What if we find nothing in our search for life beyond Earth?

"Even a single positive detection would change everything -- but until then, we need to make sure we're learning as much as possible from what we don't find," said Angerhausen.

The Challenge of Null Results

In science, sometimes, even not finding something can yield important insights. When scientists search for life on exoplanets, they often focus on specific features, such as signs of water or gases like oxygen and methane, that might indicate biological activity. But what happens if scientists don't find any of these features? Can we still learn something meaningful about how common life might be in the universe?

This study shows that if scientists examine 40-80 planets and find no signs of life, they can confidently conclude that fewer than 10-20% of similar planets harbor life. However, this depends heavily on how certain we are about each observation. Such a finding would enable scientists to put a meaningful upper limit on the prevalence of life in the universe, which hasn't been possible to date. Further, if even only 10% of planets in the Milky Way alone have some form of life, that could still be 10 billion planets or more.

"This kind of result would be a turning point," said lead author Angerhausen. "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."

Implications for Future Missions

The findings have direct implications for upcoming missions like NASA's Habitable Worlds Observatory (HWO) and the European-led Large Interferometer for Exoplanets (LIFE). These missions will study dozens of Earth-like planets by studying the planets' atmospheres for signs of water, oxygen, and even more complex biosignatures. According to this study, the number of planets observed will be large enough to draw significant conclusions about the prevalence of habitability and life in our galactic neighborhood. However, the study also points out that even with advanced instruments, these surveys will need to carefully account for uncertainties and biases and derive frameworks to quantify them to ensure their results are statistically meaningful.

Accounting for Uncertainty

One key insight from the study is that uncertainties in individual observations -- such as false negatives (when we miss a biosignature and mislabel it a dead planet) -- can significantly affect the conclusions. For example, if there's a chance that a detection instrument might miss a biosignature, this uncertainty limits how much we can trust any conclusion based on null results. Similarly, if many planets in a survey turn out to be unsuitable for life but were mistakenly included, this skews the findings.

"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," said Angerhausen. "If we're not careful and are overconfident in our abilities to identify life, even a large survey could lead to misleading results."

Asking Better Questions

The study emphasizes that framing the right questions is crucial for meaningful results. Instead of broadly asking "How many planets have life?" -- a question fraught with ambiguity -- it might be better to ask more specific and measurable questions like "Which fraction of rocky planets in the conservative habitable zone show clear signs of water vapor, methane and oxygen?" This approach helps researchers design surveys that confidently detect or rule out specific features.

Why It Matters

Even if future surveys don't find evidence of extraterrestrial life, they will still provide valuable insights into how rare or common habitable conditions are in the universe. By carefully considering uncertainties and asking precise questions, scientists can turn null results into powerful tools for understanding our place in the cosmos.

This work serves as a reminder that science isn't just about finding answers -- it's also about asking the right questions and embracing uncertainty as part of the journey.