Frost Line

The frost line, also called the snow line, is the distance from a young star beyond which it is cold enough for water to condense into ice in the protoplanetary disk.

It matters because ice greatly increases the amount of solid building material. Inside the frost line, only rock and metal are available, so planets tend to be small and rocky. Outside it, ice lets planet cores grow large enough to capture gas and become giants.

In the early solar system the frost line lay somewhere between the orbits of Mars and Jupiter, which helps explain why the inner planets are rocky and the outer planets are giants.

Read more in Step 1: Gathering the materials.