CRIRES model-based computer-generated impression of the Plutonian surface, with atmospheric haze, and Charon and the Sun in the sky. Pluto’s atmosphere consists of a thin envelope of nitrogen, methane, and carbon monoxide gases, which are derived from the ices of these substances on its surface.
The Pluto–Charon system is noteworthy for being one of the Solar System’s few binary systems, defined as those whose barycenter lies above the primary’s surface (617 Patroclus is a smaller example, the Sun and Jupiter the only larger one). This and the large size of Charon relative to Pluto has led some astronomers to call it a dwarf double planet. The system is also unusual among planetary systems in that each is tidally locked to the other: Charon always presents the same face to Pluto, and Pluto always presents the same face to Charon: from any position on either body, the other is always at the same position in the sky, or always obscured.
Credit: ESO/L. Calçada