I dont believe in aliens because, for example, a planet similar to ours capable of sustaining basic life must exist other than ours. But ours is such that if the Earth were any larger and the moon at any different location than it is now or a different size, this planet would not be capable of sustaining life due to tectonic instabilities, gravity problems etc.
As for a species evoling from bacteria on mars or Jupiter is bogus... the universe is constantly decaying, example the sun slowly burning out, the earths magnetic field reducing significantly every thousand years, so how could something already in decay and decline, magically transform it from a static lifeform to a self-aware lifeform? And dont give me this we evoled so they must have evolved bull****... evolution is a theory... a far fetched theory at that, but still a theory, nothing more.