Comment 6 by Scoundrel :

What about the radioactive decay of elements in the planets crust? Wouldnt that generate heat, making "habitable" zones basically a meaningless concept.

It would certainly complicate matters.

I'm no geologist so could be completely wrong but I see several different senarios, based on the premis that Earth has a liquid mantle due at least in part on radioactive decay.

Way too much active elements might lead to runaway vulcanism, so a crust never forms.

Somewhat less actives still causes too much vulcanism over too long a period, though a crust can form. Plate tectonics either doesn't get going or forms plates too small for continental formation. Might also be problems with fallout/decay products.

The right recipe. Just enough radioisotopes to produce something similar to Earth, in conjunction with the right distance from the right star.

Not enough radioisotopes. Mars perhaps?

There's a whole lot of variables to be taken into account, might make for an interesting bit of research.