ANN ARBOR—Like the elevation in the Rocky Mountains where the snow caps begin, a snow line in a solar system is the point where falling temperatures freeze and clump together water or other chemical compounds that would otherwise be vapor. Astronomers believe snow lines in space serve a vital role in forming planets because frozen moisture can help dust grains stick together.

Astronomers have, for the first time, directly imaged a snow line at another star. Using the new Atacama Larger Millimeter/submillimeter Array (ALMA) telescope in Chile, they obtained radio-wavelength images of the carbon monoxide snow line around TW Hydrae, a young star 175 light-years away from Earth. TW Hydrae, in the constellation Hydra, is believed to be our closest infant solar system.

"We've had evidence of snow lines in our own solar system, but now we're able to see one with our own eyes. That is exciting," said Edwin Bergin, professor of astronomy in the University of Michigan College of Literature, Science and the Arts. Bergin is co-author on a paper on the results published in Science Express on July 18.

Different chemical compounds freeze at different distances from a central star. In our own solar system, water freezes at about five times the distance from the Earth to the sun, between the orbits of Mars and Jupiter. Various chemical compounds' snow lines may be linked to the formation of specific kinds of planets. The carbon monoxide line in our system corresponds to the orbit of Neptune, and it could also mark the starting point where smaller icy bodies like comets and dwarf planets like Pluto would form, according to the National Radio Astronomy Observatory.

Until now, snow lines have only been detected by their spectral signature. They have never been imaged directly, so their precise location and extent could not be determined.

"ALMA has given us the first real picture of a snow line around a young star, which is extremely exciting because of what it tells us about the very early period in the history of our own solar system," said co-author Chunhua "Charlie" Qi, a researcher with the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. "We can now see previously hidden details about the frozen outer reaches of another solar system, one that has much in common with our own when it was less than 10 million years old."

Snow lines have been difficult to image because they only form in the relatively narrow central plane of a planet-forming disk. Above and below this region, radiation from the central star keeps the gases warm.

An outer cocoon of hot gas prevents astronomers from peering inside the disk where the gas is frozen. Instead, they hunted for a different molecule called diazenylium. Carbon monoxide gas destroys diazenylium, so it is only detectable in regions where the gas is frozen. It shines brightly in the millimeter portion of the electromagnetic spectrum, which can be detected by radio telescopes like ALMA.

By tracing the distribution of diazenylium, astronomers identified a boundary approximately 30 astronomical units from TW Hydrae. An astronomical unit is the average distance between the Earth and the sun.

"Using this technique, we were able to create, in effect, a photonegative of the carbon monoxide snow in the disk surrounding TW Hydrae," said Karin Öberg, also with Harvard but who was with the University of Virginia at the time of the observation. "With this, we could see the snow line precisely where theory predicts it should be—the inner rim of the diazenylium ring."

Öberg also points out that this snow line is particularly interesting since carbon monoxide ice is needed to form methanol, which is a building block of more complex organic molecules essential for life. Comets and asteroids could then ferry these molecules to newly forming Earth-like planets, seeding them with the ingredients for life.

ALMA, an international astronomy facility, is a partnership of Europe, North America and East Asia in cooperation with the Republic of Chile. ALMA construction and operations are led on behalf of Europe by the European Southern Observatory, on behalf of North America by the National Radio Astronomy Observatory and on behalf of East Asia by the National Astronomical Observatory of Japan. The Joint ALMA Observatory provides the unified leadership and management of the construction, commissioning and operation of ALMA.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities Inc.