Answers:
Why is disappointing from a traditional scientific point of view?
Because "Intelligent Design" is merely "Creation Science" wearin a suit instead of the usual hick overalls.
Can I be sure that life started on Earth?
Certainty is not what Science is about. Right now it is by far the most probably hypothesis. This is a borderline area where progress is slow because of the vast barrier of time between the beginning of life and today.
Can I rule out the possibility that an alien race didn't seed this solar system?
Yes. That is simply fantasy. About as likely as the Tooth Fairy. Forget it.
