Just as the solar system was forming some 4.6 billion years ago, it turned itself inside out. Some of the hottest material, residing so close to the sun that it almost vaporized, sped out to the chilliest reaches of deep space. These bits of formerly high-temperature dust ultimately became parts of the icy balls known as comets.
That startling scenario—in stark contrast to a widely held view that outlying regions grew up isolated from the inner solar system—is revealed by the first analyses of cometary-dust grains brought back to Earth by a spacecraft. NASA's Stardust craft passed through the dusty shroud of a comet called Wild 2 in 2004 and last January dropped to E