Speaking of Hubble...

Is Charon a Moon of Pluto Or a Binary Planet?

August 23, 2012 by Ray Villard
Artist's concept of Pluto and its moon Charon. (NASA)

Artist's concept of Pluto and its moon Charon. (NASA)

This month the infamous “un-planet” Pluto grabbed science headlines with the report of yet another moon whirling around it.

The Hubble Space Telescope discovery brings the tally of icy moons orbiting Pluto to five.

Or is it really four?

A reader wrote me to make the case that Pluto really has only four moons (all discovered by Hubble over the past seven years). He argued that the largest moon in the system, Charon (found in 1978), is really a planet in its own right.

Why? Because Charon is 12 percent the mass of Pluto. That may not seem like much, but our Moon is only .01 percent the mass of Earth.

The consequence is that Pluto and Charon pivot like a waltzing pair of ice skaters around a center of mass.  So do the Earth and Moon, but the center of mass, or barycenter, is inside Earth’s radius. However, alien astronomers watching Earth transiting the Sun would note the passage of our Moon as well. They might catalog Earth as a “double planet.”

That was the reader’s point. Pluto’s four outer satellites don’t really orbit the dwarf planet; they follow Keplerian orbits around the system’s center of mass, which lies between Pluto and Charon. Pluto and Charon complete one pivot around each other every 6.3 days.

When we see a pair of stars twirl around a barycenter, they are classified as binary stars. Binary systems account for at least half of the stars in our galaxy. Binary stars are born through the fragmentation of the collapsing nebula that condensed to form them.

Dozens of binary asteroids have been cataloged since 1993. They may form though the splitting of a single, fragile parent body.

So why not have binary planets too?

The popular theory is that a collision between Pluto and another icy dwarf planet spawned Charon and the other moons. A similar sort of collision has been theorized for the birth of Earth’s moon 4.4 billion years ago, though this theory has recently been questioned.

Other binary planets might be out there, though none have been uncovered in numerous surveys. They may be exceedingly rare outside of debris belts like our asteroid belt and Pluto’s Kupier belt.

Nevertheless, there could be binary planets out there that are inhabitable. The consequences would be extraordinary. The planet where intelligent life first arose would dominate the companion planet. There would be a “space race” to colonize the companion world – and no doubt the winners would subjugate whatever was living there. Travel and trade between the two worlds would become commonplace.

In 2006, the International Astronomical Union (IAU) initially considered characterizing Pluto-Charon as a binary planet.  But in all their hissy-fit fuss over what to call Pluto, Charon was simply left as a satellite of Pluto.

The IAU missed a great opportunity here to break new ground in our classification of oddball planetary bodies.