Pluto, Charon, and some TNOs, scientists may not be able to eliminate any of the completing hypotheses.
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David T. King, Jr.