Pluto and Charon

Pluto was not discovered until 1930.

Pluto's orbit averages 40 AU from the Sun.
It is highly elliptical, sometimes coming inside Neptune's orbit.

In 1978, its moon Charon was discovered.

Pluto is more like a moon.

... or, rather, two moons.
Pluto and Charon as seen by the Hubble Space Telescope.

Pluto is rather small. Its radius is 0.2 R_E.
Charon is, however, not a lot smaller. Its radius is 0.1 R_E.

Their densities are about 2000 kg/m^3, similar to Triton, suggesting a rock and ice composition.