A reader wrote to 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 one percent the mass of Earth. Pluto's four other satellites are a very tiny fraction of the mass of the system.
The consequences are 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. The four outer satellites don't really orbit Pluto; they follow strictly Keplerian orbits (the orbital period is directly related to orbit size) 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 pair of stars twirl around a barycenter they are classified as binary stars. (The photo of Pluto and Charon at left could easily pass for a binary star.)
Binary systems account for at least half of the stars in our galaxy. Binary stars are thought to be 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 through the splitting of a single, fragile parent body, or through collisions.
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 smashup has been theorized for the birth of Earth's moon 4.4 billion years ago, though this theory recently has been questioned.
There might be other binary planets out there, though none have been uncovered in numerous surveys. They may be exceeding rare outside of debris belts like our asteroid belt and the Kupier belt where Pluto can be found.
HOWSTUFFWORKS: Why is Pluto not a planet?
Nevertheless, there might be binary planets that are habitable. The consequences would be extraordinary. The planet where intelligent life first arose on would dominate the companion planet. One can imagine a "space race" to colonize the twin world - and no doubt 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 to break new ground it our classification of oddball planetary bodies.
Image credit: NASA, ESA, and M. Showalter (SETI Institute)