They don't call comets dirty snowballs for nothing. The cosmic wanderers are chock-full of fine dust particles not unlike those found in cigarette smoke. It turns out that stars are to blame. Solar wind and radiation push dust out into space, where it mixes in with the ice crystals that form comets, new research reveals.
The presence of dust in comets has stumped researchers for decades. The dust--more accurately, crystallized silicates, the constituent material of rocks on Earth--needs a lot of heat to form, but comets are mostly frozen water. So if comets contain the material, they either formed much closer to their stars than previously thought, or the silicates were kicked far into space by an unidentified force. Scientists consider the former scenario unlikely, because comets are believed to agglomerate in a solar system's outer reaches.
Today, two papers in Nature appear to solve the mystery. In the first, astronomers using the Spitzer Space Telescope report that last year they observed a massive outburst from the young star EX Lupi, located about 500 light-years away in the constellation Lupus. The outburst, which temporarily increased the star's brightness by about 100 times, produced enough heat--about 700° C--to crystallize the amorphous silicate particles floating around the inner part of the star's protoplanetary disk. This thin, rotating smear of dust and gas eventually will form a solar system around the star.
In the second paper, physicist Dejan Vinković of the University of Split in Croatia developed a model that shows how a combination of stellar wind and infrared radiation from the inner disk can blow the ultrafine crystallized silicate particles billions of kilometers from the inner regions of a protoplanetary disk to its colder outer sections, where comets and other cold but rocky objects, such as Pluto, can form.
Vinković's model "shows how dust grains propelled by radiation pressure can travel from the disk's hottest regions to its icy outer edges," says astronomer Dániel Apai of the Space Telescope Science Institute in Baltimore, Maryland. And that helps explain, he says, not only how such materials can become part of comets but also how silicate crystals appeared inside protoplanetary disks "shortly after the eruption of [EX Lupi]."