As Earth’s plates move over time, they trigger the carbon–silicate cycle, which gradually alters the amount of carbon dioxide present in the atmosphere. Over extended geological timeframes, tectonic activity locks away carbon dioxide within rock formations. “At some point, enough carbon dioxide will be drawn from the atmosphere so that photosynthesis will stop working,” said Scherf. They found that if a planet’s atmosphere contains 10% carbon dioxide, photosynthesis could be sustained for up to 4.2 billion years. Continued null results from SETI would strengthen the case that the existence of other technological civilizations is extremely rare.