Where did solid seeds for planet formation come from?
As the disk radiated away its internal heat in the form of infrared radiation (Wien’s law) the temperature dropped and the heaviest molecules began to form tiny solid or liquid droplets, a process called condensation. • There is a clear relation between the temperature and the mass of the particles that become solid (Why?). Near the Sun, where the T was higher, only the heaviest compounds condensed forming heavy solid grains, including compunds of aluminum, titanium, iron, nickel, and, at somewhat cooler temperatures, the silicates. In the outskirts of the disk the T was low enough that hydrogen-rich molecules condensed into lighter ices, including water ice, frozen methane, and frozen ammonia. • The ingredients of the solar system fell into four categories: • Metals: iron, nickel, aluminum. They condense at T~1,600 K and comprise only 0.2% of the disk. • Rocks: silicon-based minerals that condense at T=500-1,300 K (0.4% of the nebula). • Ices: hydrogen compounds like methane (CH4), am
