Water pours on young star system
 |  Nasa's Spitzer Space Telescope detected large amounts of water in the disc  |
Enlarge ImageWriting in Nature journal, astronomers detected enough water vapour to fill Earth's oceans five times over in the collapsing nest of a young star system.
Ice pours down from the outer envelope of this forming star, vapourising as it hits the dusty disc where planets form.
The team based its findings on data from Nasa's Spitzer Space Telescope.
"For the first time, we are seeing water being delivered to the region where planets will most likely form," said lead author Dan Watson of the University of Rochester in New York, US.
The young star system, called NGC 1333-IRAS 4B, is still growing inside a cool cocoon of gas and dust.
Within this cocoon, a warm disc of planet-forming materials circles the embryonic star.
Supersonic travel
The data indicates that ice from the stellar embryo's outer cocoon falls towards the forming star at supersonic speeds and vaporises as it hits the proto-stellar disc.
"On Earth, water arrived in the form of icy asteroids and comets. Water also exists mostly as ice in the dense clouds that form stars," said Professor Watson.
"Now we've seen that water, falling as ice from a young star system's envelope to its disc, actually vaporises on arrival. 
"This water vapour will later freeze again into asteroids and comets."
By analysing what is happening to the water in NGC 1333-IRAS 4B, the astronomers can learn more about its planet-forming disc.
The team calculated a density for the disc of at least 10 billion hydrogen molecules per cubic centimetre (160 billion hydrogen molecules per cubic inch).
Its dimensions could also be calculated - the disc has a radius bigger than the average distance between Earth and Pluto. The researchers also determined its temperature was -103C (-154F; 170 Kelvin).
Studying planet-forming discs at this early stage of development could determine which of two competing theories of planet formation is correct.
In the core accretion model, planets form little by little, as material slowly congeals within the disc over millions of years. 
More detailsThe disc instability model suggests that turbulence in the disc can cause matter to collapse into planets extremely quickly, forming Jupiter-like planets in just thousands of years.
NGC 1333-IRAS 4B is located in a star-forming region about 1,000 light-years away in the constellation Perseus.
Its central stellar embryo is still "feeding" off the material collapsing around it and growing in size. Astronomers cannot yet tell how large the star will ultimately become.
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