Enough water to fill the Earth’s seas five time over has been spotted raining down onto a new-born solar system. The vital ingredient for life is falling as steam and hitting a disk of dust where planets are probably forming.

The remarkable discovery, by Nasa’s heat-seeking Spitzer space telescope, is giving astronomers a first direct look at how water makes its way into planets, possibly including rocky worlds like our own.

Dan Watson, of Rochester University, New York, said: “For the first time, we are seeing water being delivered to the region where planets will most likely form.”

Scientists turned their space telescope on a stellar nursery called NGC 1333-IRAS 4B in the constellation of Perseus to make their discovery. The fledgling alien solar system, 1,000 light-years away, is still growing inside a cool cocoon of gas and dust.

Within this cocoon, circling around an embryonic star, is a warm disk of planet-forming materials. The new Spitzer data indicates that ice from the outer cocoon is falling towards the forming star and vaporising as it hits the disk.

Watson said: “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. Now we’ve seen that water, falling as ice from a young star system’s envelope to its disk, actually vaporises on arrival. This water vapour will later freeze again into asteroids and comets.”

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