When one thinks of rings in space, the mind goes immediately to Saturn and its remarkable ring structure. Jupiter, Uranus, and Neptune also have rings, so astronomers thought they were a characteristic of very large planets. But more recent discoveries put that into question. Small bodies have rings too, and now researchers know how they maintain them.
Asteroid Chariklo and dwarf planet Haumea both have rings despite lacking certain characteristics that were thought to make rings stable, such as little shepherd moons to balance out gravitational forces. As reported in Nature Astronomy, it is the odd shape of these two objects that makes them keep their rings. The irregularities of Chariklo and the unusual flatness of Haumea prevent the rings from dispersing.
“Rings appear around Saturn, Jupiter, Neptune, and Uranus, but scientists found rings around Chariklo and Haumea within the last few years. Chariklo and Haumea were the first small objects known to have rings, and we think that rings throughout the Solar System are more common than we thought,” co-author Dr Maryame El Moutamid, from the Cornell Center for Astrophysics and Planetary Science, said in a statement. “In the case of small bodies Chariklo and Haumea, gravity shepherds the rings. The rings are confined by the gravity because of the shape irregularity of their bodies.”
Chariklo is a rocky, oddly shaped object and a member of the centaur minor planets. If it was a sphere it would have a diameter of 252 kilometers (156 miles), but its dimensions vary between 200 and 300 kilometers (124-188 miles). Researchers also suspect the presence of a large mountain could be altering the gravitational field produced by the object.
Haumea, named after the Hawaiian goddess of fertility and childbirth, is an extremely elongated and fast-spinning object. It rotates on itself every four hours and this factor plays a role in its odd ellipsoidal shape. One of its axes is twice as long as the other.
These peculiarities appear to be key to the stability of the rings. Chariklo even has two. Given that more regular shapes are a prerogative of the most massive objects in the Solar System, there could be many ringed bodies orbiting the Sun. Astronomers have suspected that Chiron, another centaur, has rings as well, and this work gives theoretical backing to that idea.
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