'Snow Line' Spotted in Young Solar System


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Astronomers this week revealed that the first-ever "snow line" of a solar system has been spotted. Scientists believe that the snow line - similar to ones seen on the tops of mountains on Earth - could provide information about how planets with differing chemical compositions evolve around young stars.

In the far reaches of accretion discs around young stars, molecules such as water, carbon dioxide, and methane freeze around dust grains that will eventually coalesce into planets. The line where such behavior begins was spotted around the star TW Hydrae, a star located 175 light-years away that astronomers believe is similar to what our solar system once was. An image of the snow line was taken using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, and the results of the research have been published in the journal Science Express.

"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 Chunhua Qi, a leader of the research team and an astrophysicist at the Harvard-Smithsonian Center for Astrophysics. "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."

This is the first direct image of a snow line ever captured. As insulating clouds of warm gas cloak snow lines, the regions are normally detected using spectral signatures. Qi and his colleagues were able to use ALMA to look for a specific molecule called diazenylium, which should only appear in regions where carbon monoxide (CO) is frozen. TW Hydrae's snow line was found at around 30 astronomical units (AU, the distance from the Earth to the Sun) from the star.

"Using this technique, we were able to create, in effect, a photonegative of the CO snow in the disk surrounding TW Hydrae," said Karin Oberg, another leader in the research and an astrochemist at Harvard University. "With this we could see the CO snow line precisely where theory predicts it should be -- the inner rim of the diazenylium ring."

(Image courtesy Bill Saxton/Alexandra Angelich, NRAO/AUI/NSF