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Since the 1990s, scientists have discovered hordes of planets outside the solar system together with tantalising hints of life — or more accurately hints of hints of life. So far, however, there exists no proof that there’s life anywhere in the universe except on the earth.Does that mean our quest for alien life has failed?A team led by researchers at the Institute for Particle Physics and Astrophysics at ETH Zurich, in Switzerland, recently offered a more nuanced answer to this question — one reminiscent of an important attitude to have when doing research at the cutting edge.Writing in a recent paper in The Astronomical Journal, the team contended that a “no signs of life detected” conclusion can also offer valuable information to guide and refine future exoplanet studies. More broadly, the team emphasised the importance of recognising that every observation carries with it a degree of uncertainty and that it is important to ask the right questions.Nothing is somethingWith the ultimate goal of assessing the habitability of exoplanets and finding potential signs of life, researchers have used a statistical method called Bayesian analysis. “It’s a way of updating our understanding or beliefs based on new evidence,” Daniel Angerhausen, a scientist in the Department of Physics at ETH Zurich and lead author of the new paper, said.This is like making a first guess based on what one already believes, then fine-tuning it. For example, you could start by assuming life is very common in the universe. When you observe a hundred exoplanets without finding signs of life, you adjust your guess to accommodate factors that might explain how life can be common yet not found on these worlds. As you continue this process over time, your answer to “how common is life?” acquires a more informed shape.In the new paper, the team explored how different starting assumptions affect final estimates of how common life might be.The researchers simulated observations of 100 exoplanets, ranging from 1 to 100, to determine the minimum number of exoplanets that must be examined to conclude how many worlds are possibly habitable.Their work suggested that if scientists examine between 40 and 80 exoplanets and find no evidence of life, they can confidently conclude that fewer than 10% to 20% of similar planets are likely to support life. That is, life would be relatively rare.If the prevalence of life is indeed low, around 10-20%, it would be understandable for no signs of life to be found in a sample of 40-80 planets. But if life was more common, scientists should expect to observe some indications of it in that same sample. At least, this is the team’s argument.Need for better questionsThis key finding suggests the number of planets observed so far may suffice to establish an upper limit on the number of potentially habitable worlds. However, the authors were careful to note that “ideal” results are likely impossible because every observation has some uncertainty.This uncertainty can manifest in many ways (e.g. a false negative occurs when a significant sign of life is overlooked) and is related to the challenges in the questions researchers ask when they set out to find life signs.Angerhausen explained that the question “does this planet have life?” itself carries a significant risk of false positives. For example, a planet may have a small biosphere that doesn’t alter its atmosphere in a way that can be detected from a distance. In contrast, stipulating whether “this planet has a temperature within a specific range and concentrations of certain molecules above a defined threshold” could provide more informative data.When selecting which planets to investigate, the paper advises the importance of asking clear and specific questions. For example, instead of posing a vague question, one might ask, “of all the rocky planets in the habitable zone, how many show signs of water vapour, oxygen, and methane?” This would help create clear selection criteria for exoplanets as well as help experts avoid misinterpreting data from an alien world.When observations are filled with uncertainty, the conclusion “no life detected” can be meaningless. But if the questions are thoughtfully designed, even null results can serve as powerful tools in the search for extraterrestrial life.In sum, the effectiveness of a search depends on asking the right questions and not (solely) on the number of exoplanets observed. If scientists lack clarity on what specific indicators of life they should focus on, even the best telescopes could yield misleading results.SignificanceAngerhausen also stressed that in addition to the technological sophistication in upcoming projects like the Large Interferometer for Exoplanets (LIFE) and the Habitable Worlds Observatory (HWO) — which aim to observe dozens of earth-like planets — “our study shows that there is still a lot of work to be done on the theoretical side” and on the foundations of their knowledge. That is, how do we know a certain signal is truly a sign of life? Or what counts as dispositive evidence of a habitable planet?The LIFE and HWO projects plan to study exoplanets for signs of water, oxygen, and other molecules that may indicate the presence of life. Angerhausen himself expressed optimism about the potential to discover habitable worlds. He said that for the first time in human history, humans will soon have the technology to systematically search for life in our cosmic neighbourhood.In the final analysis, the new paper asserts that the absence of evidence is not evidence of absence — as long as we allow the right questions to lead us.Shreejaya Karantha is a freelance science writer. Published – July 16, 2025 05:30 am IST
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