Charon

Charon matters because it makes Pluto feel less like a lone dwarf planet and more like one half of a coupled system. It is not a small accessory world: its diameter is a little over half Pluto’s, which is enormous for a moon compared with its parent body. That size gives Charon a role that is architectural rather than merely decorative.

The relationship between the two worlds is unusually intimate. Pluto and Charon are mutually tidally locked, so each keeps the same face turned toward the other while they orbit a shared center of mass outside Pluto. From either world, the other would not simply rise and set like an ordinary moon; the pair behaves more like a locked duet of icy bodies at the edge of the Solar System.

New Horizons also made Charon more than a dynamical curiosity. Its images revealed a real surface, including the reddish north-polar region called Mordor Macula and terrain that points to a more complex geological history than a plain frozen sphere. That is why Charon matters on this scale: it is small beside the major planets, but central to understanding Pluto as a system.