What if we don’t find any life on the exoplanets, Doctor Angerhausen?

Scientists are currently planning two major space missions to search for life in space: NASA’s Project HWO and Project LIFE led by ETH Zurich (see box below). If one of these missions were to discover life on an exoplanet, i.e. a planet outside of our solar system, this would be a sensational finding with implications far beyond science and would change our entire view of the world. However, it is equally conceivable that both multi-billion-dollar missions will examine a large number of exoplanets but find no conclusive trace of life on any of them. Would this be deemed a failure? Or perhaps not?

ETH News met with Daniel Angerhausen to discuss this and other questions. In a recently published study, he reflected on what a null-result finding would mean and why it would still yield valuable insights for science.

Doctor Angerhausen, which is more likely – that we will have detected life in space within the next 15 years or that we will have thoroughly investigated dozens of exoplanets in that time but have found no trace of life?
Daniel Angerhausen: That’s the big question. Half of the astrobiologists say that life forms quite easily and is abundant in the universe. The other half believes that life only came about on Earth due to an extreme series of coincidences. How rare or common life is in the universe is precisely the question that we want to answer with missions such as LIFE and HWO. Personally, as a scientist, I try to consider this question as impartially as possible.

And what result would you like to see on a personal level?
Of course, I do hope that we find life. I would not have dedicated my career to searching for life in space if I did not at least hope that we would find it. But I’m excited by either finding. The British science fiction writer Arthur C. Clarke once put it very nicely: “Two possibilities exist: either we are alone in the universe or we are not. Both are equally terrifying.”

What do the results – finding something or finding nothing – mean for society?
Both findings have similarly far-reaching implications. In 15 years, if we know that life is something very rare and only occurred on Earth due to a massive coincidence, then this might perhaps inspire us to look after life on Earth better than we have done up to this point. We have to consider the possibility of getting a null result like this. When we put together a multi-billion-dollar space mission, it’s vital to make sure that, even if we do not detect any life, we still learn something about life in space.

Let’s suppose we study 50 exoplanets and find no life on any of them. This does not mean we can conclude that no life exists anywhere in space. Perhaps we will find it on the 51st or the 100th exoplanet.
Of course, we might have just missed the one planet that does harbour life. However, if we find nothing on 50 planets, then we do know that life is far rarer than what some optimists believe today. And we could express this rarity in the form of a number. We can’t do that today. At best, we just have a vague idea.

What kind of number would that be?
There are many parameters involved in calculating the probability of life in space. We already know how many stars there are in our galaxy and how many of these stars have planets. What we don’t know is how many of the planets that fulfil the requirements for life, for example in terms of their temperature, actually harbour life. Today, the value of this parameter is somewhere between zero (if there is no life in space) and 100 percent (if life in space is very common). If we study 50 exoplanets and are absolutely certain that there is no life on any of them, then we can refine our parameter considerably – it would then be less than 20 percent of planets. So we would know a lot more than we do today.

Both missions – LIFE and HWO – are still in the concept stage. It’s unclear whether scientists will ever get to study 50 exoplanets as part of these projects. What happens if – for technical or financial reasons – it is only possible to examine 30, or even just 15 planets?
Then we will be able to refine the parameter less precisely. If we studied 30 planets, we could say that fewer than 30 percent of planets harboured life. If we studied 15 planets, it would mean that the proportion was less than 50 percent. The number of exoplanets studied and the significance of a null result are directly linked. This is also important for our LIFE space telescope project.

What will determine how many planets can be examined as part of these missions?
There is a limited number of sun-like stars in our vicinity that can have Earth-like planets. The more exoplanets we want to study, the larger our telescopes need to be. If I only needed to examine 20 exoplanets for this study, then two-metre-diameter telescopes would suffice. But if I needed to examine 50 exoplanets, the diameter would have to be five metres. This is the question we are currently considering for Project LIFE, as the technology is beginning to be developed and the first decisions need to be made. It’s a bit like a tennis match between scientists and engineers: the scientists say what is necessary to get answers to their research questions, while the engineers say what is possible and how much it will cost. Our study is therefore coming at just the right time.