What happens if, after all our searching, we don’t find any signs of life on other planets?
A group of scientists led by Dr. Daniel Angerhausen from ETH Zurich and the SETI Institute has explored this important question.
Their study shows that even if we don’t find life, that result can still teach us a lot about how common—or rare—life might be in the universe.
The research, recently published in The Astronomical Journal, was part of the Swiss research group PlanetS.
The team used a method called Bayesian statistics to figure out how many exoplanets (planets outside our solar system) scientists would need to study in order to make meaningful statements about life elsewhere.
They found that if we carefully examine between 40 to 80 planets and see no signs of life at all, we could conclude with confidence that fewer than 10% to 20% of similar planets may have life.
In a galaxy like the Milky Way, that would still mean billions of potentially inhabited planets—but much less than what some might hope for.
However, there’s a big catch: No observation is ever perfect. When scientists look at planets, there’s always some uncertainty involved.
We might miss signs of life (false negatives) or choose planets that aren’t good candidates in the first place (sample bias).
That’s why it’s not just about the number of planets we study, but also about how we study them and what questions we ask.
Dr. Angerhausen explains that confidence in what we see—or don’t see—is key. If scientists are too sure of their ability to spot signs of life, they could be misled by the data.
A well-designed survey needs to focus on the right types of planets and ask specific questions like, “How many rocky planets in the habitable zone have water vapor, oxygen, and methane?” rather than the much broader “How many planets have life?”
This careful thinking is especially important for future missions like the LIFE project (Large Interferometer for Exoplanets), led by ETH Zurich. LIFE aims to look at dozens of Earth-like planets and search their atmospheres for signs of life-supporting elements such as water and oxygen.
According to the study, LIFE will likely observe enough planets to give meaningful answers about life in our corner of the galaxy—but only if it’s done with a clear understanding of uncertainties and proper sampling methods.
The researchers also compared two ways of analyzing data: the Bayesian method, which includes assumptions based on prior knowledge, and the Frequentist method, which doesn’t. Ph.D. student Emily Garvin helped show that for this kind of research, both methods give similar results when the sample size is big enough.
She believes these methods aren’t in competition—they just offer different ways of understanding the same data.
In the end, even if we don’t find alien life, that result will still be valuable. “A single positive detection would change everything,” says Angerhausen, “but even if we find nothing, we’ll learn just how rare—or common—life really is in the universe.”
