NASA’s Wide-field Infrared Survey Explorer (WISE) has revealed the true identity of the mysterious space centaurs. Centaurs are the diminutive celestial objects circling the sun between Jupiter and Neptune. Until now, their true identity was one of the long-lasting mysteries of astrophysics, according to the space agency. Now, a new report suggests that most centaurs are comets.

Previously, scientists speculated that centaurs might be asteroids thrown from the inner solar system or comets moving in toward the Sun from great distances. These celestial objects are named after 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. “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” suggests that a celestial object is probably constructed from the same material as a comet, likely was an active comet in a past life, and may become active again.

The results come from the biggest infrared survey to data of centaurs and their far off relatives, known as scattered disk objects. NASA reports that NEOWISE collected infrared images of more than 50 centaurs and scattered disk objects. Furthermore, NASA notes that 15 of the 52 are new findings.

According to the space agency, centaurs and scattered disk objects circle in an unstable belt. Eventually, gravity from the large planets will fling them either closer to the Sun or farther away from their current places of residence.

Previously, NASA’s Spitzer Space Telescope discovered some evidence of comets in the group and scientists spotted some centaurs with dusty halos (a typical characteristic of outgassing comets). However, astronomers had not been able to determine the numbers of comets and asteroids.

Infrared data from NEOWISE offered critical information on centaurs’ reflectivity to assist astronomers with sorting the population of celestial bodies. NEOWISE has the ability to determine whether a centaur has a dark surface or a shiny one that reflects a lot of light. By combining the reflectivity data with what was already recorded about the colors of the objects, scientists were able to solve the mystery. Visible-light data has revealed centaurs generally to be either blue-gray or reddish in hue. NEOWISE revealed that most of the blue-gray objects are dark, a clear indication that the objects are comets.

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

The findings suggest that approximately 66 percent of the centaur population are comets, which originate from the cold outer reaches of our solar system. Astronomers hope that NEOWISE will continue to unwrap the mysterious nature of centaurs.

The study’s findings are described in greater detail in the Astrophysical Journal.