Back in Chapter 1, we introduced systems thinking as a way to understand any system, especially complicated ones like planet Earth. We said that three systems questions often help us analyze the system that we are exploring. When we looked at Earth's matter, we mostly used the first systems question- "What are the parts of the system?"

To understand energy in the Earth system, we need to focus on a different systems question. Instead of looking at the parts of the Earth system, we need to ask the third system question- how is Earth itself part of larger systems?

And the answer is as simple as the question- Earth is part of the solar system. The sun provides virtually all the energy to keep our planet warm and sustain life.

Some people have called this third planet from the sun, the Goldilocks planet. In the children's story of Goldilocks and the three bears, she found the chair that was not too big or not too small and she ate the porridge that was not too hot or not too cold. Earth is not too close to the sun, not too far, not too hot, and not too cold. Earth is just right.