H. Hsieh and D. Jewett/University of Hawaii

Three of a kind.
All three Main Belt Comets eject dust and gas.

Hidden Comets Tell Icy Tale

Astronomers have confirmed that icy comets are orbiting comfortably among their more rocky brethren in the main asteroid belt between Mars and Jupiter. The discovery challenges conventional explanations for the solar system's evolution and suggests a new source for Earth's oceans.

Traditional thinking on asteroids and comets had made them easy to distinguish up to now. Asteroids are classified as rocky bodies that stay completely within the orbit of Jupiter. Comets, on the other hand, are icy and follow highly elliptical paths that take them to the outer edges of our solar system. But a surprising find a decade ago threatened to shake up this convenient taxonomy: Astronomers spied a body among the asteroids, later designated 113P/Elst-Pizarro, which was ejecting dust and gas like a comet. Last October, another cometlike body, called P/2005 U1, also seemed to be hanging out among the asteroids.

Then, last November, using the 8-meter Gemini North Telescope on Mauna Kea in Hawaii, astronomers David Jewett and Henry Hsieh discovered the third exception that broke the rule: A body, named 118401, with the same characteristics as 133P/Elst-Pizarro and P/2005 U1. The discovery, says Jewett, means 133P/Elst-Pizarro and P/2005 U1 are not anomalies; rather, the team thinks all three objects constitute an entirely new class of comets that formed among the asteroids at the beginning of the solar system.

These so-called Main Belt Comets may shed light on where our planet got its water. Earth is thought to have formed as a hot and dry rocky body, meaning water came to the planet after it cooled. Because of their large ice content, conventional comets were the leading candidates for many years, but recent analysis of comet water has shown it is significantly different from typical ocean water on Earth. Jewett says ice from asteroids could solve the mystery; future space probes should be able to test the hypothesis.

Brian Marsden, director of the Minor Planet Center at the Harvard-Smithsonian Center for Astrophysics--which designates the names and numbers of comets and asteroids--says there's no doubt the three objects are following asteroidlike orbits. But "the question is whether some bizarre combination of effects could have caused this," he says. For example, gravitational perturbation plus the thrust of ice vaporization during a pass by the Sun might have relocated a conventional comet to the asteroid belt. "If we had just the one, we might accept that, but with three objects, Jewett's assertion becomes more reasonable," he says.

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