Washington, August 23 : Astronomers have spotted a disk of dust and water ice ringing a young star 165 light years away that is similar to the Sun's Kuiper belt, a disk of small icy bodies that extends beyond Neptune.
The icy signature of the disk and the collisions between bodies inferred to be taking place there suggest it is similar to the Sun's Kuiper belt.
"The key new word is icy," said Christine Chen, an astronomer at Space Telescope Science Institute in Baltimore, Maryland and lead author of the new study. "This is the first time there's evidence for water ice around a main sequence star," she added.
In recent years, hundreds of debris disks have been discovered around young stars.
The infrared radiation the disks emit can be used to tell their temperature, and this has shown that many are cold; but to distinguish definitively cold dust from cold ice has proved tough.
For their study of HD 181327, in the constellation of Pictor, Chen and her colleagues used several instruments, including the Hubble Space Telescope and the Gemini South telescope, an 8-metre ground-based telescope in Chile that is sensitive in the near infrared.
But the key observations were from the Spitzer Space Telescope, which has cryogenically cooled cameras that can see emitted infrared radiation hidden from earthbound telescopes by the atmosphere.
Using Spitzer, Chen and her colleagues were able to make out a tiny dip in the infrared spectrum emitted by the debris disk.
The dip was at a wavelength that would be preferentially absorbed by particles of ice.
The apparently icy belt is three times further from its star than Neptune - which marks the inner boundary of the Kuiper belt - is from the Sun.
According to Chen, for it to show up the way it does, the bodies in the belt must be colliding.
The sort of ice grains that these observations pick up would last only a bit more than a thousand years in the harsh light of their star, and so there must be a constant source of replenishment - such as collisions between larger bodies.
One side of the belt is brighter than the other, both in the way it reflects light and the way it emits in the infrared, which might mean there are more icy grains in that part of the belt.
Chen said that this suggests that bigger bodies in the belt collided recently, creating debris that has not yet had time to even itself out around the belt.