Astronomers have located over 3,500 planets orbiting stars outside of our solar system. But determining — from a distance — which of those planets actually shows signs of life is going to take a lot of time, effort, A.I., and cautious deductive reasoning. That’s why researchers at the NASA-funded Alternative Earths Astrobiology Center have started putting together their own field manual on what to look for — essentially, a planet-watcher’s guide to finding alien life in the galaxy.

“We’re moving from theorizing about life elsewhere in our galaxy to a robust science that will eventually give us the answer we seek to that profound question: Are we alone?” NASA exoplanet scientist Martin Still, Ph.D. said in a statement released Monday. The core of that science, outlined in a set of five Astrobiology papers written by scientists with NASA’s Nexus for Exoplanet System Science (NExSS), is based on what we already know about the atmospheric and surface-level dynamics on Earth during its 4.5-billion-year history. This, in turn, shapes the “biosignatures” that future research will look for on exoplanets.

This is a still from Hubblecast 102. More information: www.spacetelescope.org/images/ann1712a/ Credit: ESA/Hubble, NASA
Scientists have put together an 'Alien Playbook' -- a guide for finding extraterrestrial life.

In a paper published this month in the journal Astrobiology, Edward Schwieterman, Ph.D., a researcher at the Alternative Earths Astrobiology Center, laid out three main categories of biosignatures. The first is gaseous biosignatures, referring to the abundant presence of key compounds in an exoplanet’s atmosphere — like oxygen, methane, and nitrous oxide — that might convey the presence of some respirating organic activity on the planet. These gases, like all chemical compounds, absorb certain parts of the electromagnetic spectrum and bounce some of it back; nitrous oxide might sound like the odd molecule out here but, in addition to not having many sources on Earth outside of living organisms, it also puts out some readily detectable signals from the electromagnetic spectrum.

The second category is surface biosignatures, which refers to changes in what the surface of the planet reflects back due to a large population of organisms undergoing photosynthesis. From space, Earth has a so-called “red edge” due to plants absorbing red light but bouncing back some unused infrared light, which Schwieterman and his group suspect will also be true of exoplanets with (what I guess you can call) exoplants. The last set of tell-tale signs, according to the paper, are temporal biosignatures, which refers to evidence of seasonal changes in the atmospheric make-up or the surface biosignatures, either of which might suggest that living organisms on the planet are changing their activity in sync with their planet’s climate, like we do.

Spotting these biosignatures sounds pretty striaghforward, but it’s in confirming their authenticity where things get complicated. “The search for life using biosignatures is not as simple as looking for a single molecule or compound,” Schwieterman says. “Atmospheric oxygen, for example, could be a sign of life, but there are many nonbiological ways that oxygen gas could be produced on an exoplanet.” This is what you’d call a ‘false positive’ — an exoplanet that looks like it has life but might actually just be a barren, rusty junkyard with a lot of oxygen floating around. To make matters more complicated, there could be ‘false negatives’ too. Plenty of living species, some even living here on Earth near undersea thermal vents, don’t produce oxygen at all. How do we search for life out there that does not resemble anything that’s appeared on Earth in the past couple of years?

Dealing with that problem is why Schwieterman’s guide puts such a big emphasis on temporal biosignatures, which he says “might be more robust biosignatures in some circumstances.” In other words, life — however it’s formed — changes, moves, and generally does dynamic stuff. We’ll see those changes if we look long enough, and so might alien life, if it’s looking back.