Are We Alone? Scientists Now Know Exactly How Many Empty Planets Would Tell Us

ZURICH, Switzerland — In our quest to answer whether we’re alone in the universe, astronomers are preparing for a major advancement: new space telescopes capable of scanning distant worlds for signs of habitability and possibly life itself. But what happens if these ambitious missions find nothing?

A recent study published in The Astronomical Journal explores this scenario, revealing that even a series of negative results from upcoming exoplanet surveys could yield profound insights about life’s rarity in the universe.

The Statistical Power of Zero

The research, led by Daniel Angerhausen of ETH Zurich, tackles a fundamental question in the search for extraterrestrial life: How many planets must we examine before the absence of detections becomes statistically meaningful?

This question isn’t merely academic. With NASA’s Habitable Worlds Observatory (HWO) and the proposed Large Interferometer for Exoplanets (LIFE) on the horizon, scientists are planning missions that will examine dozens of nearby Earth-like worlds. These telescopes represent billions in investment and decades of scientific effort, making the interpretation of their results—even null results—critically important.

The research suggests that absence of evidence can become evidence of absence, given enough observations. Through Bayesian statistical analysis, the researchers found that observing 20-50 terrestrial exoplanets with no signs of habitability would allow scientists to draw significant conclusions about how rare habitable environments might be, regardless of their initial assumptions.

Researchers simulated observational scenarios where scientists survey up to 100 planets. They focused specifically on scenarios with no detections of habitability or life—the “null result” case. Their analysis revealed how many planets would need observation before scientists could make statistically sound inferences about the prevalence of habitable or life-bearing worlds.

If future missions survey 40 Earth-like planets and find no biosignatures, astronomers could conclude with 99.9% confidence that the fraction of terrestrial planets with detectable life is less than 20%. This would reshape our understanding of life’s rarity and potentially point to cosmic “filters” that make the emergence or persistence of life exceptionally uncommon.

For context, while thousands of exoplanets have been cataloged since the first discovery three decades ago, scientists haven’t yet confirmed a truly Earth-like world with conditions suitable for life as we know it. The next generation of space telescopes aims to change that by directly analyzing the atmospheres of nearby rocky planets orbiting in their stars’ habitable zones.

This scientific effort connects to one of humanity’s deepest questions, addressed in the famous Drake Equation, which attempts to estimate the number of communicative civilizations in our galaxy. Two critical terms in this equation are the fraction of habitable planets that develop life and the fraction where intelligence evolves. The missions discussed in this paper may provide the first empirical constraints on these previously speculative values.

Future Missions and Potential Discoveries

Two upcoming missions feature prominently in this research: NASA’s Habitable Worlds Observatory (HWO) and the proposed Large Interferometer for Exoplanets (LIFE). The findings align with the planned capabilities of these future missions. The LIFE telescope concept, in its extended configuration, could potentially characterize around 38 terrestrial planets in the habitable zones of nearby stars. HWO might examine about 20 Earth-like worlds for signs of oxygen or water.

The study confirms these sample sizes, while modest compared to the billions of planets in our galaxy, should be sufficient to provide meaningful statistical constraints on the prevalence of habitable environments and life.

However, these calculations assume scientists can definitively confirm or rule out habitability markers with complete confidence. In practice, interpretation is never so straightforward.

The research reveals that interpretation uncertainty poses a more significant limitation than sample size. If scientists are only 80% confident in each negative observation (a 20% false-negative rate), then even with hundreds of observations, they couldn’t confidently conclude that the frequency of habitable or life-bearing planets is less than 20%.

Life As We Don’t Know It

The findings connect directly to the “Great Filter” hypothesis, which proposes that some developmental stage between simple chemistry and advanced civilization must be extraordinarily difficult, explaining why we haven’t detected alien civilizations despite the apparent abundance of potentially habitable planets.

If future missions survey dozens of suitable planets but find no signs of life, it would suggest that abiogenesis—the emergence of life from non-living matter—might be this filter, an extremely rare event despite suitable conditions. Conversely, if simple life appears common but complex multicellular life or intelligence remains absent, it would indicate the filter operates at a later evolutionary stage.

Such discoveries would have profound implications for humanity’s future. If simple life proves common throughout the galaxy, but technological civilization is unique to Earth, it could suggest that civilization-ending catastrophes are nearly inevitable, painting a sobering picture of our species’ long-term survival chances.