New Star's 'Snow Line' Reveals Clues About Planet Formation

Astronomers have identified the point where carbon monoxide (CO) freezes in the disk around a sunlike star — information that could help them understand how planets form.

A team of international scientists has calculated the CO "snow line" for a star called TW Hydrae, determining that the gas solidifies at about the distance of the orbit of Neptune, where it could help feed the formation of the outer edges of the system.

"The CO snow line is interesting, not only because CO is abundant in the disks, but its snow line is the most accessible to direct observations due to its low freeze-out temperature — it's farther away from the star," said principal investigator Chunhua Qi of the Harvard-Smithsonian Center for Astrophysics.

"It could mark the starting point where smaller icy bodies, like comets, and dwarf planets, like Pluto, would begin to form."

Volatiles like carbon monoxide freeze at a range of temperatures, and each can have its own impact on the growth of orbiting bodies.

The location of the snow lines for volatiles can affect planetary formation. 

Recent research has indicated the water snow line, shown here, lies farther out than previously suspected.

Credit: NASA, ESA, and A. Feild (STScI)

Tracer ions
Stars form when a disk of dust and gas collapses in on itself due to gravity. The remaining material continues to orbit the newly formed object in a disk of material.

As dust and gas particles pass through the disk, scientists say, they come together to form larger and larger clumps that can eventually grow into planets. Frozen volatiles help this process along.

"The snow line provides more sticky solid grains, and enhances the planet formation efficiency and grain growth," Qi told reporters.

But determining the location of these grains can be a challenge. Emission from volatiles along the outside of the disk can make it difficult for scientists to image the telltale signs of frozen compounds.

To locate the region of the disk where CO freezes, Qi and his team utilized a new technique. Using the Atacama Large Millimeter Array (ALMA) in Chile, they searched for the ion diazenylium (N2H+), rather than the hard-to-find carbon monoxide, around TW Hydrae, which lies 176 light-years from Earth.

"N2H+ is easily destroyed in the presence of CO gas, and is abundant where CO has frozen out," Qi said.

The new technique should be helpful for studying the CO snow lines of other stars, which will provide more insight into how outer solar-system objects form.

"As long as the disk is gas-rich, so that there should be enough N2H+, we can use this ion to image the CO snow line," Qi said.