Why the search for life in space begins with ancient Earth

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They will then use a spectrograph to try and find key molecules such as oxygen or methane. How much they find of each determines what they look for next, such as carbon dioxide or ozone. (Photosynthesis that might occur on other worlds produces oxygen. Oxygen-using organisms typically produce carbon dioxide and water, while some types of microbes, such as bacteria, produce methane.)

Best to rate Everybody of these potential biosignatures, if possible, and not just one. But depending on the range of wavelengths a telescope’s spectrograph is sensitive to, it will be able to measure the abundance of some molecules better than others. Plotting all these paths on Young’s decision tree will tell astronomers whether they’re looking at a world that resembles today’s Earth, or a past version of our planet, or something completely different.

You might be wondering why the search for alien life is so focused on… well, Earth, and not, say, gas giants like Jupiter, or ocean worlds like Saturn’s largest moon, Titan, or its sister moon Enceladus. “It makes strategic sense to look for life as we know it. We only have one example of a habitable planet, despite tantalizing hints here and there,” says Ken Williford, an astrobiologist at the Blue Marbles Space Institute of Science in Seattle.

He is working with NASA’s Perseverance rover, which is looking for signs of past life on Mars and will later head for what scientists believe is the shore of a former body of water. If Mars were like ancient Earth, the remnants of the shallow marine environment could give the rover a chance to unearth the fossilized “microbial mat,” a multi-layered community of microorganisms.

But, inevitably, anyone who follows Young’s flowchart will find some planets that return mixed results: some encouraging signs, but also uncertainty. It’s important to avoid false positives if seemingly life-friendly signatures are actually from non-biological origins, such as methane-generating volcanoes, says Maggie Thompson, an astronomer at the University of California, Santa Cruz, who also presented her work at the astronomy conference at this week.

For example, Titan’s atmosphere is saturated with methane, but is probably lifeless due to low temperatures and lack of water. (Although that’s just “probably.” Titan could be home to some really strange microbes we’ve never seen before, able to survive in methane lakes, feed on acetylene, and breathe hydrogen instead of oxygen. But we won’t know more until NASA sends its Dragonfly rotorcraft for investigation.)

However, methane may still be a key biosignature on more hospitable exoplanets, especially warmer ones with water. “The most interesting thing about methane is that it can be a relatively simple thing that life uses and produces,” says Thompson. The Webb telescope that just spotted it first exoplanet, will prove useful in this endeavour, thanks to a 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 particular biosignature,” she says.

This illustration shows the possible surface of TRAPPIST-1f, one of the recently discovered planets in the TRAPPIST-1 system. Scientists using the Spitzer Space Telescope and ground-based telescopes have discovered that there are seven Earth-sized planets in the system.

Provided by NASA/JPL-Caltech

Young is looking forward to Webb’s successor, the Habitable Worlds Observatory, which will be tasked with searching for signs of life on Earth-sized planets around sun-like stars. (Until now, astronomers have found it easier to find gas giant planets orbiting more dangerous active red dwarfs.) In December, the head of NASA Bill Nelson announced plans to develop the observatory in the 2030s. Depending on how sensitive the new telescope is, Young’s simulations show it could cover dozens of Earth-like worlds.

She also keeps an open mind to life, as we do. not know it. The decision tree includes branches for planets that are unlike any other stage in Earth’s history. “We want to be prepared for surprises, strange occurrences that we can’t classify,” she says. “Let’s put them in the ‘ambiguous planets’ category and mark them as interesting targets.”

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