Snow Line Observed in Distant, Infant Planetary System

In a far-off and infant planetary system, astronomers have for the first time observed a snow line. The find promises to reveal new information about the formation of planets and comets, as well as factors that determine the composition of the cosmic masses.

On Earth, snow lines can commonly be observed along high mountain peaks, where snow-covered rock suddenly ends and rocky face begins. Lower temperatures at higher altitudes turn moisture in the air into snow on the mountains, but the higher temperature at lower altitudes keeps snow from forming.

Snow lines in planetary systems -- while on a much bigger scale than mountains -- form in a similar way. In an emerging planetary system, conditions are warmer close to the central star. As galactic dust drifts away from the star, the temperature will decrease. First water will freeze, which is the start of the snow line. As distance from the star increases, more and more molecules will react to the cold, some of which will eventually freeze and turn into snow. Carbon dioxide, methane and carbon monoxide will all turn into their own form of snow when the temperature is low enough.

These different snows cause the surrounding galactic dust to become sticky, allowing it clump together, eventually forming the building blocks of future planets and comets. The snow also increases how much solid matter is available and may dramatically speed up the planetary formation process.

Each of these elemental snow lines -- water, carbon dioxide, methane and carbon monoxide -- can be linked to the formation of particular kinds of planets. Around a Sun-like star solar system like our own, the water snow line would correspond to a distance between the orbits of Mars and Jupiter, according to the European Southern Observatory. The carbon monoxide snow line would correspond to the orbit of Neptune.

Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) were able to image the snow line in infant planetary system around the star known as TW Hydrae, a star no too unlike our Sun.

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 the Solar System, said Chunhua "Charlie" Qi, of the Harvard-Smithsonian Center for Astrophysics.

"We can now see previously hidden details about the frozen outer reaches of another planetary system similar to our own."

Of particular interest to Qi and his research colleagues is the observation of the carbon monoxide snowline. Carbon monoxide ice is needed to form methanol, which is a building block of the more complex organic molecules that are essential for life. The researchers speculate that if comets ferried these molecules to newly forming Earth-like planets, these planets would then be equipped with the ingredients necessary for life.

Imaging the snow line was a made possible by ALMA, which has unique sensitivity and resolution which allowed the astronomers to trace the presence of the molecule diazenylium and find a clearly defined boundary snow line approximately 30 astronomical units from the star. The snow line could not be observed directly, but the bright glow of diazenylium at the millimeter portion of the light spectrum was indicative of the snow line, the researcher report, noting that diazenylium is easily destroyed in the presence of carbon monoxide gas, so would only appear in detectable amounts in regions where carbon monoxide had become snow and could no longer destroy it.

Michiel Hogerheijde from Leiden Observatory, the Netherlands, said the observations were made using only 26 of ALMA's full array of 66 antennas.

"Indications of snow lines around other stars are already showing up in other ALMA observations, and we are convinced that future observations with the full array will reveal many more of these and provide further, exciting insights into the formation and evolution of planets. Just wait and see."

Qi and his colleagues work is published in the journal Science Express.