For millennia, comets were believed to be omens of doom. Instead, solving the mysteries regarding these "dirty snowballs" could help reveal the part they played in the birth of life on Earth, as well as secrets concerning the rest of the galaxy.

Did comets help create Earth's seas?

For years scientists thought comets slamming against the newborn Earth helped deliver water to a once dry planet. But roughly a decade ago this view was shaken by the discovery that the water in comets and Earth's oceans did not match up in terms of hydrogen isotopes.

Calculations then showed it was highly improbable that enough icy rocks from the suspected homes of comets — the Kuiper belt past Neptune and the Oort cloud past that — could have collided with Earth to supply its oceans.

In the last two years, however, researchers have discovered comets in the outer part of the asteroid belt. These "main-belt comets" may have the right levels of hydrogen isotopes, and are perhaps close enough to Earth to have realistically brought us the seas that life emerged from.

"No one knows for certain yet where Earth's oceans came from," said University of Hawaii astrophysicist David Jewitt. "Earth's oceans are likely a mixture of water from all sorts of places, but the main-belt comets are very likely one of them."

Where do comets come from?

The suspected homes of comets include the Oort cloud, the Kuiper belt and now the asteroid belt. But are there more reservoirs of comets yet to be found?

The Oort cloud is a theoretical cloud of icy rocks roughly 4.6 trillion miles (7.5 trillion kilometers) from the sun thought to be the source of long-period comets — that is, ones that take more than a few centuries to complete their orbits. It was once thought the original home of short-period comets as well, until calculations suggested that was impossible.

About 20 years ago, the Kuiper belt roughly 4.6 billion miles (7.5 billion kilometers) from the sun was then proposed to be the home of short-period comets. "But measurements taken in the last few years raise some doubts about that," Jewitt explained. "Maybe there are other reservoirs of comets yet to be discovered."

Secrets regarding the birth of the solar system?

Comets were long thought to be primordial relics, pristine leftovers from the protoplanetary disk that once surrounded the newborn sun. As such, it was supposed they might hold secrets untouched for billions of years regarding the birth of our solar system.

Increasingly, however, it looks as if the comets we see are anything but unspoiled. Instead, "there is good evidence that many of them are nearly burned-out hulks, with neither the size, mass, shape nor spin they might have had before entering the solar system," Jewitt said.

Still, "since comets are icy, they're not entirely cooked, and we may learn a lot regarding the formation of the solar system from chemicals trapped in their ice," he added.

Comets so close to the sun?

The main-belt comets are themselves a mystery. Until their discovery, researchers had largely supposed no comets could have lasted that close to the sun without getting baked away after a few centuries or millennia.

Dirt coatings on main-belt comets could have protected them from sunlight for billions of years. Every now and again boulders a yard or larger tumbling around the asteroid belt might hit these comets, uncovering their ice and triggering the plumes of gas and dust that got them discovered in the first place.

"We expect to soon find many hundreds or thousands of main-belt comets," Jewitt said.

Interstellar comets?

As our solar system formed, calculations predict the gravitational pull of the planets would have scattered 90 to 99 percent of all comets that once orbited the sun away toward the stars, never to be seen again. "If every star does that, you would expect some of their comets to come toward us, but no such object has ever been seen," Jewitt said.

Still, as astronomical telescopes and techniques improve, Jewitt remains optimistic that such interstellar comets will be detected fairly soon. These comets would prove quite distinctive, zipping at great speeds and following trajectories completely unlike the orbits our comets follow.

"We could see interstellar comets for the first time in the next few years," Jewitt predicted. "It would be great if we saw one, especially so if we had the wherewithal to launch a mission to one, to get samples and study the diversity of comets in an interstellar and galactic context. But we have to find one first."