Hot water vapour has previously been detected in the inner part of the planet-forming discs of nascent, alien solar systems. But this is too close to the central star to be incorporated into the forming planets.
By contrast, the new observations are of water in the form of ice grains, which can exist only in the frigid outer reaches of a planet-forming disc. It is there that they can ultimately coalesce into planets and comets.
Before planets form, the disc of material surrounding young stars is mostly gas. Astronomers can probe the contents of that gas by analysing the spectra of the light it emits. Earlier observations had found organic materials like carbon monoxide and cyanide in such discs, but because Earth's own atmosphere is so damp, it interferes with the detection of alien water from the ground.
To solve this problem, Michiel Hogerheijde of the Leiden Observatory in the Netherlands and colleagues used the Herschel space observatory to get above Earth's clouds and observe a young star called TW Hydrae, which has just over half the mass of the sun and is 175 light years away.
Earlier models suggested that there should be water in the outer frigid regions of TW Hydrae's disc, where it would be locked up in ice and therefore invisible to Hershel's infrared eyes. But when those dust grains get hit by ultraviolet photons from their host star, they give off little puffs of water vapour that emit light in the wavelengths Herschel can see.
Hogerheijde and colleagues found a spectral signature that can be attributed to ice reservoirs about five one-thousandths of the mass of Earth's oceans. But for every gram of vapour they spotted directly, there should be thousands of grams still frozen. The team inferred that the total ice reservoir in the disc should amount to several thousands of Earth's hydrosphere.
The team further confirmed that the water they were seeing came from the frigid, comet-forming region of the disc via the signatures of two different types of water molecule, which form at different temperatures. If the two hydrogen atoms in water have the same quantum spin, the water is called "ortho", and if they're different, it's called "para". The ratio of these two versions of water in TW Hydrae's disc suggested that much of it formed cold – in other words in the frigid outer regions of the disc.
Some astronomers think Earth got its oceans from comets that smashed into the infant planet after it had cooled. The discovery of cold water around TW Hydrae, combined with the recent discovery of a comet storm in a young planetary system suggests this scenario is a distinct possibility.
That could be good news for the prospect of life on dry exoplanets that are waiting for water. "If this is true in all systems, there is certainly a lot of water around," said Rachel Akeson of the NASA Exoplanet Science Institute at the California Institute of Technology, who was not involved in the new study. "It may increase the chance that life can develop on these planets."