The point is that we severely limit ourselves when seeking possible life outside our planet when we assume that how it happened here MUST BE the only way for it to happen. There are countless alternatives which would each lead to its own set of conclusions.
Are there? Can you name at least one?
Only silicon and carbon have the potential to create chains complex enough.
And only carbon is responsive enough to interact with several other elements...
As long as we dont discover some new elements, life has to be made out of carbon.
And if its made out of carbon, its also likelly that it will need water.
About the numbers of potential earths in the milky way...
Water is way more common in the univers, as the majority thinks.
Other things are way more rare.
The new earth would have to be near a star as large or a bit smaller as our sun (too large and the star would die before life could get created - too small and the earth would have to be so close ti the star that always only one side would show to the star (just like the moon does to our earth))
The star shouldnt be too close to the centre of the milky way (too mutch radiation)
It also shouldnt be too far away from the centre (too less heavy elements, like iron)
The star shouldnt be in an arm of the milky way (too many other stars, the gravity would distroy the system of planets)
At it best would the new earth have a moon - it stables the rotation of the planet
Some large planets would be needed in the sun system (they would catch the majority of comets)
All this would be needed to create intelligent life
, maybe surround the vehicle with an artificial atmosphere similar to earths, so if something did hit it, the small stuff would burn up 
