At first glance, the search for extraterrestrial intelligence may seem like a purely scientific task that requires a clear mathematical and physical approach. However, if you look more broadly at this search, you can see that much of it concerns philosophical issues that force you to look at various phenomena that a person encounters on the way to search for extraterrestrial intelligence in a different way. One of these things is our attitude to the “null” result. What if, despite extensive efforts, humanity fails to find any signs of life beyond Earth?
A research team led by Dr. Daniel Angerhausen — a physicist from ETH Zurich’s Institute for Particle Physics and an affiliate of the SETI Institute — explored this scenario. Their study, published in The Astronomical Journal, investigates what a lack of detected life could reveal about the prevalence of life in the cosmos. The research employs Bayesian statistics to determine how many exoplanets must be studied to draw meaningful conclusions about the rarity or commonness of habitable worlds.
The study suggests that if 40 to 80 Earth-like exoplanets were examined without any signs of life, scientists could confidently state that fewer than 10–20% of such planets host life. Extrapolated to the Milky Way, this would mean roughly 10 billion potentially inhabited planets — still a staggering number, but far fewer than some optimistic estimates. This approach would finally allow researchers to establish an upper boundary for life’s prevalence in the universe, a question that has remained unanswered.
However, a “perfect” null result comes with complications. Every observation carries uncertainty, which can affect the reliability of conclusions. Some of the most important types of uncertainty that are worth paying more attention to are the risk of false negatives, where a planet with life is mistakenly classified as lifeless due to undetected biosignatures, and if the studied exoplanets are not truly representative, such as including planets that don’t meet basic habitability criteria.
These insights are crucial for upcoming projects such as the “Large Interferometer for Exoplanets (LIFE)“, an ETH Zurich-led initiative aiming to analyze the atmospheres of dozens of Earth-like exoplanets for water, oxygen, and other biosignatures. The study confirms that LIFE’s planned observations will be sufficient to make statistically significant claims about life’s prevalence in our galactic vicinity.
The researchers emphasize that precise, measurable questions — such as “What percentage of rocky planets in habitable zones show signs of water vapor, oxygen, and methane?” — are more effective than vague inquiries like “How many planets host life?”
The study underscores the importance of correct interpretation of the obtained (or missing) results, well-defined research questions, robust methodologies and careful sampling strategies. This research not only refines the search for extraterrestrial life, but also provides a statistical roadmap for future missions, ensuring that even null results contribute meaningfully to science.