'Stardust motes' carried to Earth
Seven visitors from outside the Solar System have been carried to Earth by a Nasa spacecraft, scientists believe.
If their identity is confirmed, it will be the first time scientists have ever caught and studied genuine motes of stardust.
"They are very precious particles," said lead researcher Dr Andrew Westphal, from the University of California at Berkeley.
"Fundamentally, the solar system and everything in it was ultimately derived from a cloud of interstellar gas and dust.
"We're looking at material that's very similar to what made our Solar System."
The American space agency Nasa's Stardust spacecraft was launched in 1999 to fly through the coma - the envelope of gas and dust - surrounding comet Wild-2.
Using a "fly paper" technique, it was designed to catch cometary dust in tiles of soft aerogel separated by pieces of aluminium foil.
A similar collector was placed on the rear of the probe to collect particles from streams of interstellar dust flowing through the galaxy.
Both collectors were dropped by parachute and recovered after Stardust returned to Earth in 2006.
Volunteers with home computers known as "dusters" were invited to help the professionals scan more than a million dust-track images.
The citizen science project, Stardust@home, proved critical to the needle-in-a-haystack search.
Scientists identified just seven particles that were likely to have had an interstellar origin.
Three either lodged or left traces within the aerogel while the remains of four others were found in pits in the aluminium foil.
These particles were a tenth of the size of comet dust, measuring up to two thousandths of a millimetre across, and varied in composition.
Some of the larger ones had a snowflake-like "fluffy" structure, said the researchers, writing in the journal Science.
"The fact that the two largest fluffy particles have crystalline material - a magnesium-iron-silicate mineral called olivine - may imply that these are particles that came from the disks around other stars and were modified in the interstellar medium," said Dr Westphal.
"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."
Three particles also contained sulphur compounds which some astronomers argue should not be found in interstellar dust.
Describing the dust residues discovered in the foil, Dr Rhonda Stroud from the US Naval Research Laboratory said: "They were splatted a bit but the majority of the particles were still there at the bottom of the crater.
"Their diversity was a surprise, but also these fluffy particles, sort of like a tossed salad, were complex, an agglomeration of other particles, rather than one dense particle suggested by the simplest models of interstellar particles."
Interstellar dust thrown out by exploding stars is believed to scatter the heavy elements necessary for life, such as carbon, nitrogen and oxygen, across the galaxy.
In this sense, everyone on Earth is partly made from stardust.