Alien Life, Why not?

Titles can take many forms depending on the writer's preference and style. However, it is probably good practice to keep the form consistent for any particular title. So, in your title, I would suggest making all of the words begin with a capital letter, not just the first three words. Other ways that the title could be written (although not necessarily better than yours) are:
Alien Life, Why Not?
Alien Life? Why Not?
Alien Life? Why Not!
Alien Life ... Why Not!
etc.

I always have thought that if can exist life in our planet, then why not could exist in others.

To make your sentence flow better, you could consider writing it as:
I've always thought that if life can exist on our planet, then why could it not exist on others.
... or even:
I've always thought that if life can exist on our planet, then why can't it exist on others.

Watching diversity of life in Earth, life in other worlds will be even more different and odd.

Try:
Considering the diversity of life on Earth, any life on other worlds could be even more different and odd.

Provably, the first prove of life that we meet it will be in a microorganism form.

Alternatively:
Probably, the first life that we meet will be in microorganism form.

It can be very funny but isn't nothing serious nor credible, to think in the aliens with a human form.

I wasn't sure what you wanted to say here. However, here is a possible alternative:
It's peculiar, and perhaps incredible, to think of aliens taking on a humanoid form.

If humanity last enough, is a matter of time, to find any form of alien life.

Consider:
If humanity lasts long enough, perhaps it is only a matter of time before we find any form of alien life.