For years, the small objects orbiting the Sun between Jupiter and Neptune have remained a mystery to astronomers. Dubbed “centaurs,” researchers were unsure what exactly the objects were. Now, new observations using NASA’s Wide-field Infrared Survey Explorer (WISE) have determined that many, if not most, of the objects are comets.

The new observations were taken from an infrared survey made by WISE. According to NASA, the orbiting observatory was used to observe 52 centaurs and other “scattered disk objects,” 15 of which were newly discovered. The survey found the reflectivity of the centaurs, which astronomers compared to the color of the objects, which are generally blue-gray or red. Most of the objects that are blue-gray were found to be dark, which suggests that they are comets. The observations are detailed in a new paper, published this week in the Astrophysical Journal.

“Comets have a dark, soot-like coating on their icy surfaces, making them darker than most asteroids,” said Tommy Grav, a co-author of the study and an astronomer at the Planetary Science Institute. “Comet surfaces tend to be more like charcoal, while asteroids are usually shinier like the moon.”

The study found that around two-thirds of centaurs are likely comets, and the identity of the remainder cannot be confirmed. According to NASA, this means the centaurs came from far outside the solar system and became caught. The centaurs orbit in an “unstable” belt, and will eventually be thrown either closer to or away from the sun by Jupiter or Neptune.

“Just like the mythical creatures, the centaur objects seem to have a double life,” said James Bauer, lead author of the study and an astronomer at 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.”

(Image courtesy NASA/JPL-Caltech)