Discover Magazine has a great article summarizing the evidence for life on Mars. It’s long because it goes into a lot of detail examines quite a variety of possibilities, but it ties quite nicely in with the questions we are asking about what is life and where it can be found.
The article also mentions a study titled, “The Limits of Organic Life in Planetary Systems” produced by the National Research Council for NASA. It can be found for free online, with an associated podcast that much more accessible to middle schoolers. The report is extremely open to the possibility that extra-terrestrial life exists, and could exist even without water and might even be silicon based.