Image: Diagram of forming star
Nasa / Jpl-caltech  /  NASA / JPL-Caltech
This diagram shows how stars and planets are formed. Stars start as icy cocoons of gas and dust that form a disk — and clump together into planets. NASA's Spitzer Space Telescope spotted water vapor as it smacks down on a disk circling a forming star called NGC 1333-IRAS 4B. The liquid water is a sign that life could form there.
updated 8/29/2007 2:45:32 PM ET 2007-08-29T18:45:32

NASA's Spitzer Space Telescope has revealed a dusty star system being soaked with a "steamy rain" of water vapor.

The water, pulled from gassy stellar leftovers into a dusty disk, provides what astronomers think is the first direct look at how the life-giving liquid makes its way into planets. The disk is the same sort of thing that forms around many stars and, in the case of our sun, was the seedbed for planet formation.

The amount of water in the newly observed disk is thought to equal more than five times that of all oceans on Earth.

"For the first time, we are seeing water being delivered to the region where planets will most likely form," said Dan Watson, an astrophysicist at the University of Rochester in New York.

Watson and his colleagues' work will be detailed in the Aug. 30 issue of the journal Nature.

Steamy surprise
Water is abundant throughout our universe, existing as ice or gas around stars and in the space between stars, but rarely as a liquid.

"On Earth, water arrived in the form of icy asteroids and comets," Watson said. "Water also exists mostly as ice in the dense clouds that form stars."

Astronomers found the watery evidence in a young star system called NGC 1333-IRAS 4B, located 1,000 light-years away in the constellation Perseus. The system still grows inside a cooled cocoon of gas and dust, and Spitzer data show that ice is falling from the cocoon into a warm disk of potential planet-forming materials circling the star.

As the ice smacks into the dust, it vaporizes.

"Now we've seen that water, falling as ice from a young star system's envelope to its disk, actually vaporizes on arrival," Watson said. "This water vapor will later freeze again into asteroids and comets."

Dry search
Watson and his team's discovery comes after a detailed look at 30 similarly young star systems with Spitzer's infrared spectrograph, an instrument that reveals "fingerprints" of molecules like water. Of the 30 stellar embryos investigated, only NGC 1333-IRAS 4B harbors significant amounts of water.

The dry search, however, may not be due to a lack of water in the other star systems, the astronomers explained. NGC 1333-IRAS 4B is in just the right orientation for Spitzer to view its dense core and, they added, such a watery phase is short-lived and hard to catch.

"We have captured a unique phase of a young star's evolution, when the stuff of life is moving dynamically into an environment where planets could form," said Michael Werner, a project scientist with the Spitzer mission at NASA's Jet Propulsion Laboratory in Pasadena, Calif.

The astronomers explained that water serves as an important tool for studying the planet formation process, which is not very well understood.

"Water is easier to detect than other molecules, so we can use it as a probe to look at more brand-new disks and study their physics and chemistry," said Watson. "This will teach us a lot about how planets form."

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