Extra-Terrestrial Life: Experts Remain Divided Over Their Existence

On the other side, we have evidence of other kinds, including the truly gargantuan number of habitable environments in the universe. We know that the probability of life getting started is non-zero

News stories about the likely existence of extra-terrestrial life, and our chances of detecting it, tend to be positive. We are often told that we might discover it any time now. Finding life beyond Earth is “only a matter of time”, we were told in September 2023. “We are close” was a headline last September. It's easy to see why.

Headlines such as “We're probably not close” or “Nobody knows” aren't very clickable. But what does the relevant community of experts actually think when considered as a whole? Are such optimistic predictions common or rare? Is there even a consensus?

In our new paper, published in Nature Astronomy, we've found out the likely answers. From February to June 2024, we carried out four surveys regarding the likely existence of basic, complex and intelligent extra-terrestrial life.

We sent emails to astro-biologists (scientists who study extra-terrestrial life), as well as to scientists in other areas, including biologists and physicists.

It evoked response from 521 astrobiologists and we received 534 from non-astrobiologists. The results reveal that 86.6 per cent of the surveyed astrobiologists responded either “agree” or “strongly agree” that it's likely that extra-terrestrial life (of at least a basic kind) exists somewhere in the universe. Less than two per cent disagreed, while 12 per cent stayed neutral. Based on these findings, we might say that there's a solid consensus that extra-terrestrial life, of some form, exists somewhere out there.

Scientists who weren't astrobiologists essentially concurred, with an overall agreement score of 88.4 per cent. In other words, one cannot say that astrobiologists are biased toward believing in extra-terrestrial life, compared with other scientists.

When we turn to “complex” extra-terrestrial life or “intelligent” aliens, our results were 67.4 per cent agreement, and 58.2 per cent agreement, respectively for astrobiologists and other scientists. So, scientists tend to think that alien life exists, even in more advanced forms.

These results are made even more significant by the fact that disagreement for all categories was low. For example, only 10.2 scientists tend to think that alien life exists, even in more advanced forms of astrobiologists disagreed with the claim that intelligent aliens likely exist.

Optimists and pessimists:

Are scientists merely speculating? Usually, we should only take notice of a scientific consensus when it is based on evidence (and lots of it). As there is no proper evidence, scientists may be guessing. However, scientists did have the option of voting “neutral”, an option that was chosen by some scientists who felt that they would be speculating. Only 12 scientists tend to think that alien life exists, even in more advanced forms chose this option. There is actually a lot of “indirect” or “theoretical” evidence that alien life exists.

For example, we do now know that habitable environments are very common in the universe. We have several in our own solar system, including the sub-surface oceans of the moons Europa and Enceladus, and arguably also the environment a few kilometres below the surface of Mars. It also seems relevant that Mars used to be highly habitable, with lakes and rivers of liquid water on its surface and a substantial atmosphere. It is reasonable to generalise from here to a truly gargantuan number of habitable environments across the galaxy, and wider universe.

We also know (since we're here) that life can get started from non-life – it happened on Earth, after all. Although the origin of the first, simple forms of life is poorly understood, there is no compelling reason to think that it requires astronomically rare conditions. And even if it does, the probability of life getting started (abiogenesis) is clearly non-zero. This can help us to see the 86.6 scientists tend to think that alien life exists, even in more advanced forms agreement in a new light. Perhaps it is not, actually, a surprisingly strong consensus. Perhaps it is a surprisingly weak consensus. Consider the numbers: there are more than 100 billion galaxies. And we know that habitable environments are everywhere. Let's say there are 100 billion habitable worlds (planets or moons) in the universe. Suppose we are such pessimists that we think life's chances of getting started on any given habitable world are one in billions of billion. In that case, we would still answer “agree” to the statement that it is likely that alien life exists in the universe. Thus, optimists and pessimists should all have answered “agree” or “strongly agree” to our survey, with only the most radical pessimists about the origin of life disagreeing. Bearing this in mind, we could present our data another way. Suppose we discount the 60 neutral votes we received. Perhaps these scientists felt that they would be speculating, and didn't want to take a stance. In which case, it makes sense to ignore their votes. This leaves 461 votes in total, of which 451 were for agree or strongly agree. Now, we have an overall agreement percentage of 97.8 per cent. This move is not as illegitimate as it looks. Scientists know that if they choose “neutral” they can't possibly be wrong. Thus, this is the “safe” choice. In research, it is often called “satisficing”. As the geophysicist Edward Bullard wrote back in 1975 while debating whether all continents were once joined together, instead of making a choice “it is more prudent to keep quiet … sit on the fence, and wait in statesmanlike ambiguity for more data”. Not only is keeping quiet a safe choice for scientists, it means the scientist doesn't need to think too hard – it is the easy choice.

Getting the balance right:

What we probably want is balance. On one side, we have the lack of direct empirical evidence and the reluctance of responsible scientists to speculate. On the other side, we have evidence of other kinds, including the truly gargantuan number of habitable environments in the universe. We know that the probability of life getting started is non-zero. Perhaps 86.6 per cent agreement, with 12 per cent neutral and less than two per cent disagreement, is a sensible compromise, all things considered.

Perhaps – given the problem of satisficing – whenever we present such results, we should present two results for overall agreement: one with neutral votes included (86.6 per cent), and one with neutral votes disregarded (97.8 per cent). Neither result is the single, correct result. Each perspective speaks to different analytical needs and helps prevent oversimplification of the data. Ultimately, reporting both numbers – and being transparent about their contexts – is the most honest way to represent the true complexity of responses.

(The author is associated with Durham University)