“We may find it before 2030. The James Webb Space Telescope (currently slated for launch in two years) and the Wide-Field Infrared Survey Telescope (scheduled for launch in the mid-2020s) will both be able to observe the spectra of exoplanets. Finding an earth-like or super-earth planet with free oxygen in its atmosphere would be a pretty compelling argument for life. Oxygen is highly reactive. Unless it’s constantly being generated, it will soon (in planetary terms) become bound up in oxides in the planet’s crust.

This planet did not have an oxygenated atmosphere when it formed. Life evolved here in a reducing atmosphere. The first oxygenating photosynthetic life gave us oxygen, and also exterminated almost every living organism on the planet—oxygen, being highly reactive, tends to kill anaerobic organisms.

This event in Earth’s history is often called the Oxygen Catastrophe. Wikipedia, in a bizarre fit of political correctness, calls it the “great oxygenation event”—but I digress. Point is, an oxygenated atmosphere isn’t natural without life. If all life on earth died, the earth’s atmosphere would quickly (again in planetary terms) lose all its oxygen. So if we see oxygen in the air on another planet, odds are very good we’ve seen extraterrestrial life. This next decade is going to be a very exciting time, a very exciting time indeed. What an extraordinary, splendid time to be alive!”