Probable number of alien civilisations is 36

Earth calling: Waiting for aliens to break the great silence. © Alamy

Would the discovery of alien life change anything? A philosopher says it would have huge implications for us and for our ideas of morality, and that aliens may have kept quiet for a reason.

The girl sitting her Alien History and Culture GCSE in 2025 grins at the next question. “What is the most likely number of contactable civilisations in the Universe? Show your working.” Easy! Anyone with the faintest knowledge of extraterrestrial life knows that the answer is 36. All she has to do now is prove it using the Drake Equation.

Her calculations follow those made in 2020 by two scientists at Nottingham University, Christopher Conselice and Tom Westby.

They are based on the assumption – called the Astrobiological Copernican Principle – that, if intelligent life on Earth is the product of the natural law of evolution, rather than something that appeared at random, then there is no reason why the same thing should not have happened on similar planets.

The question, if we want to contact those intelligent lives, is how far in the evolutionary process they have got, and how far away they are.

In 1961, the astronomer Frank Drake argued that there were seven things we needed to know in order to count these civilisations, including the average number of stars formed in the galaxy each year and how long the aliens could be expected to send out signals for.

If the Astrobiological Copernican Principle is added, it narrows down the possible answers considerably.

Conselice and Westby have worked out that, if life emerges between 4.5 and 5.5 billion years after a star is formed – as it did on Earth – then there are between four and 211 civilisations in the Milky Way that could communicate with us, with 36 as the most likely total.

However, as the nearest ones could be 17,000 light years away, we will not be able to exchange signals with them for at least 6,000 years.

These conclusions could help answer a fundamental question. Are values simply a human invention, or would things like the difference between right and wrong exist even if we did not? Discovering aliens, the philosopher Tim Mulgan believes, would support the latter view.

Mulgan argues that, if we find one alien civilisation, there is reason to suppose that many more exist. But why have they not made contact? He suggests that they have chosen not to because they know better than we do what is best for the Universe

Would the discovery of alien life change anything?

Off the radar

Some say, no: it would be wrong to assume that the aliens had kept themselves to themselves as a matter of principle. It might be that they cannot communicate because they are not as intelligent as us, or their technology is not yet advanced enough. Alternatively, their civilisation might already have been destroyed by war, or they might have run out of resources.

Others, including Tim Mulgan, argue that if there are many alien civilisations, it is highly unlikely that none of them can contact us. It is equally unlikely that none of them want to contact us. So, if all advanced alien races – however unlike each other – have decided it would be wrong, they must have shared values – ones which are part of the fabric of the Universe and exist without humans.

You Decide

  1. Would you rather be able to communicate with an alien or with an animal here on Earth?
  2. If the Astrobiological Copernican Principle is correct, would you expect aliens to look and think like us?


  1. On two sides of a piece of paper, write a message to an alien explaining what life is like on Earth.
  2. Imagine that you belong to an alien race which has realised it can contact humans. Hold a debate with a member of your family as to whether you should do so or not.

Some People Say...

“In the deepest sense, the search for extraterrestrial intelligence is a search for ourselves.”

Carl Sagan (1934-1996), American astronomer

What do you think?

Q & A

What do we know?
The question of why – if there are a huge number of potentially inhabited planets – we have seen no sign of aliens is called the Fermi Paradox after the physicist who first posed it. Those who think it is because the aliens cannot make contact are called Cantians (as in the word “can’t”). Those who believe they simply choose not to are called Wontians (as in the word “won’t”). The absence of communication has been termed “the Great Silence”.
What do we not know?
Whether God comes into all this. Tim Mulgan posits the idea of Kantian Wontianism, based on Immanuel Kant’s argument that any rational being who discovers universal values will be guided by them. The aliens’ decision to leave us alone could be based on the discovery of a divine being that has given the Universe its morality. If that is so, they may have reached “some threshold of wisdom or intellect that humans could never attain” – meaning that they might be more important to God than we are.

Word Watch

Beyond the Earth. The film ET the Extraterrestrial is about an alien who makes friends with a group of children.
Astrobiology is the study of life in the Universe, including how it originated and evolved, and whether aliens exist.
Frank Drake
An American scientist who has pioneered the search for extraterrestrial intelligence.
Nicolaus Copernicus (1473-1543) was a Polish astronomer who discovered that the Earth goes around the Sun. Because this contradicted the Church’s teaching that the Earth was the centre of the Universe, his book on the subject was not published in full until the year of his death.
Milky Way
The galaxy that contains our solar system. It has the structure of a disc, but looks to us like a band because we are inside it.

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