An imagined view of the Oort cloud, a massive field of icy bodies at the outer edge of our solar system. Lines illustrate the theory that some asteroids were flung out to this distant region in the early days of the solar system's formation.

An imagined view of the Oort cloud, a massive field of icy bodies at the outer edge of our solar system. Lines illustrate the theory that some asteroids were flung out to this distant region in the early days of the solar system's formation.

Claus Lunau/FOCI/Bonnier Publications/Science Source

Mysterious Oort cloud objects shed light on early solar system

A cometlike object that originated in the farthest reaches of our solar system is not what it should be. Unlike other objects from this vast field of icy bodies at our solar system’s outer edge, known as the Oort cloud, the body resembles those that formed much closer to the sun. The finding supports the idea that large planets like Jupiter moved around a lot during the chaotic early days of the solar system, flinging asteroids outward as they went. That hypothesis also suggests icy bodies were thrown inward, which could explain how water got to our planet. 

“It’s pretty exciting,” says Michael Kelley, an astronomer at the University of Maryland, College Park, who was not involved with the research. “If this one asteroid in particular really is made of inner solar system material that got out to the Oort cloud and has worked its way back in,” he says, it could refine theories about how the solar system formed.

Comets from the Oort cloud have been spotted before (including Hale-Bopp’s spectacular 1997 flyby), but they’re usually cloaked in gas and dust that make it hard to see what’s underneath. Astronomers spotted one of two new Oort cloud objects in August 2013 using Pan-STARRS1, a telescope on Maui designed to survey the sky for potentially dangerous asteroids. It had the orbit of a long-distance Oort comet, but it looked like an inert piece of rock, similar to an asteroid, not a volatile, icy comet.

Even though astronomers had never seen a rocky, asteroidlike object come from the Oort cloud before, the finding wasn’t a complete surprise. Astronomer Jan Oort (who theorized the existence of the Oort cloud and later had it named after him) had predicted in 1950 that distant comets might burn off their covering of ice and dust on their first trip around the sun. “I thought, ‘Wow, maybe we are seeing these things that he predicted and nobody ever saw before,’ ” says team leader Karen Meech of the University of Hawaii Institute for Astronomy, Manoa.

When the researchers used the 8-meter Gemini North telescope on Mauna Kea for follow-up observations, they saw extremely faint traces of a classic comet’s tail, bolstering the theory that the object may have been a burned-out comet. They also obtained a spectrum of light coming from its surface that appeared very red, similar to objects from the Kuiper belt—a less distant cloud of objects that starts just outside the orbit of Neptune. This provides further evidence that the object was made of materials drawn from the outer reaches of the solar system.

But while the scientists were analyzing the results, they spotted a second rocky object on a long-period orbit from the Oort cloud. This time, follow-up observations with the Canada-France-Hawaii Telescope on Mauna Kea found an unexpected result: a surface color more similar to an asteroid from the inner solar system. Even more puzzling, this object also had faint traces of a cometary tail, the team reported this week at the annual meeting of the Division for Planetary Sciences of the American Astronomical Society in Tucson, Arizona.

Astronomers have never seen an object that looks like an asteroid but casts off dust like a comet and has a comet’s orbit, says astronomer Yanga Fernández of the University of Central Florida, Orlando, who was not involved in the research. “It’s a disconnect between two things we thought were pretty solid,” he says. “It’s a very unusual result, and it will be extremely interesting if people get data that corroborate it.”

In addition to upsetting long-held assumptions about how asteroids and comets behave, the finding could challenge what we thought we knew about the composition of the Oort cloud, which astronomers previously believed was made up almost exclusively of icy bodies. If the object turns out to be an inner solar system asteroid, it could indicate that there’s much more of that material in the Oort cloud. One possible explanation is that large planets like Jupiter migrated significantly while the solar system was forming, an idea that has gained currency through observations of extrasolar planetary systems, which appear to have giant planets in a wide range of orbits. Such movements would likely throw asteroids outward into the far solar system.

“The origin of the Oort cloud is going to tell you something about how things were moving around in that era right after the planets started to form,” Fernández says. “That’s a very long-term goal.” The observation is so unusual, Kelley adds, it’s likely some astronomers won’t believe it. “It needs confirmation, for sure.”

Meech is now hoping to get telescope time in the next month—before the object becomes too faint to observe—to get a spectrum of the second object that would more definitively show whether its surface properties match those of inner solar system asteroids. But even if they miss their window, astronomers may get another chance to solve the mystery. Meech says a number of similar objects have been spotted in recent decades, but scientists never paid attention to them.

“Now that we know what to look for,” she says, “it may be that we start seeing a lot more of them.”

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