Dawn of a disk: water vapor pours down on embryonic star.

Even as it forms within a cloud of gas and dust, a nascent star develops a doughnut-shaped disk around it. This is the "proto-planetary disk" that might spawn planets. Using an infrared telescope to peer inside a dusty stellar womb 1,000 light-years from Earth, astronomers say that they have found evidence of such a disk in one of its earliest stages of development.

The observations suggest that large amounts of water from the star-making cloud are crashing onto the disk, spurting its growth and providing a source of water that might later be incorporated into planets. If liquefied, the water would fill Earth's oceans five times over, Dan Watson of the University of Rochester in New York and his colleagues report in the Aug. 30 Nature.

Watson's group found the infalling water vapor while surveying 30 of the youngest known stars--each still residing at the core of its natal cloud--with the orbiting Spitzer Space Telescope. Only one of the embryonic stars, known as NGC 1333-IRAS 4B, shows a strong infrared signal indicative of water.

The temperature of the water molecules indicates that they are closer to the natal cloud's warm stellar core than to its distant icy reaches. Moreover, the strength of the signal indicates that the vapor is spread over a large region.

The most likely explanation for the infrared signals, Watson's team says, is that they arise from ice in the cloud falling onto a large disk circling the budding star. The heat generated by the collision would turn the ice into steam.

"We believe ... that we're seeing material raining down on a protoplanetary disk" early in its development, says Watson. Until now, he adds, "we didn't know anything about [the material] in disks at such an early age."

The team calculated that the disk is about as big as Pluto's orbit around the sun and has an average temperature of 170 kelvins. Accumulating in the disk at the rate of one ten-thousandth of the sun's mass per year, the water quickly refreezes. As ice, it could be incorporated into asteroids, comets, and planets, if the disk lasts long enough for such objects to form. In our own solar system, astronomers say, asteroids and comets delivered water to the early Earth.

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Although the notion of water delivery described by the researchers may be intriguing, "the real value of what they have found is [in] using the water vapor as a diagnostic of disk formation," says theorist Alan Boss of the Carnegie Institution of Washington (D.C.). Spitzer isn't a powerful enough telescope to image the disk of NGC 1333-IRAS 4B. But its observations, combined with the team's modeling, "make a strong case" that the water vapor comes from star-forming material slamming into a disk, Boss adds.

The vapor is merely a tracer for much larger amounts of infalling molecular hydrogen, notes Watson. Such an infall may last only 10,000 years, explaining why just one of the observed young stars showed the water. In addition, the orientation of NGC 1333-IRAS 4B provides a dustfree line of sight to Earth, which helped Spitzer pick up the watery signal.