Helium is the second most abundant element in the universe. But it is an ideal gas, and very light, which means it is hard for a terrestrial planet, like earth,

Helium is the second most abundant element in the universe. But it is an ideal gas, and very light, which means it is hard for a terrestrial planet, like earth, to retain it. Unfortunately bringing helium to earth from the gas planets isn't really economically viable. Helium is also a byproduct of both nuclear fission and nuclear fusion. Although I don't know how viable it is to capture it.