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POSSIBLE ÉMIGRÉA new proposal suggests the largest of all asteroids, Ceres (pictured here as seen by the Hubble Space Telescope), is actually an émigré from the Kuiper belt, a reservoir of frozen bodies that includes Pluto.J. Parker/Southwest Research Institute, NASA, ESA

If planetary scientist Bill McKinnon’s hunch is right, the largest asteroid in the solar system isn’t an asteroid at all. Ceres, as the 470-kilometer-wide object is called, may be a relative of Pluto that formed at the solar system’s fringes but came in from the cold several billion years ago.

McKinnon, based at Washington University in St. Louis, said he was first struck by Ceres’ unusually low density — more similar to icy comets from the outer solar system than the rocky bodies found in the asteroid belt that lies between the orbits of Mars and Jupiter. The density of Ceres, referred to as a dwarf planet, is only slightly higher than that of Pluto. Models suggest Ceres “looks remarkably Pluto-like,” McKinnon says.

But it was a recently developed model of the early solar system that prompted McKinnon to formally propose that Ceres might be an escapee from the Kuiper belt, an outer solar system reservoir of frozen bodies that includes Pluto. He presented his proposal July 15 in Baltimore at the Asteroids, Comets, Meteors conference.

According to the model, developed by researchers including Hal Levison and Bill Bottke of the Southwest Research Institute in Boulder, Colo., and Alessandro Morbidelli of Observatory of the Côte d’Azur in Nice, France, the orbits of the outer four planets — Jupiter, Saturn, Uranus and Neptune — were initially packed much closer together than they are today.

Beyond these planets resided a band of dust, ice and gas particles. Over time, as some of these particles leaked inward, their gravitational tug lengthened the distance between the orbs. For instance, Jupiter migrated inward, while Saturn moved outward.

At some point, according to the theory, Saturn reached a gravitational sweet spot: The time it took to go around the sun became exactly twice that of Jupiter’s. That interplay strengthened the planets’ mutual tug, and ultimately hurled Uranus and Neptune into the outlying band of dust, ice and gas. The entry of Uranus and Neptune scattered debris from the chilly band, sending some of its denizens into the inner solar system.

That’s how Ceres might have migrated from the outer solar system into the asteroid belt, McKinnon suggests.

“We are saying that many objects from the outer solar system — what we call the primordial disk of comets that went on to produce the Kuiper belt — are captured in the outer part of the asteroid belt as a byproduct of the model,” Bottke says. He and Levison presented updated versions of the theory at the meeting just before McKinnon’s presentation.

“I consider McKinnon’s idea as something of a thought balloon to stimulate thinking,” Bottke says. “It is indeed possible that he is correct, but I would not bet for it at this point.”

Additional information on Ceres’ composition, to be gathered by NASA’s Dawn spacecraft when it visits Ceres in 2015, could clarify the body’s origin. But proof may require measuring the ratio of hydrogen to its heavier isotope, deuterium, in the ices or water vapor venting from the body, which would require a mission beyond Dawn, McKinnon says. If the ratio matches that observed in comets, “the case is closed” for Ceres being an émigré to the asteroid belt, he says.

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