Astrophys. J. 683, 722–749 (2008)

M31, the spiral galaxy nearest the Milky Way, and NGC 205, a nearby dwarf elliptical galaxy, appear to be stuck in an eternal pas de deux. At least, that is what it looks like through a telescope. But Kirsten Howley of the University of California at Santa Cruz and her colleagues have used what is known as a genetic algorithm to determine that NGC 205 is actually swinging around M31.

The algorithm sifted through more than a billion trillion possible orbits for NGC 205, identifying which of them best fitted the galaxy's observed motions and light characteristics.

Howley's team found that NGC 205 was zipping past M31 at hundreds of kilometres per second, close to its escape velocity. NCG 205's motion is perpendicular to, and therefore independent of, a streamer of stars previously thought to be associated with it.