This guided inquiry tutorial for upper-division physics and/or astronomy introduces students to various factors that influence the average surface temperature of a planet. First, students consider principles of blackbody radiation, using a crude model to deduce the relationship of equilibrium surface temperature of a planet and the intensity of solar radiation from its host star. The effects of albedo and greenhouse gases are then introduced into the conceptual model. Finally, students consider the "faint young sun paradox" -- how could liquid water have been present on an early Earthwhile the luminosity of the sun was significantly less than it is currently? Mathematics is integrated into each step of the tutorial. The tutorial also introduces the Proton-Proton chain through detailed illustrations to enhance understanding of energy conversion within the core of Earth's sun.