Comets Posing as Asteroids Might be Source of Earth’s Water

Comets Posing as Asteroids Might be Source of Earth’s Water
Images of the three known main-belt comets (at center of each panel). Other objects shown are stars and galaxies smeared by the motion of the telescope while tracking each comet. (Image credit: H. Hsieh and D. Jewitt/Univ. Hawaii.)

Icy comets embedded the in belt of asteroids between Mars and Jupiter may point to the origin of Earth's own water supply, scientists said Thursday.

Three comets exhibiting quite uncomet-like behavior have been found orbiting the Sun in a manner more expected of rocky asteroids. Dubbed "main-belt comets" by researchers, the objects suggests that comets and asteroids share more in common than previously thought, and that water found on Earth may have also taken root in the asteroid belt.

"It's just the tip of the iceberg," said University of Hawaii astronomer David Jewitt in a telephone interview. "We said in our study there may be hundreds of them out there, but there could be thousands."

The difference

Asteroids tend to be made of rock and metal. Comets, which typically spend most of their existence beyond Neptune and visit the inner solar system infrequently if ever, hold more water ice and other icy chemicals and are often called icy dirtballs.

Jewitt and graduate student Henry Hsieh, who led the study, used the Gemini North Telescope atop Hawaii's Mauna Kea to find that on object dubbed Asteroid 118401 was not really an asteroid at all.

Their observations found the "asteroid" was ejecting dust like a comet. The Nov. 26 discovery came one month after a similar object was discovered by the Spacewatch project in Arizona. Astronomers have known of a third comet-like object, 133P/Elst-Pizarro, for about 10 years; it no longer seems an isolated oddball.

"The main-belt comets are unique in that they have tight, circular, asteroid-like orbits, and not the elongated, often tilted orbits characteristic of all other comets," Hsieh said in a statement. "At the same time, their cometary appearance makes them unlike all other previously observed asteroids."

The research was detailed March 23 in the online version of the journal Science.

Watering a planet

Astronomers believe that the Earth formed under hot and dry conditions, relying on ice from comets to build up its stores of water and become habitable. But studies of traditional comet ice have found that their water composition is quite different that that of Earth's oceans, Jewitt said.

Main-belt comet ice appears to have formed while the solar system was still a vast protoplanetary nebula of raw material under much warmer temperatures than the conditions where traditional comets formed, out in the Kuiper Belt on the frigid fringe of our planetary neighborhood, Jewitt said.

Because of their proximity, main belt comets could have served as a major source for Earth's water, the researchers said.

More study is needed to determine if the main-belt comet ice is chemically similar to that found on Earth. To do that, Jewitt says, a full-sky survey is required to scan the myriad asteroids in the for other comet-like objects.

Since main-belt comets are somewhat predictable in their orbits and relatively close to Earth, they might even prove attractive targets for future missions.

"These guys are close astronomically, so yes, we could probably do something with a spacecraft," Jewitt said.

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Tariq Malik

Tariq is the Editor-in-Chief of and joined the team in 2001, first as an intern and staff writer, and later as an editor. He covers human spaceflight, exploration and space science, as well as skywatching and entertainment. He became's Managing Editor in 2009 and Editor-in-Chief in 2019. Before joining, Tariq was a staff reporter for The Los Angeles Times covering education and city beats in La Habra, Fullerton and Huntington Beach. In October 2022, Tariq received the Harry Kolcum Award for excellence in space reporting from the National Space Club Florida Committee. He is also an Eagle Scout (yes, he has the Space Exploration merit badge) and went to Space Camp four times as a kid and a fifth time as an adult. He has journalism degrees from the University of Southern California and New York University. You can find Tariq at and as the co-host to the This Week In Space podcast with space historian Rod Pyle on the TWiT network. To see his latest project, you can follow Tariq on Twitter @tariqjmalik.