RE: Planetary gravity question.

G'day,

> Too much of the heavy stuff and it gets hard expecting that there 
> will be enough silicates in the mantle for conventional vegetation to 
> take hold.

In a terrestrial sense... something could look Earth-like without it
relying
on conventional vegetation. Though Johan probably does mean conventional
with regard to being close to Terran analogs. 
 
> I've seen references to the fact that a moderate amount of isotopes 
> is necessary to have enough mutation in order to have evolution 
> working well. Too much and nothing lives very long due to radiation 
> exposure.

There's still some arguments about that with regard to what we're
finding
some groups can survive now that we've stopped assuming they couldn't ;)

Cheers

Beth