Earth went on to become a wet, mild world ripe for hosting life. "You really need to have a way to create climate stability for life to get started," says Michael Way, a physicist at NASA's Goddard Institute for Space Studies. On Earth, plate tectonics, which cycles carbon in and out of the atmosphere, make that stable climate possible. When volcanoes erupt, they shoot carbon dioxide out from the planet's interior. This greenhouse gas traps heat, keeping Earth toasty enough to support life. If carbon dioxide were allowed to build up, you'd get a hothouse like Venus. However, the Earth slowly recaptures carbon dioxide when it dissolves in rainwater, flows into the ocean, and is used to build carbonate rocks like limestone on the seafloor. As pieces of Earth's outer shell shift and grind together, they carry carbon back into the mantle.