Pluto and Charon

In 1978 the planet Pluto was observed to be not one but two distinct objects. The larger, with a diameter equal to 66% of the Moon's, is now considered to be the planet Pluto. The smaller, 37% of the Moon's diameter, is considered to be Pluto's moon, and is called Charon. They are very close together, separated by only nine times Pluto's diameter.

Pluto and Charon

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Pluto and Charon revolve around each other once every 6.4 days. This revolution is unusual in that the plane of their orbit around each other is almost perpendicular to the plane of their orbit around the Sun. This suggests that their association may be the result of a collision, rather than co-formation, which would result in close alignment of the two planes.