why do we call other living organisms on a different planet, aliens?

Question: why do we call other living organisms on a different planet, aliens?
- Keywords:
  alien,
  organism,
  planet
Asked by nisma to Artem, Caroline, John on 21 Jun 2012. This question was also asked by hepworth99.
- Artem Evdokimov answered on 21 Jun 2012:
  
  The study of alien biology is typically referred to as xenobiology or exobiology. Right now it’s a science without a practical subject (unless Area 51 rumors are true and there are alien corpses on ice somewhere already).
- John Short answered on 22 Jun 2012:
  
  We would have to call them something, but mostly because they did not originate on Earth. They most likely have a different physiology/ DNA biochemistry etc, although the differences may or not be not be that great depending on how they evolved.

Question: why do we call other living organisms on a different planet, aliens?