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.

And onto that disk, a rain of ice crystals is falling, slamming into the disk at supersonic speeds and vaporizing to form a cloud containing five times the water in Earth's oceans. What the astronomers are convinced will happen next is that the water will re-freeze into ice particles and eventually form comets — which in turn will crash down onto whatever planets they find, forming oceans that the future scientists of these worlds will someday be scratching their heads over.

If the Rochester astronomers are right, the idea that comets are pristine remnants of the material from which our solar system originally formed isn't going to hold up. Indeed, last year, comet-dust particles brought back by NASA's Stardust mission showed that the particles had been heated to high temperatures sometime in their lives, which implied that at least the dust in comets might not be primordial. And now it looks like the ice in them isn't either.