Our gas giants strip the icy surface off minor planets to form miniature versions of Saturn’s famous rings throughout the outer solar system, according to researchers from Kobe University.Artist’s impression of the rings around Chariklo. Image credit: ESO / L. Calcada / Nick Risinger, skysurvey.org.Artist’s impression of the rings around Chariklo. Image credit: ESO / L. Calcada / Nick Risinger, skysurvey.org.
Whilst Saturn’s are the arguably most exquisite, all the gas giants are adorned with ring systems.Not just beautiful, these discs provide an astronomical Petri dish where the processes that formed the planets billions of years ago play out today for us to study.“Looking at these systems you can understand the origins of the solar system,” says Ryuki Hyodo from Kobe University’s Department of Planetology, who studies these rings.
Until recently it was thought only the gas giants could support rings. However discs have recently been found encircling far smaller bodies.Chariklo and Chiron are centaurs, blocks of rock and ice that form further out in the solar system, but today find themselves orbiting between Jupiter and Neptune.Estimates suggests there may be 44,000 centaurs larger than 1 km across, though Chariklo, the largest at 250 km in diameter, and Chiron remain the only two around which rings have been observed.Many more centaur discs may exist.
However, as theories of origin for the only two observed centaur rings shifted between tidal forces from planetary close encounters, ejection material from smaller object impacts, and degassing caused by rising temperatures, it remained difficult to make conclusive estimates as to how common they are.
Doubts that these small bodies could retain impact ejection material, and observations of ring structure that ruled out a solely gaseous origin led Hyodo to run detailed simulations of the team’s preferred mechanism: a close passing with a gas giant creating tidal forces that remove the outer shell of the centaur.Key to their model was the assumption that centaurs were differentiated through heating by radioactive isotopes, allowing denser rocky material to sink to the core, and leaving an ice-rich mantle and surface.
“Our calculations suggests that with the right tidal forces from the right close encounter the rocky centaur core would stay intact whilst the ice-rich mantle would be stripped off forming the ring material observed around Chariklo and Chiron,” Hyodo says.Extending their simulations to the entire centaur system, the team believes around 10% of differentiated centaurs would form rings through this process.
Further out in our solar system rings have proved more elusive.Earlier this month Charles Ying-Tung from the Academia Sinica Institute of Astronomy and Astrophysics and co-authors discovered a rare trans-Neptunian object (TNO) they named Niku, meaning ‘rebellious’ in Chinese.Niku orbits unusually, both in its direction (opposite to the majority of the solar system) and its angle (110 degrees inclined to the general plane of orbit), characteristics that defy current explanation.However there is one area where Niku is a typical TNO.
It has no rings.“For TNOs it is harder to detect rings even if they do have them. Like observing a solar eclipse – it’s a chance lining up of our solar system and it is more difficult [to encounter] further out,” Ying-Tung explains.It was thought larger TNOs might provide the best chance for observing rings, generating hope ahead of New Horizon’s visit to Pluto, the largest TNO discovered. But the mission saw no such structures.
Ying-Tung believes it is less likely they are able to form this far out.“The density of objects for collisions is low, and it’s 50 to 100 degrees Kelvin colder than in the centaur region, making degassing mechanisms also less likely,” he says.With two of the three commonly predicted models discarded, Hyudo agrees the likelihood of rings at this distance is low.“Only centaurs could have rings through a planetary encounter because only centaurs encounter the gas giants.”