We Keep Coming Up Empty in the Search for Alien Life. Here’s What That Could Mean—and What Comes Next

Humans have been pondering the existence of life beyond Earth since ancient times. In the 1900s, scientists began actively searching for proof. Needless to say, we still haven’t found any. Although it’s only been a number of decades and technology is advancing every day, what if we continue to come up empty-handed?

In a study published Monday in The Astronomical Journal, an international team of researchers tackled this question by arguing that what we don’t find can still be deeply informative. The study suggests that identifying a certain number of lifeless exoplanets could enable scientists to confidently determine whether life beyond Earth is common or rare.

Specifically, the researchers used advanced statistical modeling to demonstrate that if no evidence of life is found in 40 to 80 exoplanets, we can infer that fewer than 10 to 20% of similar planets in the universe host life. While this might not seem that impressive, it would represent an upper limit for the prevalence of potential life beyond Earth—a limit we currently lack. But that said, even 10% of the Milky Way’s planets adds up to at least 10 billion worlds.

“This kind of result would be a turning point,” Daniel Angerhausen, lead author of the study and an astrobiologist at ETH Zurich, said in a SETI Institute statement. “Even if we don’t find life, we’ll finally be able to quantify how rare—or common—planets with detectable biosignatures really might be.”

Biosignatures are indicators of biological processes and, therefore, potential signs of life as we know it. It’s also worth noting that all planet-like bodies outside of our solar system are technically called exoplanets, whereas planets refer to the eight orbiting our Sun (though this might change soon). Some scientists, however, nevertheless refer to exoplanets as planets for simplicity’s sake.

The study’s results are particularly significant in the context of forthcoming missions aimed at investigating biosignatures on Earth-like exoplanets, such as NASA’s Habitable Worlds Observatory (HWO) and the European-led Large Interferometer for Exoplanets (LIFE). These missions might survey enough exoplanets to reach the kind of conclusion outlined in the study.

The big caveat with Angerhausen and his team’s statistical modeling, however, is that it is largely dependent on the accuracy of scientists’ individual observations. Errors such as false negative detections could significantly skew the broader results. Even the possibility of an instrument missing a biosignature would undermine the reliability of claims regarding the prevalence of extraterrestrial life.

“It’s not just about how many planets we observe—it’s about how confident we can be in seeing or not seeing what we are searching for,” Angerhausen, who works with the SETI Institute, explained. “If we’re not careful and are overconfident in our abilities to identify life, even a large survey could lead to misleading results.” In other words, upcoming surveys should carefully consider uncertainties and biases in their approach.

Ultimately, the researchers underscore the fact that all scientific results—even seemingly disappointing ones—can teach us valuable lessons if we accept uncertainty and pose the right questions.