Titan, Saturn’s biggest moon, probably tugged the ringed planet off-kilter one billion years ago.

Saturn is tilted with respect to its orbit around the Sun, by a little bit more than Earth is. Planetary scientists had thought that Saturn acquired its tilt more than 4 billion years ago, thanks to the gravitational influence of Neptune.

But recent measurements made with NASA’s Cassini spacecraft show that Titan is moving relatively rapidly away from Saturn. Melaine Saillenfest at the Paris Observatory and her colleagues capitalized on that finding to suggest that Titan is to blame for Saturn’s tilt.

Their calculations suggest that, around one billion years ago, Titan was migrating away from Saturn and reached a position where its gravitational influence made Saturn unstable. The planet began wobbling on its axis and tipped over into its current position.

Big migrating moons could similarly cause giant planets in other solar systems to keel over.