By studying the atmospheric contents of ancient and present-day Earth, scientists say they've discovered specific chemical combinations that could reveal the presence of biological activity on other planets.
These biosignatures, described in the journal Science Advances, could offer a key tool in the search for extraterrestrial life.
"There's a direct path from the conclusions of our work to the possible discovery, which would be an historic one, of life elsewhere," said senior author David Catling, a planetary scientist and astrobiologist at the University of Washington in Seattle.
Thousands of planets beyond our solar system, known as exoplanets, have been discovered in the last several years, a small number of which appear to be rocky, Earth-sized planets at the right distance from their star to hold liquid water. Studying the ones with detectable atmospheres could provide crucial clues as to whether they host life.
As powerful new telescopes start to come online, researchers are trying to figure out exactly which atmospheric chemicals they should be looking for. After all, just because a planet looks like it has the right ingredients for life doesn't mean there's actually anything living there.
Scientists have focused on a few potentially telltale molecules, such as methane. Methane is produced in large quantities by microbes on Earth (including those in the bellies of cattle). But methane can also be produced by nonbiological sources, such as volcanoes.
Molecular oxygen (two oxygen atoms bonded together) is produced in massive amounts today by photosynthesizing algae, plants and microbes. But the photosynthetic mechanism is so complicated that scientists think it evolved only once on our own planet. That means there's no guarantee of finding oxygen-producing photosynthesis on other worlds, even if life does exist there.
Thus, relying on any individual chemical could produce false positives or false negatives, said study coauthor Stephanie Olson, an astrobiologist and graduate student at the University of California, Riverside. But living things alter their environments in complex ways. What if there was a particular mixture of molecules that would not exist without life?
To find out, Catling's graduate student Joshua Krissansen-Totton led a study that examined the Earth's atmosphere in three stages of its existence: The Archean (4 billion to 2.5 billion years ago), the Proterozoic (2.5 billion to 541 million years ago) and the Phanerozoic (541 million years ago to the present).
During each of these time periods, life (and the planet itself) looked very different. Place a snapshot of each Earthly period side-by-side, and they'd look like totally different planets.