Pluto is a celestial object, recently discovered (1930s, by Clyde Tombaugh) within the outer part of the solar system. Considered to be the ninth of nine planets in the solar system until 2006, Pluto was always recognised as a body in possession of properties not typically found in the other eight planets. The discovery of Pluto's twin (Charon) was a major event, and this discovery pushed the general consensus to consider Pluto to be less of a planet than previously thought. Ultimately, the recent observation (via powerful Earth-based telescopes and the Hubble Space Telescope) and acceptance of the Kuiper Belt as the known boundary between the solar system and interstellar space, coupled with observations of discrete, sub-planet sized bodies (in and around the Kuiper Belt) led to revisions of Pluto's status as a true planet, and finally (2006, IAU conference) as one of a new group of sub-planetary bodies known as planetismals, Plutinos etc. This deviation from the planet group is based on the next scale down (mass-density wise), much like planets are a quantum matter scale below that of the stars.
Pluto-Charon is the largest known twin body within this group of objects. It is thought that the composition of such bodies comprises a more molecular-based chemistry (such as semi-stable compunds of volatiles, frozen inert gases and light transition metal-metalloid-ligand complexes). The small mass-density makeup of bodies like Pluto, coupled with the weakened influence of solar physics at such distances, results in accumulations of such matter in a way as is not possible as one moves toward the sun. However, the nature of a number of moons of the outer planets (eg. Triton around Neptune, Miranda around Uranus, many of Saturn's moons etc.) may supply many useful clues about bodies in the Kuiper Belt, including Pluto-Charon.
The current accepted definition of Pluto (as a planetismal) is so recent, based on new observations of this and other outer solar system material, that there is clearly a long way to go in gaining a true understanding of what Pluto-Charon is. What is known to be important however, is the stark difference (compositionally, structurall and spatially) between Pluto-like bodies and the larger, more obvious spheres such as planets and stars. Nevertheless, all these bodies have a common ancestry, in that they are products of previous events which yielded vast quantities of matter of all types. The eventual coalescence of this matter into its current existence is governed by the laws of physics, the relations between phases (solid-gas and interchangeable fluid and semi-stable materials like those at furthest distance from the controlling force generators like the sun) and the quantum energy regimes which reside at these distances from a central star combining with local quantum variations caused by accumulating, rotating and spinning masses.
Therefore, Pluto is a known planetismal body. It's limited as a database for cosmogeochemistry due to its remoteness, its small size and its home within a lesser observed realm (Kuiper Belt) on the edge of our solar system. Many new possibilities have arisen since studies of such remote objects have been conducted, and it is such studies which are able to open the door to the unknown nature of interstellar space beyond our sun and its sphere of influence. Moreover, the very nature of physics, chemistry, geology and all other physical sciences will be open to many new avenues via the continued observation and study of the cold outer reaches of the solar system.