![]() For many years, it has been imperative for the Intergovernmental Panel on Climate Change (IPCC) to be clear on how confident they are concerning a great many propositions about climate change. Or, rather, it is an idea borrowed from another context. Our new paper, published by the group Exploring Uncertainty and Risk in Contemporary Astrobiology (EURiCA), has come up with another proposal. But this only starts to mean something when the relevant space of different possibilities has been thoroughly explored. Level 4 in the framework requires that "all known non-biological sources of signal" are shown to be implausible. Unfortunately, the problem of unconceived alternatives remains a serious challenge. ![]() ![]() It recommends seven steps to verifying a discovery, from ruling out contamination to getting follow-up observations of a predicted biological signal in the same region. In 2021, a Nasa-affiliated group published a paper setting out the Confidence of Life Detection (CoLD) framework, designed to solve this problem. But for many others (say, a particular blend of gases) we don't know much at all. For some kinds of "biosignatures" (such as a dinosaur skeleton) we know that the probability of it occurring without life is incredibly low. It also requires a value for the probability of the phenomenon in question occurring naturally-not caused by aliens. And intuitions about that vary dramatically (estimates for the number of inhabited planets in our galaxy range from one to billions). But the Bayesian approach doesn't really help when it comes to extraterrestrial life.įor example, it requires an input for the prior probability that aliens exist. One could, optimistically, input the available evidence into the Bayes formula, and achieve as output a number between 0 and 1 (where 0.5 is a 50:50 chance that a signal is produced by aliens). Some would turn to Bayes' Theorem, a common statistical formula, which gives the probability (Pr) of something, given some evidence. But the scientific world needs to express itself with rigor, transparently conveying the degree of confidence justified by the evidence. Newspapers might capitalize on this flexibility with a cheeky headline that appears to indicate that something is a bit more exciting than it actually is. The concept of possibility incorporates these extremes, and everything in-between. By contrast, we often use the word "might" to express something that has high probability, as in "it might snow today." Many shouts of: "It might be aliens!" should be interpreted in this (strained) sense. There's a sense in which it is possible that I'll meet King Charles III today, but at the same time it is extraordinarily unlikely. ![]() The word "possible" is strange, with a rather unfortunate degree of flexibility. So how can we be sure we've come to the right conclusion for something as subtle as the presence of a certain gas or a strange looking space rock? In our new paper published in the journal Astrobiology, we have proposed a technique for reliably evaluating such evidence. But although a few of the examples are still controversial, most have turned out to have other explanations (it wasn't aliens). These are just two of the latest cases of a long list of examples of such initially promising phenomena. Notably, physicist Avi Loeb from Harvard University argued in favor of the oddly shaped 'Oumuamua being an alien spaceship.Īnd phosphine in the atmosphere of a rocky planet is proposed to be a strong signature for life, as it is continuously produced by microbes on Earth. In both cases, it seemed possible that the phenomenon indicated some kind of extraterrestrial biological source. And in 2021, it was the possible discovery of the gas phosphine in the clouds of Venus. In 2017, it was the mystery interstellar object 'Oumuamua. Recently, two very different examples sparked excitement.
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