Pluto

Pluto (Sol IX) is a dwarf planet primarily made of ice and rock located in Sol. It was the first major Kuiper Belt object to ever be discovered.

Atmospheric Composition
Pluto's tenuous atmosphere is composed of Nitrogen (N2), Methane (CH4) and Carbon Monoxide (CO) which are in equilibrium with their ices on Pluto's surface.

Charon
The gravitational influence of Charon on Pluto is such that the pair has been classified and unclassified as binary dwarf planet multiple times since its discovery in 1978.