Pluto's moons in coupled orbits
The recently discovered two small moons of the planet Pluto are believed to have formed with the larger Pluto moon Charon in a violent collision.
So far relatively little is known about the Pluto moons P1 and P2. Even their diameters are not precisely determined. Depending on the reflectivity of their surface, scientists assume values between 30 and 160 kilometers. Better information is known about their orbits. Astonishingly, the small satellites with 24.9 and 38.2 days per orbit require almost exactly four or six times a Charon month of 6.4 days. Also, all three moons move in the same plane and in the same direction around Pluto.
These similarities lead scientists to conclude that a single ancient collision may be the origin of the moons. However, this is contradicted by the fact that P1 and P2 move on orbits that lie far outside of Charon's orbit.
The astrophysicists William Ward and Robin Canup from the Southwest Research Institute have now resolved this contradiction with a model. According to their calculations, the orbits result automatically when the orbits of Charon and the minor moons are coupled. If Charon then follows an eccentric orbit, this forces P1 and P2 onto the observed circles. A mechanism very similar to that proposed for the formation and stability of Neptune's rings.