Why the Search for Life in Space Starts With Ancient Earth

Next, they’ll use the spectrograph to try to find key molecules like oxygen or methane. How much they find of each determines what they’ll look for next, like carbon dioxide or ozone. (Photosynthesis, which could arise on other worlds, produces oxygen. Organisms that use oxygen typically produce carbon dioxide and water, while some kinds of microbes, like bacteria, produce methane.)

It’s best to estimate all of these potential biosignatures, if possible, and not just one. But depending on the wavelength range a telescope’s spectrograph is sensitive to, it will be able to measure the abundance of some molecules better than others. Charting all these paths on Young’s decision tree will tell astronomers whether they’re looking at a world resembling modern Earth, or a past version of our planet, or something else entirely. 

You might be wondering why the search for alien life is so focused on … well, Earth, rather than, say, gas giants like Jupiter or ocean worlds like Saturn’s largest moon, Titan, or its sibling satellite, Enceladus. “Strategically, it makes sense to look for life as we know it. We only have one example of an inhabited planet, despite tantalizing hints here and there,” says Ken Williford, an astrobiologist at the Blue Marble Space Institute of Science in Seattle. 

He works with NASA’s Perseverance rover, which is searching for signs of past life on Mars and will later be headed for what scientists think is the shore of a former body of water. If Mars was anything like ancient Earth, the remnants of a shallow marine environment could give the rover a shot at digging up a fossilized “microbial mat,” a layered community of microorganisms.

But, inevitably, anyone who follows Young’s flowchart will find some planets that return ambiguous results: some encouraging signs but also uncertainties. It’s important to avoid false positives, if the apparent life-friendly signatures are actually due to nonbiological origins, such as methane-generating volcanoes, says Maggie Thompson, an astronomer at UC Santa Cruz who also presented her work at the astronomy conference this week. 

For example, Titan has an atmosphere smogged with methane, but it’s probably lifeless, thanks to its frigid temperatures and lack of water. (That’s just a “probably,” though. Titan could conceivably host really weird microbes we’ve never seen before, capable of surviving in methane lakes, eating acetylene, and breathing hydrogen rather than oxygen. But we won’t know more until NASA sends its Dragonfly rotorcraft to investigate.)

Nevertheless, methane could still be a key biosignature on more hospitable exoplanets, especially warmer ones with water. “The exciting thing about methane is that it could be a relatively simple thing that life uses and produces,” Thompson says. The Webb telescope, which just spotted its first exoplanet, will prove useful in this endeavor, thanks to its near-infrared spectrograph. “Methane is one of the few gases that JWST can actually detect, but JWST alone probably won’t find a planet with a definitive biosignature,” she says. 

Young’s looking ahead to Webb’s successor, the Habitable Worlds Observatory, which will be tasked with searching for signs of life on Earth-size planets around sun-like stars. (So far, it has been easier for astronomers to find gas giant planets orbiting more dangerously active red dwarf stars.) In December, NASA chief Bill Nelson announced plans to develop the observatory in the 2030s. Depending on exactly how sensitive the new telescope is, Young’s modeling shows that it could scope out dozens of Earth-like worlds.

She’s also keeping an open mind for life as we don’t know it. The decision tree includes branches for planets that don’t seem to resemble any stage of Earth history. “We want to be prepared for surprises, the weird cases that we might not be able to categorize,” she says. “Let’s put those in the ‘ambiguous planet’ category, and flag them as interesting targets.”