An innovative plan to retrieve comet particles from earth's stratosphere has hit pay dirt, with the discovery that some predate the formation of the solar system.

"It was the largest number ever found," says Dr Henner Busemann of the University of Manchester's School of Earth, Atmospheric and Environmental Sciences.

The samples were collected by a high-altitude NASA research jet flying in April 2003 as the Earth travelled through the dusty wake of Comet 26P/Grigg-Skjellerup.

"This was the equivalent of sampling a meteor shower. Nobody had previously collected samples of a comet in that way," says Professor Donald Brownlee of the University of Washington, who heads a science team analysing particles returned by the Stardust spacecraft, which flew by Comet Wild-2 in January 2004.

Scientists believe our solar system was formed out of the exploded remains of an older star. Isotopic analysis of interplanetary dust particles, culled from meteorites and other sources, show some grains are older than the 4.5-billion-year-old solar system.

Scientists don't know how long interstellar dust grains can survive in space. They are made in stars and destroyed by shock waves, says Brownlee.

Richest haul

Thousands of grains have been analysed, but so far the richest haul of pre-solar particles appears to be from the sample collected on plastic plates covered in sticky silicon oil flown outside the NASA U2 aircraft.

The Stardust team has been looking for similar particles among its samples, but so far has come up empty-handed.

"All this is quite perplexing, actually," says Brownlee.

It's possible that the comets were made at different times and formed from a different variety of materials, says Brownlee, or that pre-solar grains from Wild-2 were destroyed as they were captured by the probe.

"It's a mystery," he says, "but that's what makes science run."

Busemann presented his findings the European Week of Astronomy and Space Science conference at the University of Hertfordshire.