Question

The outer planets are mostly formed of the lighter elements, hydrogen and helium, because these elements condense at a temperature, further from the Sun.

Answer 1

The outer planets actually did start out as rocky cores. However, they were far enough from the Sun that their outer atmospheres did not warm significantly enough to let their outer atmospheres escape through Jeans Escape. Thus, they were able to keep on accreting the residual hydrogen and helium gas (which were responsible for 90% of the mass of the solar nebula - http://csep10.phys.utk.edu/astr1... ). And this residual hydrogen and helium gas significantly increased their mass and gravitational fields (which, in turn, made them accrete more of the remaining mass at a faster rate). This, in turn, made them gas giants.

Answer 2

The outer planets actually did start out as rocky cores. However, they were far enough from the Sun that their outer atmospheres did not warm significantly enough to let their outer atmospheres escape through Jeans Escape. Thus, they were able to keep on accreting the residual hydrogen and helium gas (which were responsible for 90% of the mass of the solar nebula . And this residual hydrogen and helium gas significantly increased their mass and gravitational fields (which, in turn, made them accrete more of the remaining mass at a faster rate). This, in turn, made them gas giants.