Planetary turntables

Planetary orbits tend to be nice and tidy: A star rotates, pulling planets around it in the same direction.  Moons and rings follow suit, happily spinning along. But the massive rings of J1407b, a planet almost 400 light years away from us, break the expected conventions.

The rings, which are about 200 times the size of those orbiting Saturn, have baffled scientists since they were first spotted more than a year ago. The planet has a highly asymmetrical orbit — its most distant point is eight times further from its star than its closest point. There, close enough to its star that the distance from the outermost ring to the star is less than the width of the ring itself, it’s so close that the star should rip the rings apart, yet they survive. So why has this “Saturn on steroids” been able to stay ringed even under intense gravitational pressure?

Two Dutch astrophysicists — Matthew Kenworthy of Leiden University and Steven Rieder, now at the RIKEN Advanced Institute for Computational Science in Japan — may have the answer.  In their study, published in October in Astronomy & Astrophysics, the pair successfully modeled not only how J1407b’s rings stay intact, but also how they caused the bizarrely-prolonged eclipses that led to the planet’s discovery in 2012. While the planet follows the expected pattern of orbiting and spinning in the same direction as its star, the rings surrounding J1407b spin in the opposite direction.

Kenworthy and Rieder’s models demonstrate how this backwards spin allows J1407b’s rings to stay intact even when in close proximity to the destructive gravitational pull of the star that it orbits. If the rings spun in the expected direction, they would be pulled apart within just a few 11-year orbits around the star. However, the team’s calculations demonstrate how these reverse-spinning rings can withstand the forces pulling them apart for at least 10,000 years — the limit of their simulation. That’s enough time for J1407b to make its eleven-year trip around the star almost a thousand times. The researchers found that rings spinning in reverse can be sustained at almost one and a half times the size that they would be if spinning as expected, as larger rings are more susceptible to the star’s gravitational pull.

Thanks to their backward spin, J1407b’s rings approach the star at a higher speed. That extra speed allows them to hold together, whirling around the planet to relative safety before the star’s gravity has time to shred them.

Rieder’s simulation of his findings demonstrate that rings spinning in reverse (Retrograde orbit, on the left) can resist the star’s gravitational pull.

In Rieder’s video, it becomes clear that rings spinning in reverse are able to stabilize into distinct circles. Meanwhile, if the rings spun as conventional planetary models would have predicted, they would be annihilated the first time the star sails by.