Start by asking yourself this question, "Would we even be able to identify extraterrestrial life as life? What if it had a completely different biochemistry from us?" Life at it's most basic is self replicating chemistry. Many people have speculated on what other types of biochemistry life could have other than our familiar carbon based one. Silicon biochemistry? Boron or Sulfur biochemistry? Plasma/dust based amalgamations? Don't even get me started on whether or not we could recognize if extraterrestrial/"nonterrestrial" life was sentient. Try some speculative sci fi just to get a handle on this stuff. "Solaris", "Dragon's Egg", and "The Black Cloud" are all good places to start.