Is there life on other planets?
To answer that question, you first have to answer this one: what, exactly, is life? We may have a general sense of life as it exists here on Earth -- one that involves a given object's ability to metabolize and grow, to respond to stimuli, to maintain homeostasis, to reproduce. But that textbook definition of life -- "life," that is, "as we know it" -- is itself entirely defined by the particularities of Earth. It's a set of criteria determined by a planet that happens to be terrestrial, that happens to be covered in salt water, that happens to orbit a yellow dwarf star ... and that happens to be shielded from said star by a protective ozone layer.
Life as we know it, in other words, is entirely contingent on its environment. As the Harvard astronomy professor Dimitar Sasselov puts it: "We have life here on Earth that has its roots in the chemistry of the planet."