Saturn's largest moon is moving away from the planet 100 times faster than thought—a finding that helps cement a new theory on how moons migrate. NASA explains why moons move at all: A moon's gravity tugs on the planet it is orbiting, "causing a temporary bulge in the planet as it passes. Over time, the energy created by the bulging and subsiding transfers from the planet to the moon, nudging it farther and farther out." Our own moon moves about 1.5 inches from Earth each year, per CBS News. Titan—which is three times farther from Saturn than our moon is from us—is drifting about 4 inches per year, according to data from NASA's Cassini spacecraft. That's a big surprise and suggests Titan started out much closer to its host planet billions of years ago, "which would mean the whole system expanded more quickly than previously believed," per Space.com.
"Most prior work had predicted that moons like Titan ... were formed at an orbital distance similar to where we see them now," says Caltech theoretical astrophysicist Jim Fuller, co-author of a new study in Nature Astronomy. "This implies that the Saturnian moon system, and potentially its rings, have formed and evolved more dynamically than previously believed." The findings sync with Fuller's 2016 theory that outer moons move at a similar rate as inner moons despite being further away from the host planet's gravity. He now believes these interactions could "apply to many systems … even binary star systems, where stars orbit each other," per CNN. Scientists hope to learn more when NASA's Dragonfly arrives for a three-year study of Titan around 2034. (Read more space stories.)