With the appearance of the first planets, the Universe became much more complex. Planets, which formed from clouds of chemicals created during the death of stars, are more chemically diverse than the stars from which they came. Planets also differ from stars because they don’t generate huge amounts of energy at their centers. As a result, their surface temperatures are much cooler. This combination of diverse chemical ingredients and less violent conditions enabled planets to become the setting for life. When life emerged on Earth, it had characteristics that made it very different from nonliving things. Many people will tell you they “know” the difference between living and nonliving things. However, biologists – the experts – have struggled to agree on a single definition of life, even though many of the proposed definitions share similarities. In this lesson, we’ll focus on understanding the characteristics all living things share.