This is the point

a) spetroscopic analysis of even the most remote star systems demostrate that they are like our sun
No unknown elements has never been found in the thousands meteorits falling every year on our earth and coming from the deep space
ammino acids have been found in meteorits

b) As ChrisB said life can be based only on carbon
A complex organism must be made of different kind of tissues
These tissues must be reasonably stable but not that stable to allow the complex bio chemical process
Only carbon has this property

If you put a and b togheter it reasonable to assume that the aliens , if they exist, are like us , at least as far as their chemistry is concerned

Of course their shape, color ,dimensions , might be quite different

Beside also their planet must be like our earth for evident reasons