Instead of calculating how many other Earth's there are, they should be scratching their heads trying to figure out how THIS ONE ever got here. It's too perfect. Everything has to work out just right. The odds of another planet that harbors intellignet life in this galaxy is just about zero. Multiply those odds by 100 billion and it is still about zero. With 100 billion galaxies in the universe, the odds that there is another Earth are still just about zero.
The Earth itself is a miracle. If it is the result of chance, it beat incredible odds, even with 100 billion galaxies in the universe.
'Tantalus'
Our situation is anything but perfect, but I am still happy about it anyway.