No Sign of Alien Life? That Alone Can Still Reveal How Rare It Is

If humanity spends decades building advanced telescopes to search for extraterrestrial life but finds nothing, would those efforts be in vain? Not necessarily, according to a recent study from ETH Zurich (Swiss Federal Institute of Technology). The research suggests that even a lack of evidence can yield valuable insights into how common—or rare—life may be across the universe.

Using statistical modeling, the researchers analyzed how many exoplanets would need to be studied to meaningfully assess the prevalence of life beyond Earth. Scientists typically search for biosignatures such as water vapor, oxygen, or methane in planetary atmospheres. The study found that if none of these markers are detected in 40 to 80 Earth-like planets, it could be reasonably concluded that less than 10% to 20% of similar planets host life. This represents the first attempt to set an upper limit on the likelihood of life in the cosmos. Even if only 10% of planets in the Milky Way support life, the number could still exceed 10 billion.

The findings have implications for upcoming missions like NASA's Habitable Worlds Observatory (HWO) and the European-led Large Interferometer for Exoplanets (LIFE), both of which aim to analyze the atmospheres of dozens of terrestrial exoplanets. However, the study cautions that observational uncertainties—such as instruments failing to detect faint biosignatures—could affect the reliability of conclusions. A rigorous statistical framework is therefore essential.

The researchers emphasize the need to refine the questions scientists ask. Rather than broadly inquiring, "How many planets have life?" they suggest focusing on more specific questions, such as, "What percentage of rocky planets in the conservative habitable zone show signs of water vapor, methane, and oxygen?" This targeted approach could make future research more effective.

Even if extraterrestrial life remains undiscovered, these investigations can still shed light on how common habitable conditions are in the universe. Scientific progress lies not just in finding answers—but in asking the right questions and embracing uncertainty along the way.