Comets are born of fire as well as ice, the first results from the US space agency's Stardust mission show.
In January, Stardust's sample-return capsule landed in Utah, carrying over a million tiny comet grains inside. Some of these grains contain material that formed at extremely high temperatures, scientists were surprised to find.
Comets formed in the cold, outer-reaches of the early Solar System, and were never exposed to such extreme heating. The Sun and the planets began forming out of a gaseous cloud called the solar nebula about 4.6 billion years ago.
This "accretion disc" consisted of a hot inner region and a cold outer region where ice was able to survive.