•Start with material of solar composition
material
–
(H, He, C, N, O, Ne, Mg, Si, S, Fe ...)
•Material starts out hot enough that
everything is a gas
–May not be exactly true but is simplest
starting point
–
•As gas cools, different chemicals condense
–First high temperature chemicals, then
intermediate ones, then ices
•Solids begin to stick together or accrete
–snowflakes Þ snowballs (“Velcro
Effect”)
•Once large enough gravity pulls solids
together into planetesimals
–planetesimals grow with size
•At some point wind from sun expels all the
gas from the system
–Only the solid planetesimals remain to
build planets
–Composition depends on temperature at that
point (in time and space)
–Gas can only remain if trapped in the
gravity of a large enough planet