system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do scientists commonly say that a planet cannot support life, if we don't know what type of life we're looking for?

When people say that a planet cannot support life, it's interpreted that it's human life that can't exist, but if we were truly looking for alien life then surely we don't know what conditions are required, so we could be overlooking planets that could posses life. Is it just a method of searching for life based on what we already know, or something else?

Edit: There have been some great answers and explanations, Many thanks to everyone!
gpt
>Is it just a method of searching for life based on what we already know

Pretty much. Our understanding of the development of life on Earth is that it required the presence of liquid water (and some other relatively common trace elements). Although forms of life exist which live outside of environments with liquid water, they all originated from the first cell. They are also almost exclusively small and boring (and their cell is still water filled).

Given that we don't know of any other way life could start, we discount planets which orbit either too far or too close to their host star for liquid water to exist on their surface. We also focus on rocky planets, although a gas giant in the "habitable zone" would almost certainly have some rocky moons which might have water. There is a growing number of candidate planets which are in, or on the edge of, the habitable zone. With a suitable greenhouse effect, there could well be liquid water on some of them (the habitable zone is therefore a somewhat fluid term).

There's no reason that life couldn't originate elsewhere as you said, but we don't know what to look for so we don't make any assumptions/statements about planets out of the hospitable zone being capable of supporting life.