Janusian type worlds
Janusian Worlds
Image from Steve Bowers
Named after the moon of Saturn, these are co-orbiting pairs of moons or planets with 1:1 resonant orbits which regularly exchange momentum.

Usually one world of the pair has a more eccentric orbit than the other. The semi-major axis and year of one moon or planet's orbit is usually slightly greater than the other. At regular intervals, whenever the planets or moons are in closest proximity, such worlds exchange momentum and swap orbital characteristics.

Where one world is much larger than the other, the orbit of the larger world will change less than the orbit of the smaller world. Many worlds have a few minor objects in a 1:1 resonance with them, but the difference between the mass of the world and the mass of the resonant object is so great that there is almost no change to the orbit of the planet, while the asteroid regularly changes its orbital parameters quite dramatically.

Another form of 1:1 resonance that frequently occurs is that exhibited by Trojan objects, where the smaller object (usually an asteroid or small planetoid) orbits the L4 or L5 Lagrange point, which is separated from the planet by the length of the semimajor axis of that world. These distant minor objects have almost no effect on the planet's orbit.
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Development Notes
Text by Steve Bowers
Initially published on 02 November 2010.