Since the beginning of civilization, humans have looked up into the sky and wondered whether we are alone in the Universe. Since then our technology has come quite a way and it feels like we can make a better-educated guess to this question. We currently estimate that our observable Universe is home to approximately stars and planets [1].  Surely we cannot be alone in this vastness of space, right?

It is true that the vast numbers of stars and planets, and the recent discovery that even potentially Earth-like planets are abundant, all tend toward a scenario where our universe is crowded with life. These discoveries give us an insight into the total number of habitable worlds (). However, without knowing the probability of abiogenesis (), the formation of living organisms from inorganic or inanimate substances, we cannot tell whether these discoveries have a significant effect on the probability of life elsewhere in the observable Universe.

The probability of abiogenesis

Calculating the probability of abiogenesis has been proven to be very challenging for modern science, so let us focus on what we do know. We know that all life on earth uses water to survive and is carbon-based. From both of these chemicals, we know that they are common in the observable Universe. Even more complex and organic molecules like benzene, sugars and amino acids appear to naturally form outside of Earth. However, this knowledge still tells us very little to nothing about the probability of these molecules coming together to create self-replicating chemical networks, capable of Darwinian evolution, our current definition of life.

Another approach to get a better insight into the probability of abiogenesis is to recreate the environmental circumstances under which life formed on Earth, in 1952 Stanley Miller and Harold Urey [2] were the first to come up with such an experiment. Unfortunately, neither their experiment nor any experiment since, has been able to create complex proteins or any living organisms, from inorganic substances.

An often-heard counterargument is that on Earth we see life everywhere, living under the most extreme conditions. Some of these so-called extremophiles, like the tardigrade, are even able to survive in the vacuum of space. Surely, this would imply that life is easy, right? What is often forgotten is that all life forms on Earth share one common ancestor, called LUCA (last universal common ancestor), so the fact that life can adapt to live in all kinds of conditions tells us nothing about whether life can start in different conditions.

Survival bias

Another argument for life outside of Earth is the fact that life on Earth started relatively fast after the conditions became habitable for life. Surely, the early emergence of life on Earth indicates that life is a relatively easy process to get going. To disprove this argument consider the following experiment:

We have a million cells with each cell containing one person, with each person only being aware of his own cell. We give each person a hairpin to open the lock of the cell. If they succeed within a minute, then they are free to go, otherwise, they will be killed.

Suppose that we know that on average it would take a hundred hours to open the lock. If we repeat this experiment numerous times, then we will see that sometimes there is a person who manages to open the lock within a minute. Since this person is only aware of his own cell, he would probably say that the lock was quite easy to pick, this is known as survival bias.

It actually turns out that for the experiment above the time at which a person manages to open his cell tends towards a uniform distribution [3]. If we now replace each cell with a planet and each minute for the period of time that the conditions are suitable for life on this planet, it becomes clear that the probability of life starting early on Earth is roughly equal to the probability of life starting late on Earth. Therefore, this does not give us any information about the probability of abiogenesis, as we just might as well be an outlier.

Statistical reasoning

If you had to put a bet on whether we are alone in the Universe or not, most people in the field of astronomy would put their money on there being other lifeforms out there. This can be explained by the principle of mediocrity, which says that we tend to think that our experiences are typical and since life formed here, it should have also formed elsewhere. However, if we use statistical reasoning, we end up with another conclusion.

The expected number of worlds containing life () is given by

   

Suppose we have a collection of habitable worlds in our observable Universe. If the probability of abiogenesis is then we would expect

   

exactly one inhabited world in our observable Universe. Of course, we do not know , which can take any value between zero and one. The probability of being exactly is very unlikely. Chances are that either

   

or

   

In the first case, we are expected to see numerous inhabited worlds in our observable Universe. The absence of evidence for any inhabited world outside ours tends to make the second case more likely. In this case, our observable Universe is expected to have way less than one inhabited world and since we live in an inhabited world, Earth, our observable Universe is already an outlier.

Conclusion

So, are we alone then? The honest truth is that at this moment we simply do not know. There are many arguments both in favour and against the existence of life outside of Earth. Famous astronomer Carl Sagan once said that “Faith is belief in the absence of evidence.” So, next time you hear someone stating that life must or cannot exist outside of Earth, remember that it is just their faith talking.

[1] European Space Agency (ESA) https://www.esa.int/Science_Exploration/Space_Science/Herschel/How_many_stars_are_there_in_the_Universe

[2] S. L. Miller, H. C. Urey, Organic compound synthesis on the primitive earth. Science 130, 245–251 (1959).

[3] J. Chen, D. Kipping, On the Rate of Abiogenesis from a Bayesian Informatics Perspective. Astrobiology, 18, 12 (2018).