In 2004, as the Stardust space probe cruised past a comet called Wild-2, it thrust out a pair of tennis racket-sized gel trays, intended to catch particles of dust from the comet and surrounding space.
Today, in an analysis published in Science, researchers from UC Berkeley and elsewhere concluded that at least seven of the millions of particles collected are of "probable interstellar origin." If the conclusion holds up upon further testing, these will be the first objects we've ever collected that came from outside our solar system.
The particles are so tiny — a few microns wide, about a fiftieth of the width of a human hair — that just finding them in the gel has taken years. The gel trays were stored in a tiny capsule that the craft dropped as it swung back past Earth, and landed in the Utah desert in 2006.
In some cases, the tracks the particles made in the gel were spotted by volunteers, who pored over millions of microscope images as part of the Stardust@Home crowd-sourcing project. Other particles were found in the aluminum foil lining beneath the gel. And many of the gel panels still haven't even been scanned — so the scientists involved believe there are probably dozens more particles of interstellar dust to be found.
The vast majority of the particles found in the gel were determined to have peeled off the spacecraft itself. But analysis of seven of the particles' composition, size, and structure has provided evidence that they're from interstellar space.
The presence of one particular crystalline material in two of the particles suggests they might've been formed during a supernova explosion millions of years ago, and have since been warped by long-term exposure in deep space. Regardless of their particular origin, if the particles are confirmed to have come from interstellar space by future testing, they may help scientists better understand the lifecycle of stars and other solar systems.