What is a space 'Centaur'? Scientists now know the answer

Is it an asteroid? Or a comet?

NASA's Wide-field Infrared Survey Explorer (WISE) has found the answer to one of astronomy’s vexing questions: what are those strange objects wobbling around in our outer solar system?

The odd objects, called centaurs, are small, celestial bodies in an unstable orbit around the sun, passing between Jupiter and Neptune. The objects have been something of a mystery to scientists since the first one was spotted in 1920. If the hulk of material had come from far out in space, plunging inward from outside outer reaches of the solar system to get trapped between our fifth and eight planets, it would be a comet. But if it had originated here, flung outward toward the colder planets, it would be an asteroid. 

So scientists settled somewhere in the middle. Those bodies are called centaurs, after the mythical animal of ambiguous descent: the head and torso of a human and the legs of a horse. 

In the last few years, researchers had begun to report that centaurs might just be comets, no ambiguity about it. Astronomers had begun to see centaurs with halos, a feature of comets fizzing off gas and dust. And 2013 data from the Spitzer Space telescope was also suggesting that the bodies had infrared qualities similar to those seen in comets.

Still, the evidence had been sparse, and now astronomers have turned to NEOWISE, the asteroid-hunting portion of NASA’s WISE mission, an unmanned satellite in orbit around Earth and panning the solar system with an infrared-sensitive telescope. 

First, NEOWISE observed 52 centaurs and scattered disk objects, a cousin to the centaur, in the thermal infrared – in total, astronomers were looking at about a quarter of the known centaurs. Fifteen of the observed objects were new discoveries. Next, NEOWISE relayed to astronomers to the centaur’s albedo, or reflectivity – did the centaur have a matte and dark surface or a glistening, light-reflecting one? Comets, a bluish color in visible light, are matte. Asteroids, a red color, are light-reflecting.

Centaurs, NEOWISE found, are matte – providing substantial evidence that those once-ambiguous bodies are comets.The research is published in The Astrophysical Journal.

Even so, the mystery is not quite solved: NEOWISE’s data pinpoints just two-thirds of the centaurs as comets.

"The majority of the Centaurs we observed (about 2/3rds) look remarkably like comets in that they have similar colors and brightnesses.said James Bauer, a researcher at NASA's Jet Propulsion Laboratory.

"The remaining 1/3rd had a different (redder) color, and on average had brighter, or more reflective, surfaces. These redder Centaurs could be linked to other populations that are not cometary, such as the asteroids," he said. "The evidence is suggestive, but further studies need to be done."

What we could be looking at, then, is not centaurs, but a collection of people and horses, so to speak.