Giant rings orbit wrong way around exoplanet

13 October 2016

Researchers from Japan and the Netherlands who were previously involved in the discovery of an exoplanet with huge rings have now calculated that the giant rings may persist more than 100,000 years, as long as the rings orbit in the opposite direction to that of the planet around the star. Their findings have been accepted for publication in the Astronomy & Astrophysics journal.

 

The researchers Steven Rieder (RIKEN, Japan) and Matthew Kenworthy (Leiden University, the Netherlands) studied the star J1407. This young sun-like star exhibited a strange series of eclipses in 2007. In 2015 the researchers put forward an explanation: there must be a planet with a gigantic ring system orbiting the star. These rings must be over a hundred times larger than the rings of Saturn. 

Very eccentric orbit

In 2016 the researchers carried out simulations to see whether such a massive ring system can be stable over a long period. Before the simulations there were doubts about this hypothesis since the exoplanet travels in a very eccentric orbit - sometimes the planet gets close to the star, and the gravitational effect of the star could disrupt the rings.

Stable system

Now the simulations show that the system is stable and can persist for more than 10,000 orbits of 11 years. According to lead author Steven Rieder there is one premise, though: ‘The system is only stable if the rings rotate opposite to how the planet orbits the star.’

The rings around J1407b are so big that we could see them from Earth at dusk if they were around the planet Saturn. The rings can be seen here above the Old Leiden Observatory (c) M. Kenworthy / Leiden University

Divergent orbit caused by catastrophe

Rings that turn in this direction (retrograde rings) are not common. The researchers therefore suspect that a catastrophe has occurred that caused the rings (or the planet) to turn the other way around.

Rieder: ‘It might sound far-fetched - massive rings that rotate in opposite direction - but we have now calculated that a “normal” ring system cannot survive.’

Star eclipses 

The star eclipses could still be caused by a free-floating object. ‘The chance of that is minimal,’ says Rieder. ‘Also, the velocity measured with previous observations may not be right, but that would be very strange, because those measurements are very accurate.’

In future research, the astronomers want to investigate how the ring structure could form and how it changes over time.

Image at top of web page: Artist's impression of the ring system around J1407b (c) Ron Miller