  The Pluto-Charon system provides a broad variety of constraints on planetary
formation, composition, chemistry, and evolution. Pluto was the first body to
be discovered in what is now known as the Kuiper belt, its orbit ultimately
becoming a major clue that the giant planets underwent substantial orbital
migration early in Solar System history. This migration has been linked to an
early instability in the orbits of the giant planets and the formation of the
Kuiper belt itself, from an ancestral trans-Neptunian planetesimal disk that
included Pluto. Pluto-Charon is emblematic of what are now recognized as small
or dwarf planets. Far from being a cold, dead, battered icy relic, Pluto
displays evidence of a complex geological history, with ongoing processes
including tectonism, cryovolcanism, solid-state convection, glacial flow,
atmospheric circulation, surface-atmosphere volatile exchange, aeolian
processes, and atmospheric photochemistry, microphysics, and haze formation.
Despite Pluto's relatively modest scale, the combination of original
accretional heat, long-term internal radiogenic heat release, and external
solar forcing, when combined with sufficiently volatile (and thus mobile)
materials, yields an active world. Pluto may have inherited a large organic
mass fraction during accretion, which may responsible, in part, for its surface
and atmospheric volatiles. Charon, Pluto's major moon, displays evidence of
extensive early tectonism and cryovolcanism. Dwarf planets are thus truly
planetary in terms of satellite systems and geological and atmospheric
complexity (if not ongoing activity). What they may lack in mass is made up in
number, and the majority of the Solar System's dwarf planets remain
undiscovered.
