Water vapour detected in protoplanetary disks

Washington, March 19: NASA's Spitzer Space Telescope and high-resolution measurements from the Keck II Telescope in Hawaii have found water vapour in protoplanetary disks of dust and gas around two young stars, which might bring scientists one step closer to understanding water's role during terrestrial planet formation. Water is an essential ingredient for forming planets, yet has remained hidden from scientists searching for it in protoplanetary systems, the spinning disks of particles surrounding newly formed stars where planets are born.

Now, researchers from the California Institute of Technology and other institutes found water molecules in disks of dust and gas around two young stars known as DR Tau and AS 205A, which are respectively around 457 and 391 light-years away from Earth. These stars are each at the center of a spinning disk of particles that may eventually coalesce to form planets.

"This is one of the very few times that water vapour has been detected in the inner part of a protoplanetary disk - the most likely place for terrestrial planets to form," said Colette Salyk, a graduate student in geological and planetary sciences at Caltech.

Salyk and her colleagues first harnessed light-emission data captured by Spitzer to inspect dozens of young stars with protoplanetary disks. They honed in on DR Tau and AS 205A because these presented a large number of water emission lines - spikes of brightness at certain wavelengths that are a unique fingerprint for water vapour .

"Only Spitzer is capable of observing these particular lines in a large number of disks because it operates above Earth's obscuring water-vapour -rich atmosphere," said Salyk.

To determine in what part of the disk the vapour resides, the team made high-resolution measurements at shorter wavelengths with NIRSPEC (Near-InfraRed cross-dispersed echelle grating Spectrometer) for the Keck II Telescope.

NIRSPEC can resolve individual water lines in selected regions where the atmospheric transmission is good. The shape of each line relays information on the velocity of the molecules emitting the light.

"They were moving at fast speeds, indicating that they came from close to the stars, which is where Earthlike planets might be forming," said Salyk.

According to coauthor Geoffrey Blake, professor of cosmochemistry and planetary sciences and professor of chemistry at Caltech, "While we don't detect nearly as much water as exists in the oceans on Earth, we see only a very small part of the disk - essentially only its surface - so the implication is that the water is quite abundant."

"Although they have not detected icy solids in the extrasolar disks, our observations are possible evidence for the migration of solids in the disk. This is an important prediction of planet-forming models," said Salyk.

ANI