Space Station Info >> Plutos Moons

Pluto's Moons

w Pluto and its satellite Charon

Pluto’s natural satellite, Charon was first identified in 1978. Pluto and Charon are significant for being the only planet/moon pair in the solar system whose barycenter lies above the planet's surface, thus prompting some astronomers to label it a double planet.

Pluto and Charon are also remarkable among planets in that they are tidally locked to each other. This represents that Charon always presents the same face to Pluto, and Pluto also always presents the same face to Charon. Note that some binary asteroids may also possess both of these traits, and that the Jupiter/Sun barycenter is above the Sun's surface, so neither is distinctive.

The discovery of Charon permitted astronomers to establish the mass of the Pluto-Charon pair from their observed orbital period and separation by a straightforward application of Kepler's third law of planetary motion. The mass was found to be lower than even the lowest former estimates.

The discovery also led astronomers to alter their estimate of Pluto's size. Originally, it was believed that Pluto was larger than Mercury but smaller than Mars, but that calculation was based on the basis that a single object was being observed. Once it was realized that there were in fact two objects instead of one, the estimated size of Pluto was revised downward. Today, with modern adaptive optics, Pluto's disc can be determined and thus its size can be directly determined.

Charon's innovation also resulted in the estimation of Pluto's albedo being revised upward; since the planet was now seen as being far smaller than originally estimated, by necessity its capacity to reflect light must be greater than what had been formerly believed. Recent estimates place Pluto's albedo as marginally less than that of Venus, which is fairly high.

Earlier some researchers theorized that Pluto and its moon Charon were moons of Neptune that were knocked out of Neptune's orbit. Nowadays it is widely accepted that Pluto never orbited Neptune. Instead, Triton, the largest moon of Neptune, shares many atmospherical and geological composition similarities with Pluto, signifying that Triton was once a Kuiper belt object in a solar orbit.