Carbon monoxide snow line detected in exosolar system

The green is emission from a molecule revealing the location of carbon monoxide ice in a young star system. The blue line is the size of the orbit of Neptune. According to planet-formation models, the region where carbon monoxide freezes is exactly where this observation finds it.

ALMA (ESO/NAOJ/NRAO)

The formation of stars and planets is rapid in cosmic terms—measured in millions rather than billions of years—but it's still too slow on human time scales for us to witness the whole process. As a result, our best hope of understanding how the Solar System came to be is observing other systems at various stages of formation. New data from the protoplanetary disk around a relatively nearby star could reveal the edge of the region where important planetary bodies like comets could form: the carbon monoxide "snow line."

Chunhua Qi and colleagues observed a disk around a newborn star, a disk that appears similar to what we think the Solar System looked like about 5 billion years ago. It surrounds a newborn star similar to the young Sun. They looked for light emissions that only occur where carbon monoxide is frozen. The observations discovered a ring of ice where theory predicted, roughly at the same distance Neptune is from the Sun. In the Solar System, that distance marks where large planets no longer form and where the realm of the comets and Pluto-like objects begins. This is the first observation of its kind, so it should help astronomers refine planetary formation models and understand the origins of prebiotic molecules—essential chemicals for life.

Snow lines are the distances from a star where different materials freeze; there are distinct snow lines for molecules such as water and ammonia. The analogy is obviously with mountains on Earth: as you ascend, the colder temperatures make snow more likely, even in summer, with a distinct transition where the temperature crosses 0°C. The important snow lines in planet formation theory are for water, carbon dioxide (CO₂), and carbon monoxide (CO). Those distances help determine what kind of planetary body will form and how massive it can grow.