Sleet Storm in Space

Astronomers are pretty sure that most of the water on Earth — and the water that probably flowed billions of years ago on the surfaces of Mars, and maybe Venus as well — came from icy comets, raining down on the newborn planets. As for where the comets came from, it's long been assumed that they formed directly out of clouds of interstellar gas and dust. That's why scientists are so interested in them: by studying comets, they figured they could get a handle on material that's been unchanged for billions of years.

But a new study suggests it's not so simple. Published in the Aug. 30 issue of Nature, the paper shows that the creation of a comet is probably a much more complicated process than anyone thought. The evidence comes from the Spitzer Space Telescope, which uses infrared-sensitive cameras to peer through the shroud of dust that surrounds newly forming planetary systems to see what's going on inside. In one such system, known as IRAS 4B, about 1,000 light-years from Earth, astronomers from the University of Rochester have detected a disk-shaped knot of material that will one day emerge as a suite of planets orbiting a young star.

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