Earth was hot and dry when it formed, and water for its vast oceans came later when the planet cooled, scientists believe.

The source of that water remains a mystery, but a discovery by two University of Hawai'i astronomers could provide some clues.

Based on Nov. 26 observations using the 8-meter Gemini North telescope on Mauna Kea, UH graduate student Henry Hsieh and professor David Jewitt found three icy comets orbiting in the main asteroid belt between Mars and Jupiter. This group of "main-belt comets" represents a new class of celestial objects, Jewitt said yesterday.

"We've identified a new type of object that by the shape of its orbit is definitely a main-belt asteroid, but by its appearance is definitely a comet. It fits into both categories," Jewitt said. Their existence suggests that asteroids and comets are much more closely related than previously thought, he said.

The discovery was announced yesterday by the UH Institute for Astronomy and will appear in the April print edition of Science magazine.

Comets are thought to originate in the cold outer solar system beyond Neptune, and consequently contain much more ice than asteroids, which are thought to have formed closer to the sun in the asteroid belt between Mars and Jupiter, according to an institute news release. Comets also have large, elongated orbits and experience wide temperature variations as they move closer to and farther from the sun.

As it heats up on approach to the sun, a comet's ice turns to gas, giving rise to a tail and a distinctive fuzzy appearance, the release said. When it travels away from the sun, any remaining ice stays frozen until its next pass.

Objects in the asteroid belt have circular orbits within the inner solar system and are expected to be essentially rocks baked dry of ice.

Because they contain ice, Jewitt said, comets were considered more likely candidates to have brought water to Earth in collisions with the planet. But recent analysis has shown that comet water is significantly different from typical ocean water on Earth, he said.

That would seem to point to asteroids as the source of water, even though it was believed they did not contain ice. The UH astronomers' discovery of the icy main-belt comets indicates that is not the case, and that ice is on the surface of at least some objects in the main asteroid belt.

Jewitt said he expects hundreds, maybe thousands, more of these objects to be discovered using the new Pan-STARRS telescope atop Haleakala, which will provide a more panoramic view of the night sky when it begins operation this year.

Jewitt and Hsieh's work is supported by a grant from the NASA Planetary Astronomy Program of the Science Mission Directorate.

Reach Christie Wilson at cwilson@honoluluadvertiser.com.