“What if extraterrestrial life is not found?” While it may not be the ideal outcome of our current searches for signs of life elsewhere in the cosmos, it could nonetheless lead to surprising new insights and help fine-tune future searches, according to an international team of researchers in a new study.
The research was led by Dr. Daniel Angerhausen, an ETH Zurich professor and SETI institute affiliate, who had pondered what could be gleaned from surveys that fail to detect life in the cosmos. Through complex statistical analysis, Angerhausen and his team worked to develop how such a result could establish new baselines in the number and types of worlds to be studied.
One of the study’s central findings is that future researchers could use non-detections to place upper limits on the probability that certain types of planets host life. For example, if scientists investigate 40 to 80 exoplanets and find no signs of life, they could reasonably conclude that similar planets have less than a 10 to 20 percent chance of being inhabited.
Even that small percentage would still leave a substantial number of potential candidates. At a 10 percent rate, current models suggest the possibility of ten inhabited planets.
The ‘LIFE’ Mission
A major challenge in interpreting such data is that observations always carry some uncertainty. Missing a biosignature could lead to a false negative, while including planets lacking necessary features for life could result in misleadingly low detection rates.
“It’s not just about how many planets we observe – it’s about asking the right questions and how confident we can be in seeing or not seeing what we’re searching for,” says Angerhausen. “If we’re not careful and are overconfident in our abilities to identify life, even a large survey could lead to misleading results.”
Identifying those “right questions” is especially important as new life-seeking missions prepare for launch in the coming years. ETH Zurich is leading the Life Interferometer for Exoplanets (LIFE) mission—a proposed five-to-six-year endeavor to detect life in the cosmos that has been in development since 2017. LIFE will focus on Earth-like exoplanets with similar mass, radius, and temperature, analyzing their atmospheres for water, oxygen, and other biosignatures.
The spacecraft will carry a nulling interferometer, designed to cancel out dominant signals and reveal faint biosignatures in the mid-infrared spectrum, where key molecules have distinct spectral features. The planned observations will provide a large enough sample size to allow astrobiologists to begin drawing more concrete conclusions about the prevalence of life in our galaxy.
Asking Questions and Reading Results
The authors emphasize that even with advanced instruments, statistical rigor and awareness of uncertainty and bias are essential for producing meaningful results. The team recommends asking specific, measurable questions, such as, “Which fraction of rocky planets in a solar system’s habitable zone show clear signs of water vapor, oxygen, and methane?” instead of, “How many planets have life?”
Additionally, the team assessed whether assumptions from previous knowledge would affect surveys. They performed this analysis by comparing the outcomes of a Bayesian statistical analysis, accounting for prior learning, to a Frequentist approach, not doing so. The results in a sample size about what researchers expect from LIFE found very similar results between both methods.
“In applied science, Bayesian and Frequentist statistics are sometimes interpreted as two competing schools of thought. As a statistician, I like to treat them as alternative and complementary ways to understand the world and interpret probabilities,” says co-author Emily Garvin.
Planning Future Missions
“Slight variations in a survey’s scientific goals may require different statistical methods to provide a reliable and precise answer,” added Garvin. “We wanted to show how distinct approaches provide a complementary understanding of the same dataset, and in this way present a roadmap for adopting different frameworks.”
The study highlights the importance of refining methodology when exploring the cosmos by interrogating the data and the very questions guiding the analysis. As humanity searches for a potential needle in the haystack of extraterrestrial life, scientists must maintain a tightly focused approach to avoid overlooking the faint signals that could mark a groundbreaking discovery.
“A single positive detection would change everything,” says Angerhausen, “but even if we don’t find life, we’ll be able to quantify how rare – or common – planets with detectable biosignatures really might be.”
The paper “What if We Find Nothing? Bayesian Analysis of the Statistical Information of Null Results in Future Exoplanet Habitability and Biosignature Surveys” appeared on April 7, 2025, in The Astronomical Journal.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.