This artist's concept shows a centaur creature
together with asteroids on the left and comets at right.
Image Credit: NASA/JPL-Caltech
PASADENA, Calf. -- The true identity of centaurs, the small celestial bodies
orbiting the sun between Jupiter and Neptune, is one of the enduring mysteries
of astrophysics. Are they asteroids or comets? A new study of observations from
NASA's Wide-field Infrared Survey Explorer (WISE) finds most centaurs are
comets.
Until now, astronomers were not certain whether centaurs are asteroids flung
out from the inner solar system or comets traveling in toward the sun from afar.
Because of their dual nature, they 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, Calif.
Bauer is lead author of a paper published online July 22 in the Astrophysical
Journal. "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.
The findings come from the largest infrared survey to date of centaurs and
their more distant cousins, called scattered disk objects. NEOWISE, the
asteroid-hunting portion of the WISE mission, gathered infrared images of 52
centaurs and scattered disk objects. Fifteen of the 52 are new discoveries.
Centaurs and scattered disk objects 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, 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."
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.
The paper is available online at:
JPL, managed by the California Institute of Technology in Pasadena, managed
and operated WISE for NASA's Science Mission Directorate. The NEOWISE portion of
the project was funded by NASA's Near Earth Object Observation Program. WISE
completed its key mission objective, two scans of the entire sky, in 2011 and
has been hibernating in space since then.
For more information about the WISE mission, visit:
NASA
Guillermo Gonzalo Sánchez Achutegui
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