Water Vapour Detected
In Protoplanetary Disks
(18 March 2008) 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 the
detection of water vapour in the inner part of two extrasolar protoplanetary
disks brings scientists one step closer to understanding water's role during
terrestrial planet formation.
By maximising the spectroscopic
capabilities of NASA's Spitzer Space Telescope and high-resolution measurements
from the Keck II Telescope in Hawaii, researchers from the California Institute
of Technology and other institutes found water molecules in disks of dust and
gas around two young stars. DR Tau and AS 205A, respectively around 457 and 391
light-years away from Earth, are each at the centre of a spinning disk of
particles that may eventually coalesce to form planets.
"This is one of
the very few times that water vapor has been detected in the inner part of a
protoplanetary disk--the most likely place for terrestrial planets to form,"
says Colette Salyk, a graduate student in geological and planetary sciences at
Caltech. She is the lead author of a group of scientists reporting their
findings in the March 20 issue of the Astrophysical Journal
Letters.
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," says Salyk.
To
determine in what part of the disk the vapour resides, the team made
high-resolution measurements at shorter wavelengths with NIRSPEC, the
Near-InfraRed cross-dispersed echelle grating Spectrometer for the Keck II
Telescope. Unlike Spitzer, which observed water lines blended together into
clumps, 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," says Salyk, "indicating that they came from close to the stars, which
is where Earthlike planets might be forming."
"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," remarks coauthor Geoffrey Blake, professor of
cosmochemistry and planetary sciences and professor of chemistry at
Caltech.
The presence of water in the inner disk may indicate its stage
on the road to planet formation. A planet like Jupiter in our solar system grew
as its gravitational field trapped icy solids spinning in the outer part of the
sun's planetary disk. However, before Jupiter gained much mass, these same icy
solids could have travelled towards the star and evaporated to produce water
vapour such as that seen around DR Tau and AS 205A.
Although they have
not detected icy solids in the extrasolar disks, says Salyk, "our observations
are possible evidence for the migration of solids in the disk. This is an
important prediction of planet-forming models."
These initial
observations portend more to come, says co-author Klaus Pontoppidan, a Caltech
Hubble Postdoctoral Scholar in Planetary Science. "We were surprised at how
easy it is to find water in planet-forming disks once we had learned where to
look. It will take years of work to understand the details of what we
see."
Indeed, adds Blake, "This is a much larger story than just one or
two disks. With upcoming observations of tens of young stars and disks with
both Spitzer and NIRSPEC, along with our data in hand, we can construct a story
for how water concentrations evolve in disks, and hopefully answer questions
like how Earth acquired its oceans."
Other authors on the paper are Fred
Lahuis of Leiden Observatory in the Netherlands and SRON, the Netherlands
Institute for Space Research; Ewine van Dishoeck, also of Leiden Observatory;
and Neal Evans of the University of Texas at Austin.
(source: California
Institute of Technology)
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