The newly discovered comets, called "main-belt comets," hold asteroid-like orbits, and unlike other comets they appear to have formed in the warm inner solar system inside the orbit of Jupiter, rather than in the outer solar system beyond Neptune. Their existence suggests asteroids and comets might be more closely related than previously thought.

"What the (main-belt comet) discovery means, I suspect, is that the snow line was at about 3 AU (Earth distances) or closer at the time when the outer belt objects formed," research team leader David Jewett, of the University of Hawaii, told SpaceDaily.com.

The snow line is the distance from the Sun inside which water can exist as a gas or a liquid, and outside of which it exists mainly as ice. Jewett said the MBCs, as they are called, "accreted outside the snowline, like the short- and long-period comets and all the other icy things in the solar system." He added that the newly found objects suggest where the snowline was during the earliest history of the solar system.

Writing in the March 23 issue of Science Express, Jewett and Hawaii colleague Henry Hsieh said they used the 8-meter Gemini North Telescope on Mauna Kea last Nov. 26 and found that an object they designated Asteroid 118401 was ejecting dust like a comet.

Together with a mysterious comet designated 133P/Elst-Pizarro - known for almost a decade but still poorly understood - and another comet designated P/2005 U1 (discovered by the Spacewatch project in Arizona just a month earlier), 118401 forms an entirely new class of comets.

"The expert dynamicists who have looked at this have not found ways to transform short- or long-period comets into asteroidal orbits," Jewett said. "The reason is that the asteroidal orbits do not allow interaction with Jupiter, so there is no opportunity for the transformation to occur within dynamical schemes."

In other words, the MBCs are unique because they follow flat, circular asteroid-like orbits - not the elongated and often tilted orbits that characterize nearly all other comets. Moreover, their comet-like appearance makes them unlike all other previously observed asteroids.

"In fact, if the MBCs could be derived from comets, then I think one would have to conclude that the whole main asteroid belt could be derived from comets, and nobody is claiming that," Jewett said. "So, we think that they formed in place, with ice that has survived since the accretion epoch."

In both 1996 and 2002, 133P/Elst-Pizarro (named after its two discoverers) was seen to exhibit a long dust tail typical of icy comets, despite having the flat, circular orbit typical of presumably dry, rocky asteroids. As the only main-belt object ever observed to take on a cometary appearance, 133P/Elst-Pizarro's true nature remained controversial.

"The discovery of the other main-belt comets shows that 133P/Elst-Pizarro is not alone in the asteroid belt," Jewitt said. "Therefore, it is probably an ordinary (although icy) asteroid, and not a comet from the outer solar system that has somehow had its comet-like orbit transformed into an asteroid-like one. This means that other asteroids could have ice as well."

Earth is thought to have formed as a hot and dry rocky body, meaning its water content must have been delivered after the planet cooled. Possible candidates for supplying the water are comets and asteroids that impacted the early Earth.

Because of their large ice content, comets were leading candidates for many years, but recent analysis of comet water has shown it is significantly different from typical ocean water on Earth, in terms of its ratio of molecular hydrogen and its heavier isotope, deuterium.

Ice from asteroids could be the solution to the mystery � and future space probes should be able to compare asteroid ice chemically to Earth's water and either prove or disprove the hypothesis.

Jewett said a huge number of impacts from MBCs would be required to furnish all of Earth's water. "Of course, the number is not really what you care about," he added. "The MBCs must occupy a size distribution and a lot of the mass could be contained in a smaller number of objects bigger than 5 kilometers (3 miles)."

He noted that the mass of all the water on the Earth equals about 0.0005 the planet's mass, and previous calculations have suggested that external impacts could have provided that much volume.

The real importance of the discovery, Jewett said, "is that the new objects provide an additional set of icy objects, probably formed much closer to the Sun (3 AU) than either the short- (40 AU?) or long- (20-30AU?) period comets, so we can use them in the future to sample a different piece of the protoplanetary disk."

He said although there is no proof "that the oceans are melted MBCs, but this is certainly possible. Finding the MBCs gives us something to work as we try to address this interesting possibility. Also, it's not very often that you get to find an entirely new class of object in the solar system, so it's very cool from that perspective, too."

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