The Pluto-Charon system is the only planet-satellite system in our solar system that has not been explored by spacecraft. Therefore, the state of knowledge about this system is necessarily more primitive than at any other planet. Despite this limitation, however, many basic facts are established. These include the radius, mass, and density of Pluto (each known to better than 10%) and the radius of Charon (known to 5%), and the mass and density of Charon (known to about 25%). Importantly, Charon is almost precisely half the size of Pluto. Because the system barycenter is known to be outside Pluto (between the two bodies), the pair constitute a true double planet- something unique in our solar system.

Pluto-Charon orbit the Sun in an elliptical, inclined, 248-year orbit that is in the 3:2 mean motion resonance of Neptune. Perihelion was reached in 1989; the system is now receding from the Sun. The planet and satellite share a polar obliquity of 120 deg. Pluto-Charon have reached complete spin-spin-orbit synchronicity; the pair are the only fully tidally evolved planet-satellite pair in the solar system. Pluto's density, very near, 2 gm cm-3, indicates its bulk composition is dominated by hydrated rock, but contains up to 30% water ice. Light organics and other materials are predicted to be abundant minor constituents.

Pluto's surface is highly reflective, with a globally averaged normal albedo of 55%. The surface color is red, much like Triton. Reflectance spectroscopy has identified N2, CO, CH4, and H2O frosts on the surface, with N2 being the dominant constituent. Other light organics resulting from ice radiolysis and other processes are widely expected to be present. Photometric measurements have revealed a complex lightcurve with an amplitude higher than any other planet in the solar system. The surface has been mapped crudely (500 km resolution) by HST; the maps reveal polar caps and other high-contrast surface units. Thermal measurements indicate steep surface temperature gradients, with bright (presumably sublimation-cooled) areas being near 40 K, and dark (purely radiative equilibrium?) units being near 60 K.

Pluto's atmosphere was discovered by stellar occultation techniques. Its base surface pressure is at least 3 and perhaps as great as 50 microbars; the upper atmosphere has a temperature of 106 K owing to a near-surface inversion, but the details of this thermal structure are indeterminate. Hazes and/or discrete clouds may be present in the atmosphere. Model calculations predict an N2 dominated atmosphere, with traces of CH4, CO, and a complex suite of photolysis products. Owing to Pluto's high orbital eccentricity and its high axial tilt, strong thermal forcing results. Owing to coupled ice/atmosphere sublimation thermal balance, strong seasonal pressure cycles have been predicted, including possible seasonal atmospheric collapse. Escape rate calculations indicate that Pluto's atmosphere is likely to be in hydrodynamic escape, unlike any other planet (but like the archaen Earth).

Charon's surface albedo is much darker than Pluto (35%), its surface color is gray (neutrally reflecting), and it has only a low amplitude (8%) lightcurve. Its surface composition appears to be dominated by water ice, but new absorption features in the mid-infrared have been detected in recent years, indicating the presence of other, as yet unidentified, surface constituents. There has been no definitive detection of an atmosphere.

The origin of the Pluto-Charon binary is thought to have been caused by a giant impact, much like the Earth-Moon system. The evidence for this hypothesis is based on the system's high specific angular momentum, its high axial obliquity, and the large mass ratio of the binary. Pluto itself is thought to have been grown in heliocentric orbit during the epoch of planetary growth in the Kuiper Belt, some 4 Gyr ago. As such, and owing to its size, it is expected to represent a key sample of the bulk composition of planetesimals in the trans-Neptunian region.

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