The Mystery of the Centaurs Revealed: Enigmatic Space Objects are Comets After All

For years the true identity of centaurs, the small celestial bodies orbiting the Sun between Jupiter and Neptune, baffled scientists. Now, with the help from NASA's Wide-field Infrared Survey Explorer (WISE), a team of researchers believe they have solved the mystery, reporting that most are comets.

The main piece of the puzzle to solve in coming to this conclusion was whether or not the objects represent asteroids flung out from the inner solar system or comets traveling in toward the Sun from afar.

"Just like the mythical creatures, the centaur objects seem to have a double life," James Bauer of NASA's Jet Propulsion Laboratory in Pasadena, Calif., said in a press release.

However, as the lead author of the study published in the Astrophysical Journal, Bauer explains that his team's "data point to a cometary origin for most of the objects, suggesting they are coming from deeper out in the solar system."

By this the scientist means the majorityof the objects are likely made from the same material as a comet, may have been an active comet in the past and may be active again in the future.

It's true that In the past astronomers observed some centaurs with dusty halos, a common feature of outgassing comets. In addition, NASA's Spitzer Space Telescope previously uncovered evidence for comets in the group. However, scientists were unable to estimate the ratio of comets to asteroids without further information.

Infrared data from NEOWISE provided the needed insight by collecting data on the objects' albedos, or reflectivity, which allowed the scientists to sort out the population by determining that most of the blue-gray objects are dark, a telltale sign of comets.

"Comets have a dark, soot-like coating on their icy surfaces, making them darker than most asteroids," said the study's co-author, Tommy Grav of the Planetary Science Institute in Tucson, Ariz. "Comet surfaces tend to be more like charcoal, while asteroids are usually shinier like the moon."

Given this information, the scientists estimate that roughly two-thirds of the centaur population are comets, although it is not clear whether the rest are asteroids.

The findings come as a result of the largest infrared survey to date of centaurs and their more distant cousins called scattered disk objects. In all, NEOWISE, the asteroid-hunting portion of the WISE mission, gathered infrared images of 52 centaurs and scattered disk objects -- 15 of which represent new discoveries.

Going forward, the scientists hope that future research from NEOWISE may reveal the secrets of the centaurs further.