Which theory states that our inner planets are rocky because rocks and metals combine at extreme temperatures, while the outer planets are gaseous because hydrogen compounds condense at colder temperatures? hubble doppler kepler nebular

The nebular theory states that our inner planets are rocky because rocks and metals combine at extreme temperatures, while the outer (Jovian) planets are gaseous because hydrogen compounds condense at colder temperatures, and they are further from the sun. In contrast to our solar system’s gas giants, the deepest and most extensive direct imaging surveys of orbital space to date have shown that the vast majority of gas-giant planets tend to cling to their parent stars. Most of those identified tend to shun orbital zones far from their parents.

Nebular!!  That is the answer.

nuclear fission produces elements that are heavier than helium.

the elements that are used in nuclear fission and their products are much heavier than helium.