How many of the solar system's objects have ring systems? If you answered one, you're way behind the times. We now know of five, but an MIT team may have produced evidence suggesting one more.
After Huygens identified “a thin flat, ring nowhere touching” Saturn, it took 322 years to discover that Uranus also has rings. Soon after, we learned both the other gas giants, Jupiter and Neptune, are ringed too.
It was much more surprising, however, when minor planet Chariklo turned out to have rings as well. It's still mysterious how an object just 230 kilometers (140 miles) across forms and holds onto a ring system, but it seems it may not be that unusual.
In 2011, minor planet 2060 Chiron passed in front of a star. Not only was the star hidden as Chiron passed, but its light was dimmed before and afterward, indicating the presence of something 300 km (186 miles) each side of the minor planet's center. Analysis has taken more than three years, but has now been published in Icarus.
Often confused with Pluto's moon Charon, Chiron's elongated orbit brings it inside Saturn and almost as far out as Uranus. It was the first of a class of objects known as centaurs, thought to have tens of thousands of members. One of these is Chariklo, which is very similar in size and orbit.
MIT's Dr. Amanda Bosh helped observe similar passages in 1993 and '94, which allowed astronomers to estimate Chiron's diameter at 180 km (110 miles) and detect intriguing features that possibly indicate it was spouting jets of water or dust like a comet approaching the sun.
In 2011, Bosh got another chance, even though the star that Chiron passed has a magnitude of 14.9, requiring a large telescope to even see it. She used NASA's Infrared Telescope on Mauna Kea and the Les Cumbres Observatory on nearby Haleakala. "There's an aspect of serendipity to these observations," Bosh says. "We need a certain amount of luck, waiting for Chiron to pass in front of a star that is bright enough. Chiron itself is small enough that the event is very short; if you blink, you might miss it."
The pattern of dimming that Bosh and her colleagues observed suggests two rings, roughly 3 km and 7 km wide (3 miles and 4.3 miles wide), with a gap of 10-14 km (6-8.7 miles) between them. Bosh says the theory of symmetrical jets remains a possibility, but that permanent rings are also at least as likely.
Jessica Ruprecht, whose master's thesis contributed to the paper, says centaur rings may be the remnants of other objects. Alternatively, Ruprecht says, "Centaurs may have started further out in the solar system and, through gravitational interactions with giant planets, have had their orbits perturbed closer in to the sun. The frozen material that would have been stable out past Pluto is becoming less stable closer in, and can turn into gases that spray dust and material off the surface of a body."
Confirmation will require passage in front of another star, viewed from dispersed locations on Earth.
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