I don't "believe" anything about life on other planets.
From what I know of the relevant sciences, I understand that:
- the evidence indicates that life appeared on Earth almost as soon as it
formed, pretty much as soon as it had cooled enough for the necessary
chemicals to become stable;
- we are starting to detect planets orbiting other stars, suggesting that
planets are common;
- the Earth is a planet of an unremarkable star, of which we can detect
huge numbers in our galaxy;
- we can detect countless numbers of galaxies.
From this information, I conclude that it is likely that there are
staggeringly-huge numbers of planets and that it would be extremely likely
for there to be life on a number of them.
No belief, just seems a lot more than likely.