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Study Finds Most Centaurs Are “Cometary”

This artist’s concept shows a centaur creature together with asteroids on the left and comets at right. NASA/JPL-Caltech.

Centaurs are the small celestial bodies orbiting the Sun between Jupiter and Neptune. Their origin has long been mysterious. Until recently, astronomers were not certain whether centaurs are asteroids flung out from the inner solar system or comets traveling in toward the Sun from afar. Now a study has found that most centaurs are comets. The findings come from the largest infrared survey to date of centaurs by NEOWISE, the asteroid-hunting portion of NASA’s Wide-field Infrared Survey Explorer (WISE) mission.

Centaurs take their name from the creature in Greek mythology whose head and torso are human and legs are those of a horse. “Just like the mythical creatures, the centaur objects seem to have a double life,” said James Bauer of NASA’s Jet Propulsion Laboratory in Pasadena, California. Bauer, who did his PhD work at IfA, is the lead author of the paper about this study. “Our data point to a cometary origin for most of the objects, suggesting they are coming from deeper out in the solar system.” “Cometary origin” means an object likely is 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.

IfA astronomer Karen Meech is also a member of the study team. She noted, “These objects are exciting because they are active at large distances from the Sun, and they can give us a glimpse of the types of ices other than water ice, which is the most abundant ice in inner solar system comets. Water can only be released from a comet when it gets closer to the Sun than about the orbit of Jupiter. Outside of this distance, it is too cold.”

Centaurs orbit in an unstable belt. Ultimately, gravity from the giant planets will fling them either closer to the Sun or farther away from their current locations. Although astronomers previously observed some centaurs with dusty halos, a common feature of outgassing comets, and NASA’s Spitzer Space Telescope also found some evidence for comets in the group, they had not been able to estimate the numbers of comets and asteroids.

Infrared data from NEOWISE provided information on the objects’ albedos, or reflectivity, to help astronomers sort the population. NEOWISE can tell whether a centaur has a matte and dark surface or a shiny one that reflects more light. The puzzle pieces fell into place when astronomers combined the albedo information with what was already known about the colors of the objects. Visible-light observations have shown centaurs generally to be either blue-gray or reddish in hue. A blue-gray object could be an asteroid or comet. NEOWISE showed that most of the blue-gray objects are dark, a telltale sign of comets. A reddish object is more likely to be an asteroid.

 “Comets have a dark, sootlike 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, Arizona. “Comet surfaces tend to be more like charcoal, while asteroids are usually shinier like the Moon.” Grav is a former IfA postdoctoral researcher.

The results indicate that roughly two-thirds of the centaur population are comets, which come from the frigid outer reaches of our solar system. It is not clear whether the rest are asteroids. The centaur bodies have not lost their mystique entirely, but future research from NEOWISE may reveal their secrets further.