Scientists don't know how it got there, but after spending about 2 million years in the relative warmth of the inner solar system, a particle of matter was heaved out into the frigid nether-regions where it eventually became part of a comet.
The speck was among the samples returned from the comet Wild-2 (pronounced "Vilt-2") by NASA's Stardust science mission. It is the first of what scientists hope will be many particle lineages unraveled as analysis of the microscopic grains continues. Compiled, these stories may begin to shed light on how the solar system took shape.
Based on detailed chemical analysis, Jennifer Matzel, a researcher at Lawrence Livermore National Laboratory, and colleagues figured out that the particle, which measures about 5 microns — about the width of a single human red blood cell — spent a fair amount of time in the inner solar system before it was transported out beyond Neptune and became incorporated into the comet.
Scientists have long believed comets are outer system bodies that contain the pristine remains of material from which the solar system formed. But ongoing analysis of samples from Wild-2 have thrown some kinks in that theory. Evidence is now showing that some comets contain fragments that formed closer to the sun and after the first material in the solar system was created.
"It is thought that the solar system formed from a cloud of gas and dust (the solar nebula) that began collapsing, possibly due to a shock wave from a supernova," Steven Smith, senior scientist at the University of Chicago's Department of Geophysical Sciences, wrote in an email to Discovery News.
This shock wave could have delivered short-lived radionuclides, vestiges of which were discovered in the comet Wild-2 sample, to the outer solar system.
"As the sun grew, its gravity drew in more material and the nebula collapsed down to a disc, but there would have been a return flow of material, I think mainly out of the poles," Smith added.
The small particle, called Coki, was captured and returned to Earth by NASA's Stardust mission. The spacecraft flew by the comet Wild-2 in 2004 and then swung past Earth to deliver the samples it had collected, ejecting a capsule that parachuted down to the Utah desert.
The particle has daughter particles of a long-extinct aluminum isotope, Matzel told Discovery News. "That's how you know it dates back to the early solar system."
Tracing back the processes the particle underwent before arriving at its present state, scientists determined it had spent some time heated in the inner solar system, though exactly how far it was from the sun hasn't been determined.
"I'm just really amazed that we have these samples here to work with," said Matzel. "I think it's neat to be able to put some boundaries on what we know about the inner solar system."
Matzel's research was published in Science.