Given the existence of life in such extreme environments on Earth, the likelihood of microorganisms on other worlds, even within our solar system, is a likely possibility. However, the chances of there being more complex life out there become much more slim, given the precise conditions required for a planet to sustain complex life forms.
The Earth is within a narrow range of distance from the Sun that permits liquid water to exist in most places. Also, the stable rotation and nearly circular orbit disallows extreme temperature differentials in a given place on Earth. The Earth has an atmosphere that regulates temperature and prevents an extreme margin in temperature between day and night for a given location. Lastly, the Earth contains an abundance of elements necessary for life, at least by our understanding, to exist. These are plentiful amounts of carbon, oxygen and nitrogen.
But as I said before, the conditions necessary for simple life forms are not difficult to achieve. But despite the extremely slim chances of complex, albeit sentient life in the universe, the sheer scale of the universe gives those chances a significant boost.
