Centaurs: Cross-Dressing Comets That Go As Asteroids

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Who’d have thought it? Cross-dressing is no longer the preserve of humans, cameleons, magnetars and wolves in sheep’s clothing. Comets are now in on the act.

New research announced today describes the finding that many of the mysterious objects that populate orbits between Jupiter and Neptune are in fact comets. Cross-dressing comets! Live and let live is what I say.

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In all honesty, astronomers have always been a little dubious about these objects’ origins, hence why these objects were named “centaurs.” Centaurs in Greek mythology are part human, part horse. Celestial centaurs therefore always had some odd characteristics that fell into the comet and asteroid category.

However, at least part of the centaur debate has been settled by NASA’s Wide-field Infrared Survey Explorer (WISE).

During the latter stages of the WISE mission, the orbiting space telescope entered the “warm” phase of its extended mission when the onboard coolant ran dry in October 2010. In an attempt to catalog previously unnoticed near-Earth objects (NEOs), WISE was used to spy out the weak infrared signal these objects emit. As part of the four-month NEOWISE mission, centaurs (and their scattered-disk object cousins) were surveyed and at least two thirds of the objects were found to be comets, not asteroids.

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“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., in a JPL news release. “Our data point to a cometary origin for most of the objects, suggesting they are coming from deeper out in the solar system.” Bauer is lead author of a paper published online July 22 in the Astrophysical Journal.

52 centaurs and scattered disk objects were surveyed, 15 of which were newly discovered by NEOWISE.

Previous observations of centaurs have uncovered comet-like behavior. For example, other observatories have imaged what appeared to be halos — cometary outgassing — around individual objects in the past. But NEOWISE was able to survey dozens of centaurs and in doing so spotted a pattern.

NEOWISE was able to gather albedo data — a measure of how shiny an object is — on the surveyed centaurs. Combined with previous albedo and color data, a picture emerged.

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“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.”

NEOWISE found the majority of centaurs to be dark, a tell-tail sign that they are more likely comets and not asteroids.

This finding adds to the rich tapestry of our understanding of the history (and future) of our solar system. Centaurs and scattered disk objects exist in very precarious orbits, having migrated from the furthest-most reaches of the solar system, sandwiched between the orbits of the massive outer planets. The planets’ domineering gravity will eventually destabilize the centaurs, propelling them toward the sun or ejecting them from the solar system entirely.

Image: Artist’s concept of a mystical centaur creature amongst celestial centaurs. Credit: NASA/JPL-Caltech