The Moon Charon (2) ...

The figure adjacent right shows a more recent Hubble Space Telescope image of Pluto and its moon Charon. In this image, the planet and moon are separated by only 0.6 seconds of arc.

The Orbit
The geometry of the Pluto-Charon system is illustrated in the figure shown below. The orbit is almost circular, but looks elliptical from Earth because it is inclined by a large angle relative to our line of sight. The orbit of Charon is tilted by 118 degrees relative to the plane of the orbit of Pluto around the Sun (which is itself tilted by 17 degrees from the ecliptic plane). The diameter of the orbit is about 20,000 kilometers, which is only about three times the diameter of the Earth.

Masses from Kepler's Third Law
Pluto and Charon are tidally locked to each other, with each keeping the same face turned toward the other. The period and separation of Pluto and Charon can be used in conjunction with Kepler's third law to determine the masses of the planet and the moon. Pluto is found to contain about 0.002 Earth masses, and Charon is about six times less massive than Pluto. Because this difference in mass between planet and moon is much smaller than normally encountered, one sees appreciable effects from the revolution of each around the center of mass. In particular, this means that the common center of mass around which Pluto and Charon orbit lies about 2700 km from the center of Pluto, well outside its radius of about 1150 km.

Revision of Pluto's Mass

With the presence of a moon, it was now possible to determine the mass of Pluto with much better precision than was possible before. This caused a drastic decrease in the accepted value for the mass of Pluto, which had been assumed previously to be as large as 10-100% of Earth's mass.

Diameters and Density
By observing the changes in brightness caused by Charon moving across the disk of Pluto and then behind it, the diameter of Pluto and of Charon have been inferred. Pluto is found to be 2300 km in diameter and Charon is about 1300 km in diameter. Thus, Pluto is about two-thirds the diameter of Earth's Moon and Charon is almost a factor of two smaller than that. From the masses and diameters, the densities are found to be 2.0 g/cm3 for Pluto and 2.2 g/cm3 for Charon. This suggests that Pluto and Charon are roughly 70-80% rock and 20-30% ice (recall that ice has a density near 1 g/cm3 but rock has a larger density).
Origin of Pluto and Charon
The origin of Pluto and Charon is a mystery. They have much more in common with the large icy moons of Neptune than they do with the gas giants, their nearest planetary neighbors. It has been suggested that they might be escaped moons of Neptune but, as noted earlier, there is some doubt about this now. It is possible that Pluto and Charon, and the large moons Triton and Nereid of Neptune that have unusual orbits, are related in some way (and that they may be members of a new class of objects with other members yet to be discovered). However, even if this conjecture is correct, the relation among Pluto, Charon, Triton, and Nereid remains unclear.