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Snow found around infant solar system

Jenny Winder, News Writer
Jul 20, 2013, 7:00 UTC

Sen—Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) telescope have taken the first-ever image of a snow line in an infant solar system. This is thought to play an essential role in the formation and chemical make-up of planets around a young star.

Snow lines form around young stars in the distant, colder reaches of the disks from which solar systems form. Depending on the distance from the star more exotic molecules can freeze and turn to snow.

Water ice freezes out first, then moving outward, other abundant gases like carbon dioxide (CO2), methane (CH4), and carbon monoxide (CO) freeze, forming a frost on dust grains, which are the building blocks of planets and comets.

ALMA spotted a CO snow line around TW Hydrae, a young star 175 light-years from Earth.

"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 "Charlie" Qi, a researcher with the Harvard-Smithsonian Center for Astrophysics.

Snow lines form exclusively in the relatively narrow central plane of a protoplanetary disk. Above and below this region, stellar radiation keeps the gases warm, preventing them from forming ice. Until now, snow lines have only been detected by their spectral signatures; they have never been imaged directly, so their precise location and extent could not be determined.

Astronomers were able to pierce the intervening CO fog by hunting for a different molecule known as diazenylium (N2H+) which is easily destroyed in the presence of CO gas, so would only appear in detectable amounts in regions where CO had frozen out. Diazenylium shines brightly in the millimeter portion of the spectrum, which can be detected by a radio telescope like ALMA here on Earth.

ALMA's unique sensitivity and resolution allowed the astronomers to trace the presence and distribution of diazenylium, finding a clearly defined boundary approximately 30 astronomical units (AU) from TW Hydrae.

Snow lines help dust grains overcome their normal tendency to collide and self-destruct by giving the grains a stickier outer coating and may dramatically speed up the planet formation process. Since there are multiple snow lines, each may be linked to the formation of specific kinds of planets.

CO snow line compared to orbit of Neptune. Image credit Karin Oberg, Harvard University/University of Virginia.

Around a Sun-like star, the water snow line would correspond to the orbit of Jupiter and the CO snow line to the orbit of Neptune. The transition to CO ice could also mark the starting point where smaller icy bodies like comets and dwarf planets like Pluto would form.

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

These observations were made with only a portion of ALMA's eventual full complement of 66 antennas. The researchers hope future observations with the full array will reveal other snow lines and provide additional insights into the formation and evolution of planets.