Does Uranium 235 come from outer space?
Having a longer half-life than the earth is old has nothing to do with existing longer. The half-life of a quantity whose value decreases with time is the interval required for the quantity to decay to half of its initial value. Essentially, ALL elements came from outer space, its just helium fused together in the stars over millions of years or created in the hot fires of a billion super novae. Therefore it is normal for radioactive materials to have a longer half-life than the earth is old.
