Cometary Origins For Most Centaurs: WISE
July 26, 2013

Many Of Our Solar System’s Centaurs Are Actually Comets

April Flowers for - Your Universe Online

The small celestial bodies orbiting between the sun and Jupiter and Neptune are known as centaurs, and their true identity has been one of the enduring mysteries of astrophysics. Previously, scientists have questioned, are they asteroids or comets? A new study, published in the Astrophysical Journal, using observations from NASA's Wide-field Infrared Survey Explorer (WISE) finds most centaurs are comets.

Astronomers have been unsure whether centaurs are comets traveling in toward the sun from afar or asteroids flung out from the inner solar system. This dual nature is the inspiration for the name centaur. A centaur was a creature from Greek mythology whose head and torso were those of a man and the legs were 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 (JPL). "Our data point to a cometary origin for most of the objects, suggesting they are coming from deeper out in the solar system."

An object with a "cometary origin" is most 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.

The data used to calculate the new findings was collected as part of the largest infrared survey to date of centaurs and their more distant cousins, called scattered disk objects. The asteroid-hunting portion of the WISE mission, called NEOWISE, gathered infrared images of 52 centaurs and scattered disk objects, fifteen of which are new discoveries. The centaurs and scattered disk objects orbit in an unstable belt. At some point in the future, gravity from the giant planets will fling them either closer to the sun or farther away from their current locations.

Astronomers have previously observed centaurs with dusty halos - a common feature of outgassing comets - and some evidence for comets was found in the group by NASA's Spitzer Space Telescope. However, the astronomers were not able to estimate the numbers of comets and asteroids.

NEOWISE infrared data provided information on the observed objects' reflectivity, called the albedo. This information helps astronomers to sort the population. NEOWISE allows the scientists to tell whether a centaur has a matte and dark surface or a shiny one that reflects more light. When the astronomers combined the albedo information with what was already known about the colors of the objects, the puzzle pieces fell into place. Centaurs are normally either blue-gray or reddish in hue in visible light observations, and blue-gray objects can be either an asteroid or a comet. The NEOWISE data revealed that most of the blue-gray objects are dark, which is a telltale sign of comets. A reddish object, on the other hand, is most likely an asteroid.

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

The study findings suggest that roughly two-thirds of the centaur population is made up of comets. These comets come from the frigid outer reaches of our solar system. So far, it is unclear whether the final third of the population are asteroids. The centaurs still remain mysterious, but future NEOWISE data may reveal further secrets.