This graph is from the grade 5-8 lesson Where To Look For Life?, which addresses the possibility of abodes of life in the Solar System. The lesson contains two inquiry-based activities, the first of which—Happy Places— explores temperatures across the Solar System.

Students first explore what they know about the behavior of an object’s temperature as it is placed at different distances from a heat source. They then extrapolate the behavior to a small black rock (a physicist calls it a black body) at different distances from the Sun as a means of predicting how temperature varies from planet to planet.

Students also explore the fundamental requirements for life, which includes the presence of liquid water, the medium in which biochemistry takes place.

This graph is generated by the students, and appears in the teacher answer key. The curve represents the temperature of a black body at different distances from the Sun. The horizontal lines define the temperature range for liquid water on the surface of Earth, 0-100°C (32-212°F). Liquid water can exist at lower temperatures than 0°C if the water has a high salinity, and at temperatures higher than 100°C if under high pressure, e.g., at ocean depths. But the observed temperature range for life as we know it is very close to 0-100°C. The space between the horizontal lines therefore represents the ‘happy place’ for life as we know it.

A look at the curve seems to indicate that only at Earth’s and Venus’s locations from the Sun are temperatures in the happy place. Students then plot the actual observed range in temperature for each planet, which are the vertical bars on the graph, and find that a planet does not behave like a rock at that distance from the Sun. In fact Venus is far hotter than predicted, and Mars can actually attain temperatures in the ‘happy place’. Students learn that actual temperatures reflect things like greenhouse warming, rotation rate, and thickness and composition of atmosphere. The conclusion—there may be many happy places across the solar system—many abodes of life—due to the complexity of worlds.