It could be the most exotic material on the planet. Seven particles of dust brought back to Earth by a spacecraft nearly a decade ago appear to have come from beyond our solar system.

The specks have all the hallmarks of being created in interstellar space. If confirmed, it would make them the first material from outside the solar system to be brought to Earth for study.

Scientists found the tiny particles – including some shaped like fluffy snowflakes – on detectors carried by Nasa's Stardust probe which launched in 1999 on a mission to capture dust from interstellar space and the tail of comet Wild-2.

The detectors were dropped to Earth by parachute when Stardust flew past in 2006. Each detector worked like cosmic fly-paper and collected particles as they hurtled past the spacecraft.

An optical microscope image of a track through aerogel made by Orion, one of the dust particles believed to be from interstellar space. Photograph: D Frank/Nasa/JSC

The dust might have been created in a supernova explosion millions of years ago and shaped by their exposure to the harsh extremes of space. "These are very precious particles," said Andrew Westphal, a physicist at the University of California in Berkeley, who worked on the dust.

The two largest fluffy particles contain a crystalline magnesium-iron-silicate mineral called olivine, which suggested that they came from the discs around stars and were altered by the interstellar environment, Westphal said.

If the nature of the dust is confirmed, then studies of the material could shed light on the origins of interstellar dust. Almost everything known about interstellar dust has come from observations, either with ground-based or space-based telescopes. "We seem to be getting our first glimpse of the surprising diversity of interstellar dust particles, which is impossible to explore through astronomical observations alone," Westphal added.

The international team of scientists sought help from more than 30,000 citizen scientists to scan thousands of microscope images in search of the particles. The largest of the particles was only a few thousandths of a millimetre across, considerably smaller than this full stop. Most of the specks weighed a few millionths of a millionth of a gram.

Two particles, named Orion and Hylabrook by their discoverers, were found from the tracks they left in detectors made from aerogel, an ultra-light and porous material. Scientists found a third track from a particle moving in the same direction, but it was evidently moving so fast, at more than 15km per second, that it vapourised on impact.

Four more particles, with the right chemical make-up for interstellar dust, were found at the bottom of pits left in thin aluminium foils built into the detectors. "They were splattered a bit, but the majority of the particles were still there at the bottom of the crater," said Rhonda Stroud at the Naval Research Laboratory in Washington DC. More tests are planned on the particles to confirm or rule out their interstellar origins.

Besides the exotic dust particles, researchers identified more than 50 other particles of spacecraft debris in the Stardust detectors, according to a report in Science.

Anton Kearsley, a microanalyst who took part in the study at the Natural History Museum in London, said recognising interstellar dust was a huge challenge.

"In the end, 30,000 people around the world worked through thousands of digital microscope images of the main part of the collector, the aerogel, and eventually found the tracks that included interstellar dust particles," he said.

"As the results came in, the numbers and sizes of dust grains were not what we'd expected, and many seemed to have come from strange directions," he added. "Only by careful plotting of impact directions was the team able to identify the seven particles that must have come from outside the solar system."