Are we alone in the universe? The search for extraterrestrial life in four stages

Little green men in a flying saucer, the ET calling home, hideous monsters bursting from people's chests: Pop culture is home to countless aliens. Science, however, has yet to discover any. But the search for them is in full swing.

The field of research that investigates the existence of extraterrestrial life is astrobiology, and it's constantly making headlines.

This spring, a report from researchers at the University of Cambridge attracted worldwide attention. Using the James Webb Telescope, they had studied the exoplanet K2-18b. An exoplanet is a planet orbiting a star other than the sun. The researchers received an unusual signal from K2-18b. In the exoplanet's atmosphere, they found a sulfur compound that is produced only by living organisms on Earth. K2-18b could be an aquatic world teeming with life, speculated the study's lead author.

In the weeks that followed, however, other research groups significantly downplayed this spectacular announcement. Some suggested that the signal could indicate a completely different molecule. Others even found only meaningless noise in the data.

The observations of the researchers from Cambridge are therefore still far from providing evidence of extraterrestrial life.

How far exactly? And what would have to happen for an observation to be considered evidence of life on other planets? NASA designed a scale for this purpose in 2021. It resembles a staircase. The further you climb the staircase, the more certain you are of life.

Currently, humanity is still at the foot of the stairs. To be able to speak of aliens, we would have to climb at least the first four steps. What this might look like in concrete terms can be clearly illustrated by the example of the controversial sign of life from the exoplanet K2-18b.

The exoplanet K2-18b is 124 light-years from Earth and orbits a star in the sign of Leo. If life does exist on its surface, we could only detect it indirectly: through the traces it leaves in the planet's atmosphere.

Every time K2-18b passes on its orbit exactly between its star and Earth, the light from its star shines through the planet's atmosphere to us. The gases in the atmosphere then leave a kind of fingerprint in this light.

The James Webb Telescope has captured the light from K2-18b. But the signal is very weak. Depending on how you analyze it, you might recognize it as a fingerprint—or just a statistical outlier. More observations of the planet are needed to interpret the data more clearly.

Even better would be data from a new generation of telescopes. Unlike the James Webb Telescope, for example, the Extremely Large Telescope is explicitly designed to search for signs of life on distant planets. It is scheduled to be built in Chile by 2030.

Such new telescopes could examine many more planets for signs of life than before. And they would provide significantly better data, in which the fingerprints of gases would be more clearly visible. If such a clear signal were to reach us, the first step toward detecting extraterrestrial life would have been taken.

Once a robust signal has been detected, the fingerprint must be correctly assigned. Just like with humans, the fingerprints of gases are all different, but can look similar.

Astrobiologists are particularly interested in gases that indicate the presence of life forms. These are called biosignatures. One possible biosignature is the sulfur compound dimethyl sulfide (DMS). On Earth, it is produced exclusively by living organisms such as phytoplankton. The researchers from Cambridge believe they have found precisely this molecule on the exoplanet K2-18b. However, so far, the signal is so unclear that it cannot be definitively attributed to DMS.

There are many other gases that could, in principle, be considered biosignatures. In Earth's atmosphere, for example, the combination of oxygen and methane would probably stand out. Astrophysicist Sara Seager is a professor at the Massachusetts Institute of Technology (MIT) and a pioneer in the study of exoplanets. She says: "We don't yet agree on what constitutes good biosignatures. But it's exciting to be the first generation to argue about it."

If we could clearly assign the fingerprint in the light of the exoplanet K2-18b to a biosignature gas, this would mean taking the second step.

Not too hot, not too cold, a rocky surface and a gaseous envelope: the conditions on Earth are apparently well suited for the development of life.

But is life also possible on K2-18b? The exoplanet is seven to ten times as massive as Earth and orbits its star at a much closer distance. K2-18b most closely resembles a small Neptune.

Accordingly, it's difficult to make definitive statements about its habitability. While there are calculations suggesting that the surface of K2-18b could be covered by oceans full of water, it's equally possible that the surface consists of liquid lava , or even that there is no solid surface at all . Life would be difficult to imagine on such a planet. Based on the limited data available from K2-18b, none of these scenarios can be proven so far.

Some planned missions therefore focus on searching for planets that are very similar to Earth. "With an Earth twin, we can be much more certain about habitability," says Seager. With such planets, the third step would be a piece of cake.

Just because a certain substance is only produced by living organisms on Earth doesn't mean that's the only possible way. Conditions on alien planets may be completely different from those on Earth. It's possible that the same gases could be produced entirely without the intervention of living organisms.

This could be precisely the case with DMS, the sulfur compound that researchers discovered on K2-18b. The substance has also been found on a comet in our solar system. "It's pretty clear that life can't exist on this comet," says Susanne Wampfler, an assistant professor of astrophysics at the University of Bern and involved in the comet study. "So, in the universe, these molecules can arise through non-biological means."

To date, we still know very little about the chemistry of alien planets. This makes the step to the fourth level particularly difficult.

The path up the stairs is long and arduous. But there are situations in which a signal could theoretically climb several or even all of the steps at once. This would be the case if we discovered traces of a technological society: radio signals, for example, or the remains of a spaceship.

Experts call these things technosignatures. "They're obviously much less likely to be found," says Wampfler. "But they would be relatively unambiguous."

Instead of hoping for such extremely unlikely scenarios, astrophysicist Sara Seager has a different plan: She's banking on finding life in our own solar system. "Only if we take a sample from a planet or a moon and directly see cells in it can we be sure we've discovered life," she says. Such direct sampling isn't possible for exoplanets many light-years away.

That's why Seager is now leading a mission to send an unmanned spacecraft to Venus. Researchers have already detected a faint signal of a possible biosignature in the atmosphere there. Seager and her team suspect that bacteria could be living in Venus' clouds—even though the clouds are made of toxic sulfuric acid. More detailed on-site investigations should now determine whether this is a false alarm.

The results of this research could also influence the search for life outside the solar system. If bacteria exist under the seemingly hostile conditions on Venus, many more exoplanets could be considered candidates for extraterrestrial life. It could also be an indication that life is far more common in the universe than previously thought.

While finding a microbe on Venus may sound less spectacular than the alien encounters in most science fiction films, proving that life isn't confined to Earth would revolutionize our worldview.

An article from the « NZZ am Sonntag »