The comets that come streaking through Earth's neighborhood are visitors from frigid, distant regions in the outer solar system. The icy, dusty bodies formed there billions of years ago, far from the heat and radiation of the sun, so it was long thought that they comprised unsullied scraps left over from the solar system's formation. But a new analysis of a particle from Comet Wild 2 indicates that the mote formed close to the sun and then migrated outward to be captured by the comet millions of years after the solar system began taking shape. Along with similar investigations of other samples, the new finding reinforces the theory that comets originating in the Kuiper belt, the distant field of icy debris where Pluto orbits and beyond, contain fragments that formed somewhat later than the solar system's primordial grains, and much closer to the sun.

Jennifer Matzel, a cosmochemist at Lawrence Livermore National Laboratory in California, and her colleagues backed up this timeline with a bit of astrochemical forensics. Matzel and her collaborators analyzed the small particle, called Coki, captured and returned to Earth by NASA's Stardust mission. Stardust visited Wild 2 (pronounced "Vilt 2") in 2004 and is now on its way to a 2011 encounter with Comet Tempel 1. But in 2006 the spacecraft swung past Earth to deliver the samples it had collected from Wild 2, ejecting a 46-kilogram capsule that parachuted down to the Utah desert.

Some of the samples, including Coki, bear the mark of a high-temperature formation process, resembling the calcium- and aluminum-rich inclusions (CAIs) seen in some meteorites. Those CAIs are thought to have formed exposed to the heat of the sun in the very early solar system more than 4.5 billion years ago, and heat-forged particles like Coki indicate that comets contain not just the raw materials from which the solar system formed but also some materials that were at least partially processed in the inner solar system before migrating out to the Kuiper belt.

In a study published online by Science on February 25, Matzel and her co-authors present the results of dating the particle's formation by searching for the decay products of a radioactive isotope of aluminum. (Isotopes are species of an element with different numbers of neutrons and hence different atomic masses.) Aluminum 26, which has one less neutron than stable, ordinary aluminum 27, decays to magnesium 26 with a half-life of roughly 700,000 years—a relatively short span in astronomical time. But although the Coki particle contains aluminum 27, it contains no detectable magnesium 26 from the radioactive decay of aluminum 26. So Matzel and her colleagues conclude that the particle formed after all the aluminum 26 had decayed out of the solar nebula, the primordial cloud surrounding the newborn sun.

That dates Coki to roughly 1.7 million years or more after the earliest CAIs formed, which gives it a fairly early placement on the time line of our 4.6-billion-year-old solar system. But the fact that Coki formed millions of years into the solar system's evolution and later migrated out to reach the Kuiper belt is somewhat surprising, indicating that some outward transfer process may have been at work for millions of years. "The new and interesting thing about this paper is it's the first time we've been able to get some estimate of the timing," Matzel says. "Even though it's a very old object and it formed very close to the sun, it had some longer history in the inner solar system before it got flung out to the comet-forming region."

Joseph Nuth, an astrochemist at the NASA Goddard Space Flight Center in Greenbelt, Md., who did not contribute to the new research, says that before this analysis there had not been firm evidence to support such a late outward migration. At that stage in the solar system's history there would be large bodies between the sun and the Kuiper belt whose gravity would make them difficult to skirt. "One of the things that happens one million or two million years into the nebula is Jupiter forms," Nuth says. "And Jupiter would be a big hurdle to jump."

Nuth calls the isotope analysis "great work" but notes that the interpretation—that materials moved outward across the solar system for a surprisingly long time—relies on the crucial assumption that aluminum 26 was evenly distributed throughout the nebula. If there were variations in the concentration of aluminum 26 across space or time, then the isotope's use as an atomic clock would be muddied. Some models, in fact, propose that aluminum 26 may have been inserted into the solar nebula by a nearby supernova or some other process, meaning that Coki could simply have formed before the isotope was available.

"This is not a definitive time frame unless you choose one of those particular models, and so the one they chose basically means that transport is done for several million years in the nebula," Nuth says. "But if they chose a model where the aluminum 26 and other radioactive elements were injected sometime later, it's possible that the CAI fragment that they analyzed could have been one of the very first formed CAI fragments."

Matzel says that spatial variations in isotope concentration are more of a concern than temporal ones. "Just from looking at the meteorites, I think we have some pretty good evidence that there wasn't a huge later injection," she says. "I think the worry is maybe that if aluminum 26 wasn't homogeneously distributed, there may be some parts of the nebula that just never had any and never will." Although Matzel says that scenario cannot be ruled out, "every time we analyze new things we seem to get a very consistent story."

Whatever the implications, Nuth says that the Coki analysis provides a significant data point for unraveling the history of the solar system. "Getting things that are formed at very, very high temperatures transported out to that distance is a really big deal in terms of the chemistry of the solar system," he says. Nuth adds that the duration and efficiency of that transport, as well as the volume of material it moved, have implications for what kinds of compounds seeded the solar system and perhaps even how precursors to life were distributed to the planets.