Binary Planet: Pluto & Charon
The term “binary planet” describes a pair of objects, which have similar masses while orbiting in a gravitational balance point that is between them. This is the center of mass location point. The only recognized binary planet in our Solar System is Pluto and Charon. Pluto’s companion Charon was discovered in 1978 by U.S. Naval Observer James Christy and Robert Harrington. Both Charon and Pluto’s gravity locks them in a synchronous orbit, which means that Pluto keeps the same hemisphere facing Charon at all times. In other words, Charon does not move across Pluto’s sky, like Earth’s moon, but stays in one location. Although Pluto and Charon are the only binary planets, there are many binary asteroids and binary Kuiper Belt Objects.
Charon
Charon, which is from Greek mythology, was the boatman who ferried souls of the dead across the river into the underworld. (The underworld is ruled by Hades in Greek mythology or Pluto in Roman mythology). Unfortunately, there is not a lot of information on Charon. The composition of this icy companion may be covered by water ice (instead of Pluto’s frozen nitrogen, methane, and carbon dioxide ices) and has an approximate diameter of 750 miles (1,210 km), which is 52% of Pluto. Charon also has an orbital period of 6 days because it has a synchronous orbit with Pluto. Furthermore, some scientists theorize that binary KBOs, like Pluto and Charon, formed from collisions with other objects.
Pluto's Moons
Nix and Hydra were discovered in the summer of 2005 during preparations for the launch of the New Horizons probe. Nix is from Egyptian mythology representing the goddess of darkness who is also the mother of Charon (Nyx in Greek, which already names another KBO). Hydra is a giant monster with the body of a serpent and nine heads (honoring the former ninth planet), who keeps watch at the gates into the underworld. Again like Charon, very little is known about Pluto’s newest moons. The diameter of Nix is around 40 to 130 kilometers and it takes 25 days to complete its orbital rotation. Hydra orbits 38 days and has a diameter of about 45 to 160 kilometers.
More Resources
I.) Arnett, Bill. "Pluto." http://www.nineplanets.org/pluto.html (accessed 3. December, 2006).
Bill Arnett's website on the nine planets has a web page devoted to Pluto and Charon, with a little information on Nix and Hydra.
II.) Alexander, Amir. "Pluto's Moons Named: Nix and Hydra Join Pluto's Family ." The Planetary Society. http://www.planetary.org/news/2006/0622_Nix_and_Hydra_Join_Plutos_Family.html (accessed 2. December, 2006).
This educational article from The Planetary Society, discusses Pluto's newest moons Nix and Hydra.
The pictures of Pluto and Charon and Pluto and all of its moons are the property of the New Horizon's Website.