Although in general space is just a few degrees above absolute zero, because of our Sun’s radiation the young inner solar system was too warm for volatile molecules like methane and water to condense. Thus planetesimals accreting there were largely constituted of rocky or metallic compounds with high melting points. And because such compounds formed less than 1% of the solar nebula, such bodies were small. They eventually coalesced into the so-called terrestrial planets: Mercury, Venus, Earth and Mars.

Between Mars and Jupiter, large coalescing bodies were easily torn apart or destabilized by Jupiter’s gravitational stresses. The result is the asteroid belt (page 116), comprised of millions of small bodies orbiting between Mars and Jupiter. Their total mass is about 4% of our Moon’s; 220 are more than 100 km. in diameter; 26 are more than 200 km. in diameter. The largest, Ceres (the only dwarf planet within Pluto’s orbit), was discovered in 1801, is 933 km. across, and accounts for ¼ the total mass of all the asteroids.

Moving outwards from the protostar, we eventually reach a distance where the solar nebula is cool enough for volatile hydrogen compounds like water, ammonia and methane to condense into solid grains of ice. This is a very significant boundary, for such compounds comprise most of the solar nebula and beyond this "frost line" solid ice grains are more available to accrete into planets. It is here that the giants Jupiter, Saturn, Uranus and Neptune have formed; within this frost line the terrestrial planets reside.