Until fall 2005, Epimetheus orbited on the inside track and, because it was closer to Saturn, the moon traveled faster than Janus. As the distance between the moons decreased, Epimetheus pulled on Janus from behind just as Janus tugged Epimetheus. This interaction caused the moons to exchange angular momentum. Epimetheus gained momentum, moved out from Saturn slightly, and moved slower in its orbit, while Janus lost momentum, dropped toward Saturn, and sped up.
The moons’ latest orbital swap, which takes about 100 days to complete, concluded in late January. Their orbits are stable because resonant effects from the gravity of Saturn and its other moons force the issue and lock the “co-orbital moons” into these paths.
About 1,800 asteroids share Jupiter’s orbit. They lead and follow the giant planet by 60°. So, a pair of planets could develop a similar relationship, especially if, like Jupiter and its swarm of asteroids, one object were many times more massive than the other.
It’s unclear how stable such configurations can be, especially if other massive planets orbit the star. Their gravitational influence could eventually upset the delicate balance, and the planets would go their separate ways.
— FRANCIS REDDY, ASSOCIATE EDITOR