Ask A Scientist© Astronomy Archive

Question:  Do you believe extraterrestrial life forms exist... and why do you
believe this? (Evidence is preferable.)I am a sophomore in Accelerated Chemistry 
class doing a presentation on Tuesday 1/10/95. From my readings, I believe
extraterrestrial life forms exist.
Thanks for your time.
Sobby



Answer:  Actually, sometimes I'm certain they must exist, and sometimes I'm
certain they don't. The evidence that they don't exist is that if they did, we
would probably have seen them do something or communicate with us by now (UFO's
are not believable because the things UFO's do are simply not logical things one  would expect
intelligent aliens to do). But maybe they do exist and they know about us and
they are deliberately ignoring us, or leaving us alone to figure things out for
ourselves.

The big question is, we don't know how likely it is for life to begin. Simple
arguments suggest that it is astronomically unlikely, so there could be only one 
planet in the visible universe with life on it (ours). Other arguments suggest
that life began in simple stages that led very naturally to the later complex li fe forms (the very
early algae on earth) and that life might arise separately in many different
places. We do know (or think we know) that life started only once on earth,
which already makes it more probable that life is generally rare in the universe .
It's possible that life began somewhere else and "seeded" the earth a few
billion years ago - life started here remarkably soon after the earth was ready
for it!
Asmith