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Celestial Centaurs are Likely Comets, Researchers Find

July 25, 2013
Centaurs, which orbit between Jupiter and Neptune, resemble both asteroids and comets. This artist's concept shows a Centaur creature together with asteroids on the left and comets on the right. Credit: NASA/JPL-Caltech
Tucson, Ariz. -- Observations from NASA's Wide-Field Infrared Survey Explorer (WISE) suggest that most Centaurs – small celestial bodies orbiting between Jupiter and Neptune – are comets rather than asteroids.
Centaurs, named after the mythical human-horse creatures because of their dual comet-asteroid nature, are comets traveling in towards the Sun rather than asteroids flung out from the inner solar system, according to a new paper "Centaurs and Scattered Disk Objects in the Thermal Infrared: Analysis of WISE/NEOWISE Observations" appearing in the Astrophysical Journal.
NEOWISE can tell whether the Centaurs are matte and dark like charcoal, or shiny, reflecting more light as is the case with our Moon. A better understanding of Centaurs was gained by combining this information with what is already known about their colors, said Tommy Grav, a Research Scientist at the Planetary Science Institute and co-author on the paper. Roughly two-thirds of Centaurs are likely comets hailing from the frigid outer reaches of the Solar System, the paper states. The origin of the remaining Centaurs is still unknown.
"Just like the mythical creatures, the Centaur objects seem to have a double life," said James Bauer of NASA's Jet Propulsion Laboratory, and the paper's lead author. "Our data point to a cometary origin for most of the objects, suggesting they are coming from deeper out in the solar system."
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 component of the WISE mission, gathered infrared images of 52 of these objects, 15 of which are new discoveries. The objects orbit in an unstable belt where gravity from the giant plants will ultimately fling them out.
"Comets have a dark soot-like coating on their icy surfaces, making them darker than most asteroids," Grav said. "Comet surfaces tend to be more like charcoal, while asteroids are usually shinier like a sidewalk. The results add to earlier results from the NEOWISE projects showing that all observed populations of small bodies outside the Main Asteroid Belt are dominated by objects with dark surfaces."
"That means the small body populations found beyond the Main Asteroid Belt, like the Hildas, Jupiter Trojans and Centaurs, were either formed where they currently are, or they were inbound objects coming from the far reaches of the Solar System that settled into their current orbits," he said. "This all helps us understand how the Giant Planets were formed and evolved in the earliest times of the Solar System."
Grav was funded under a subcontract to the Planetary Science Institute from the Jet Propulsion Laboratory/California Institute of Technology, which manages NEOWISE.
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