Seven grains of stardust that are believed to come from outside our solar system are revealing new hints about what the universe is made of, scientists said Thursday.
Some are fluffy like snowflakes, not dense like experts expected, according to the study in the US journal Science that describes the first close look at what astronomers have only been able to view from afar -- until now.
The grains were painstakingly isolated from a collector on NASA's Stardust spacecraft, a probe that launched in 1999 to take a sample of a comet's dust and bring back to Earth a tiny taste of the interstellar world.
The Stardust probe "traveled halfway to Jupiter to collect the particle samples from the comet Wild 2. The spacecraft returned to Earth's vicinity to drop off a sample return capsule eagerly awaited by comet scientists," NASA said on its website.
The probe also sampled the stream of fine dusty material that enters the solar system from interstellar space.
Though the stardust sample returned to Earth in 2006 while the unmanned spacecraft continued on its journey, it has taken a team of international scientists eight years to narrow down the search for particles that came from beyond our solar system on the collector's aerogel and aluminum foil surfaces.
Most of the specks of material it returned were actually not that exotic, researchers said.
"After analysis by no less than six synchrotron particle accelerators and numerous X-ray microanalyzers, it became obvious many of the captured particles were tiny fragments of the spacecraft," said Anton Kearsley, microanalyst at Britain's Natural History Museum.
However, some of the "dust grains were not what we'd expected, and many seemed to have come from strange directions," he said.
"Only by careful plotting of impact directions were the team able to identify the seven particles that must have come from outside the solar system."
Scientists also did not want to destroy the stardust, so they took special care to analyze the grains -- less than a thousandth of a millimeter across.
- Global science effort -
The research team involved 66 scientists in seven different countries. Some 30,000 citizen scientists also helped through a web campaign called Stardust@home, working through thousands of digital microscope images.
"The analysis of these particles captured by Stardust is our first glimpse into the complexity of interstellar dust, and the surprise is that each of the particles are quite different from each other," said lead author Andrew Westphal, physicist at the University of California, Berkeley's Space Sciences Laboratory.
Some had a fluffy makeup, similar to a snowflake, he said.
The particles also contained "crystalline material called olivine, a mineral made of magnesium, iron, and silicon, which suggest the particles came from disks or outflows from other stars and were modified in the interstellar medium," said the study.
Three of the particles detected were complex and contained sulfur compounds, which surprised some astronomers.
The findings are still preliminary and it remains to be confirmed whether the particles really are from beyond our solar system, researchers said.
A series of 12 papers detailing their methods are being published in the journal of Meteoritics & Planetary Science.
The Stardust spacecraft is no longer in operation. It sent its last signal to Earth in 2011, after NASA said it ran out of fue