In a recent study announced by MIT, astronomers have posited that a “lost planet” may have pulled Saturn from Neptune’s gravitational influence and formed its rings after grazing the planet and being torn apart a few hundred million years ago.
Around 2000, the prevailing theory behind Saturn’s tilt was that it was in gravitational resonance with Neptune, with the blue planet’s pull angling Saturn’s axis by 27 degrees. But after NASA’s Cassini spacecraft found that Saturn’s primary satellite, Titan, was escaping Saturn’s orbit, scientists theorized that it was Titan’s gravitational pull causing the tilt. However, that explanation had one major unknown variable: Saturn’s moment of inertia. A planet’s moment of inertia determines how quickly a planet spins and the location of its axis based on how the planet’s mass is distributed. Without knowing Saturn’s moment of inertia, the theory around Titan could not be confirmed or invalidated.
Determining the moment of inertia became one of the key goals of lead author Jack Wisdom, professor of planetary sciences at MIT, and his co-authors. By constructing a model of Saturn’s interior and mass distribution based on Cassini’s map of the planet’s gravitational field, Wisdom and his team were able to specify the moment of inertia. They found that it placed Saturn outside of resonance with Neptune—which means that Neptune is not responsible for Saturn’s tilted axis.
After running some simulations to see if existing moons may have affected the tilt but finding no such evidence, Wisdom and his colleagues reevaluated the equations describing how a planet’s axis of rotation shifts over time, known as precession. It was then they realized that one factor in the equation may need to be adjusted: the contributions from all a planet’s satellites. If one satellite was removed from the equation, the precession would change accordingly. With this realization, the researchers ran additional simulations to extrapolate the properties of a missing moon and what it must have done to knock Saturn out of Neptune’s resonance. The result, dubbed Chrysalis, was a moon approximately the same size as Saturn’s third-largest existing moon slamming into Saturn and fracturing into pieces between 100 and 200 million years ago, thus knocking it out of Neptune’s influence and creating the rings.
“It’s a pretty good story, but like any other result, it will have to be examined by others,” Wisdom says. Drawing comparisons to the unpredictable destruction of chrysalises when the butterfly suddenly emerges, he continues, “But it seems this satellite was just a chrysalis, waiting to have its instability.”