This guided-inquiry tutorial is the linchpin of the Habitable Zones DigiKit. It introduces factors that influence average surface temperature of a planet as students work with a rudimentary model using spheres and principles of blackbody radiation. They calculate total power absorbed and radiated per unit area to deduce that equilibrium temperature is not dependent on the radius of the sphere. In Section 2, students consider effects of albedo and greenhouse gases. Finally, they will explore the "faint young sun paradox" -- how could liquid water have been present on an early Earth while the luminosity of the sun was significantly less than it is currently? Mathematics is integrated into each step of the tutorial, which also introduces the Proton-Proton chain through detailed illustrations to enhance understanding of energy conversion within the core of Earth's sun.

Note: This tutorial is intended for upper division courses in physics or astronomy.

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