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Astronomers find first asteroid with rings


LOS ANGELES » Scientists have discovered a double ring system around a dark, icy asteroid in the outer solar system.

The discovery marks the first time that rings have been found around any object that is not a planet. The findings were published Wednesday in the journal Nature.

"It is amazingly cool — until now, we had no idea that anything but the giant planets could have rings," said Amy Mainzer, an astronomer at the Jet Propulsion Laboratory who was not involved in the study.

Observations made with seven different telescopes in June 2013 suggest that the two rings are dense and thin. One is about 2 miles wide. The other is about 4 miles wide. They are separated by a gap of about 5.5 miles, and they are likely made of water ice.

The two rings circle a large asteroid known as Chariklo that is 155 miles across. Chariklo is a type of asteroid known as a centaur — a class of objects that orbit the sun between Jupiter and Neptune. It is shaped like an M&M, and its surface is darker than asphalt.

Because Chariklo is so far away and so dark, it is very difficult to image from ground-based telescopes. The discovery of the rings was made when the asteroid was observed passing in front of a distant star. By taking careful measurements of how long the light of the star was blocked by the asteroid, an international team of scientists led by Felipe Braga-Ribas of the Observatorio Nacional in Rio de Janeiro was hoping to get a more precise idea of Chariklo’s size.

But something unexpected happened. The star’s light went out briefly twice, and then for a long time, and then briefly twice again. Something around Chariklo was clearly blocking the light too. The only explanation for the symmetry? Rings.

"It is a similar technique to how the rings of Uranus were discovered and the ring arc of Neptune," said James Bauer, a planetary astronomer at the Jet Propulsion Laboratory who was not involved in the study. "This is a great technique for finding faint rings, and the evidence is very credible."

Nobody knows exactly how these rings formed, but the authors of the Nature paper suggest their origin is likely tied to some sort of collision. One possibility is that another asteroid slammed into Chariklo, knocking ice off its surface that reformed as rings. Another possibility is that the rings are made of the remains of that other body, blown to smithereens by the impact. Or perhaps two small moons of Chariklo collided, forming the rings.

"We observe asteroids impacting other asteroids in the Main Belt, so perhaps it stands to reason that the phenomenon occurs in the more distant parts of the solar system as well," Mainzer said.

Now that scientists know there are rings around Chariklo, Bauer said, he expects the next step is to get some images of them.

"Now that we know the faintness and the distance, there may be ways of trying to image this object either from space or from ground base using enhanced imaging techniques," he said.

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