snow line

The snow line (also called the frost line, ice line, or water line) is the distance from a protostar at which ice can form, which is also roughly the distance where ammonia (NH₃) and methane (CH₄) can condense. A theorized temperature is 150 K for a typical protoplanetary disk, and alternate theories generally put it in the 145-170 K range. The line's distance from the star depends upon the star's temperature and luminosity. Such a distance-calculation can be carried out for stars of any age, but the result is likely different from that when it was actually a protostar. The solar system's current snow line is about 5 AU, nearly to Jupiter. In a general way, the snow line forms the outer bound of the habitable zone.

An analogous line further out is an ice transition radius (amorphous-to-crystalline ice transition radius or amorphous ice snow line) is the line between the ring-shaped region within the disk where water forms crystal ice versus that beyond where it forms amorphous ice. The difference can affect the tendency for the ice to clump and form planetary embryos.

Other common volatiles, such as carbon monoxide (CO), carbon dioxide (CO₂), ammonia, and methane have analogous lines (encircling lines of some radii) within protoplanetary disks for each, based upon their condensing and freezing temperatures, such as a CO ice line and a CO₂ ice line.