Lyons’ own research takes a similar perspective to Young’s: He investigates whether an alien astronomer observing Earth could have correctly discerned that it’s a life-friendly place. That means inferring the contents of our atmosphere throughout the past 4 billion or so years that our world has hosted life, and then determining whether the levels of biologically derived gases would be detectable from space. (Another team has tried to suss out whether beings on other planets could spot us by using the technique we use to find rocky exoplanets in other solar systems. With the right vantage point, these researchers think, aliens could detect Earth as it transits in front of the sun, briefly dimming its light and offering a clue to our presence.)
Left, an image of Earth from the DSCOVR-EPIC camera. Right, the same image degraded to a resolution of 3 by 3 pixels, similar to what researchers will see in future exoplanet observations.
Courtesy of NOAA/NASA/DSCOVR
Currently, when evaluating the life-friendliness of an exoplanet, scientists first examine its host star to make sure it’s not spewing lots of stellar flares. Then they check its orbit, to assess whether it’s stable and in a “Goldilocks zone” that’s neither too hot nor too cold to allow liquid water on the surface.
Then the harder part begins. With Young’s decision tree, astronomers would try to see whether there’s a significant amount of water vapor in the atmosphere—a sign there’s actually water down below. That means using a spectrograph, like the one carried on the JWST, to scan a planet’s atmosphere at infrared wavelengths.
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.
