This is a lesson about characteristics necessary for life. Learners will identify the top candidates for life in the solar system by examining Habitability Cards, which discuss each planet and the six large moons in terms of water temperature, atmosphere, energy, and nutrients. A math extension is provided on the Inverse Square Law. Includes background reading for teachers, student activity guide, reflection questions, and blackline masters. This is activity 3 of 5 in the educators guide, Astrobiology in your Classroom: Life on Earth..and Elsewhere?