Move Over Saturn: Astronomers Discover Miniature Planet With Rings
Saturn, Jupiter, Uranus, and Neptune have something in common besides being enormous balls of gas: they were the only objects known to have orbital rings of rock, ice, or dust. But a new discovery announced Wednesday has changed the picture of our solar system.
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Astronomers say they saw a halo of ice and dust floating around a giant rock between the orbits of Saturn and Uranus. The rock, a kind of asteroid-comet hybrid called Chariklo, is the smallest object and the first non-planet found to have rings. "We were not even looking for rings, because they had never been observed around small objects like Chariklo," says Uffe Gråe Jørgensen of the Niels Bohr Institute, who co-authored a study published Wednesday in Nature.
The way the scientists describe it, this ring system would be beautiful if you could see it up close. Chariklo, referred to sometimes as a miniature planet, is only about 155 miles wide. To drive all the way around its surface would be like driving from New York City to Cleveland. And if you could look up into the night sky, they said, you'd see two glittering rings — one brighter, one more faint — 12 miles wide and hovering about as high up as the International Space Station is above the Earth.
The best picture of Chariklo we can see from down here, however, is a subtle dimming of background stars as the mini-planet passes in front of them. That's the only way scientists at the Niels Bohr Institute and the European Southern Observatory were able to detect it, and purely by chance.
The telescopes in South America that spotted Chariklo's rings are deep-space exoplanet hunters, trained to find other planets in the Milky Way galaxy by watching their sillhouettes slide across their host stars. When they have some down time they look at other things; this time they happened to see Chariklo pass in front of a distant star. The star dimmed for Chariklo, but it dimmed around it, too. "The discovery — and the amazing amount of detail we saw in the system — came as a complete surprise," says Felipe Braga-Ribas, the lead author on the paper, in a statement.
How the rings got there is another question. No one knows for sure. Their best guess, though, is that Chariklo collided with a planet a long time ago, ejecting a bunch of debris out into space. Some of that debris was trapped by the object's weak gravitational pull. Scientists believe this is the origin of the Earth's moon — that during the chaotic birth of the solar system, some planets formed rings that eventually condensed into moons. "So, as well as the rings, it's likely that Chariklo has at least one small moon still waiting to be discovered," Braga-Ribas says.
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