Snow Line 
As the temperature creeps upward during spring, something else also creeps upward: the snow line -- the altitude on a mountain where snow can fall.
Star systems have a snow line, too. It’s the distance from the star where it’s cold enough for certain compounds to freeze -- like water. It’s also the dividing line between dense, rocky planets and bigger, puffier planets. In our own solar system, it’s in the middle of the asteroid belt.
When the planets were taking shape, it was too hot for ices to form close to the Sun. That left only grains of rock to clump together to make planets -- worlds like Earth. Farther away, the grains of rock mixed with grains of ice. That allowed the cores of the planets to grow much larger, and to sweep up some of the gas leftover from the Sun’s birth.
Just because a planet was born above the snow line doesn’t mean it has to stay there, though. Astronomers have discovered giant planets quite close to their stars. These worlds formed beyond the snow line but were somehow pulled closer in.
It’s also possible that changes in the snow line could create giant ocean worlds. Such a planet would be bigger and heavier than Earth, and would have been born with a lot of ice. As its parent star settled into maturity, though, the environment around the planet would grow warmer. That would melt some of the ice, creating a planet topped by deep oceans of liquid water -- a world inside the snow line.
More about “exoplanets” tomorrow.
Script by Damond Benningfield, Copyright 2012