An exoplanet's huge rings may orbit in the opposite direction to the planet's rotation around its star.
Steven Rieder from the RIKEN Advanced Institute for Computational Science in Japan and Matthew Kenworthy at Leiden University in the Netherlands carried out simulations of a suspected planet orbiting the star J1407 – a planet with a gigantic ring system that periodically eclipses its star.
They found the rings could only remain stable if they turned against the rotation of their planet's journey around J1407. The calculations were accepted for publication in the journal Astronomy and Astrophysics.
Before the simulations, there were doubts about this hypothesis – mainly because the exoplanet orbits in a very eccentric orbit.
This means sometimes the planet gets very close to the star, and the star's gravitational effects could disrupt the rings (which are 100 times larger than Saturn's rings).
The simulations show that the system is stable and can persist for more than 100,000 years – or 10,000 orbits, where each orbit takes 11 years.
There is one premise, though, Rieder says: "The system is only stable when the rings rotate opposite to how the planet orbits the star."
Rings that turn in this sense (retrograde rings) are not common so the pair suspects that there has been a catastrophe that caused the rings (or the planet) to turn the other way around.
The star eclipses could still be caused by a free floating object. But "the chance of that is minimal", says Rieder.
In future research, the astronomers want to investigate how the ring structure could form and how it changes over time.