Ars Technica: To understand how our solar system formed over billions of years, researchers have been studying snapshots of nearby systems in various stages of formation. New observations of one such system have revealed the first evidence of a “snow line” for carbon monoxide around another star. Snow lines are the distances from the star at which various substances, such as water and ammonia, freeze. They take their name from a feature of mountains. A team led by Chunhua Qi of Harvard University examined images of the protoplanetary disk around a relatively nearby star similar to the Sun 5 billion years ago. They found a ring of ice showing the reflected emissions of frozen carbon monoxide at a distance from the star roughly the same as the distance of Neptune from the Sun. That observation matched with theoretical predictions. In our solar system, that snow line marks the end of the large planets and the beginning of the region filled with frozen Pluto-like planets and comets.