Once thought to have been born in isolation on the cold outskirts of our solar system, new evidence indicates that comets may actually have had company when they formed. Recent analysis of comet dust, the first ever collected and brought back to Earth, revealed a mix of particles from the region beyond Neptune as well as the area closer to the sun. The finding is significant because it indicates that more mixing of ingredients occurred when the solar system was forming than scientists originally believed.

Comets formed nearly 4.6 billion years ago, at the same time as the sun and planets. Some comets, such as Wild 2 and Tempel 1, were thought to have developed exclusively in the outer solar system from ice and dust particles similar to those found in interstellar space. After forming, they were pulled into orbit in the planetary neighborhood where some comets are visible to us here on Earth. But when researchers recently analyzed the comet dust, they discovered that it contained minerals that likely originated in the inner regions of the solar system as well as particles typically found in materials formed farther away.

One of the grains analyzed also contained minerals formed under superhigh temperatures found close to the sun. The discovery supports earlier suggestions that materials that developed there were mixed together with others in the far reaches of the solar system where these comets formed.

"There really is a lot of mixing of material at the microscopic level" over a large distance, says Michael A'Hearn, an astronomer at the University of Maryland, who was not involved with the new research but studies comet material. He adds that this was "always a possibility but this is the first clear evidence."

The evidence came from comet dust collected by NASA's Stardust mission. As icy comets travel toward the sun, their dusty ice is heated and some evaporates, leaving a cloud of gas and dust. When the Stardust spacecraft flew by the 4.8-kilometer-wide comet Wild 2 in 2004, it captured about 10,000 of these tiny particles in a specialized porous glass material. The samples were returned to Earth in a capsule early this year and scientists around the world began to analyze the comet material. Their findings, published in the December 15 issue Science, forced scientists to re-consider what they could learn about the outer solar system from comets, A'Hearn says, because they now must account for the mingling materials "Basically," he says, "it complicates the picture."

While researchers continue to analyze the hundreds of remaining particles collected by Stardust, another spacecraft is journeying toward the comet 67P/Churyumov-Gerasimenko. The European Space Agency's Rosetta spacecraft will be the first to land a probe on that comet in 2014 in hopes of collecting more information about the origin and evolution of our solar system. Looking beyond that mission, A'Hearn says it will also be important to bring back a large sample of cometary material that is collected cold. He adds that analyzing a chunk of a comet as well as the microscopic particles collected by Stardust could reveal how intimately materials mixed to form our solar system.