Snow Line
The distance from a star where it's cold enough for water ice and other volatile compounds to condense and remain solid in a protoplanetary disk.
The boundary in a protoplanetary disk beyond which temperatures drop low enough for volatile compounds like water, methane, and ammonia to freeze into ice. Closer to the star, heat vaporizes these materials; farther out, they solidify. This line marks a key transition in planetary formation, influencing whether rocky or icy bodies form in different orbital zones.
What this means in real life
In our solar system, the snow line sits roughly between Mars and Jupiter; rocky planets formed inside it, while icy bodies and gas giants formed beyond it. This explains why Earth is dry rock while Neptune is an ice giant.
What it isn’t
It is not a literal visible line or boundary you could photograph. It's a theoretical temperature threshold that varies depending on the star's luminosity and the disk's composition, and it shifts over time as the disk evolves.
Commonly misused online
People sometimes treat it as a fixed, universal distance (e.g., 'the snow line is always at 3 AU'), when in reality it depends entirely on the star's brightness and the specific protoplanetary system.