Question

What was the frost line of the solar system?

Answer 1

The frost line is a boundary between mostly ice-covered objects and mostly rock-covered objects. It is about 5 AU from the Sun.

Explanation:

Inside the frost line surfaces exposed to the Sun are warm enough for water ice to melt or sublime readily, leaving exposed rock like our Moon. Ice can still exist in shadowed or covered regions such as deep crater bottoms, or there may be liquid water given the right combination of temperature and pressure (Earth).

Outside the frost line ice remains stable even in direct sunlight, so it remains on most surfaces like most moons of the outer planets. There are again exceptions, primarily the gas-covered giant planets themselves and Jupiter's moon Io (where water had been driven away by volcanic action). But the frost line reliably predicts whether most surfaces are rocky or icy.

Because of this stability of water ice, objects outside the frost line tend to contain a lot of water, and some of that may be liquid below the icy surface. As a result most likely candidates for life elsewhere in the Solar System are outside the frost line.