If their identity is confirmed, it will be the first time scientists have ever captured genuine stardust.
"We're looking at material very similar to what made our solar system," wrote Dr Andrew Westphal of the University of California, in the journal Science.
Nasa's Stardust spacecraft was launched in 1999 to fly through gas and dust surrounding comet Wild-2.
Using a "fly paper" technique, it was designed to catch cometary dust, which was recovered in on Stardust's return in 2006.
Volunteers with home computers helped professionals scan more than a million images. The project proved critical to the search, and scientists have now identified seven particles likely to have interstellar origin.