For the first time, scientists have identified and analysed single grains of silicate dust formed in ancient stars from before our Solar System was created.
By Dr David Whitehouse
BBC News Online science editor
The researchers say the discovery provides a new way to study the history of the Universe.
The new ion probe found telltale signatures in six grains
The silicate grains were bound up within interplanetary dust particles collected from high up in the Earth's atmosphere using a modified spy plane.
"The stardust grains we discovered are typical of the kinds of dust that were available at the beginning of our Solar System; these were the building blocks of the Sun and the planets," says Lindsay Keller, a US space agency (Nasa) co-researcher on the study.
Most stardust is made of tiny silicate grains, much like dust from rocks on Earth.
It comes from dying and exploded stars. Because new stars form from the wreckage of previous ones, some of this ancient dust would have become trapped inside asteroids and comets when our own Sun was born.
Although silicate grains are abundant in the cosmos, to date no grains formed in the atmospheres of other stars have been found in meteorites, possibly because they were easily destroyed when the Solar System was young.
In the new research, published in the journal Science, astronomers describe the discovery of six stardust grains.
The identification is based on measurements of the different types, or isotopes, of oxygen atoms in the grains using a new ion probe instrument. The results are unlike anything one would expect to find in material from our Solar System.
The researchers found the stardust trapped in tiny dust particles collected 20 kilometres above the Earth by Nasa's ER-2 aircraft, a modified U2 spy plane.
Until recently, ion probes had to analyse dozens of grains at one time and so were able to deduce only the average properties of a sample.
However, the new NanoSims probe can resolve individual grains, "so like the Hubble telescope, the NanoSims allows us to see things on a much finer scale than ever before", says Scott Messenger, of the Laboratory for Space Sciences, at Washington University in St Louis.
Using the NanoSims probe, Messenger and colleagues measured the relative amounts of three isotopes of oxygen in more than a thousand grains.
The data told them which grains had come from ancient stars. "Finding something that people have been seeking for such a long time was incredibly exciting," Messenger says.
Because the silicate grains are relatively delicate, the authors propose that they may have been captured by and preserved in comets instead of meteoroids. Material in these bodies is less likely to have undergone change in the early Solar System.
"We found that one percent of the mass of these interplanetary dust particles was stardust," Messenger says. "So stardust is about 50 times as abundant in these particles as in meteorites, which suggests that it comes from far more primitive bodies."
Of the six grains, three appeared to have come from red giant stars, a late stage in stellar evolution. A fourth was from a star containing little metal. The fifth and sixth possibly came from a metal-rich star or a supernova (exploding star).