When you think of a celestial ring system, the beautiful ringed planet Saturn will likely jump to mind. But for the first time astronomers have discovered that ring systems aren’t exclusive to planetary bodies — asteroids can have them too.
Announced on Wednesday, astronomers using several observatories in South America, including the ESO’s La Silla Observatory in Chile, have discovered that distant asteroid Chariklo possesses two distinct rings. Chariklo, which is approximately 250 kilometers (155 miles) wide, is the largest space rock in a class of asteroids known as Centaurs that orbit between Saturn and Uranus in the outer solar system.
“We weren’t looking for a ring and didn’t think small bodies like Chariklo had them at all, so the discovery — and the amazing amount of detail we saw in the system — came as a complete surprise!” said lead researcher Felipe Braga-Ribas, of the Observatório Nacional and MCTI, in Rio de Janeiro, Brazil.
Due to the asteroid’s remote location, direct observations of Chariklo’s rings are not possible. Instead, astronomers planned to watch the asteroid drift in front of a distant star — an event known as an occultation, when starlight is blocked for a few seconds. As the asteroid drifted in front of the star UCAC4 248-108672 on June 3, 2013, the astronomers noticed something strange happen to the starlight a few seconds before and a few seconds after occultation: there was a double-dip in starlight brightness.
Astronomers have interpreted this double-dip signal as the presence of a double ring system — the first ever ring system discovered around an asteroid.
“For me, it was quite amazing to realize that we were able not only to detect a ring system, but also pinpoint that it consists of two clearly distinct rings,” said team member Uffe Gråe Jørgensen of the Niels Bohr Institute, University of Copenhagen, Denmark. “I try to imagine how it would be to stand on the surface of this icy object — small enough that a fast sports car could reach escape velocity and drive off into space — and stare up at a 20-kilometre wide ring system 1000 times closer than the moon.”
The astronomers have been able to read incredible details from this observation, including information about the ring system’s shape, orientation and width. There are two sharply confined rings 7 kilometers (4.3 miles) and 3 kilometers (1.9 miles) wide separated by a 9 kilometer (5.6 miles) gap. The rings are being nicknamed Oiapoque and Chuí, two rivers in Brazil.
The ring gap and the ring system’s defined shape is suggestive of the presence of a small satellite in orbit around the asteroid.
“So, as well as the rings, it’s likely that Chariklo has at least one small moon still waiting to be discovered,” said Braga-Ribas.
All this information points to the ring system and hypothetical moon as being debris from an asteroid impact. It seems possible that the moon may be formed out of the ring debris, hinting at the moon formation process that may have spawned Earth’s moon and other moons found throughout the solar system.
This discovery has been published in the March 26 online edition of the journal Nature.