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 evolution, rather than something that appeared at random, then there is no reason why the same thing should not have happened on similar planets.

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.

Philosopher Tim Mulgan believes that, if we find one alien civilisation, there is reason to suppose many more exist. But why have they not made contact? He suggests that they have chosen not to because they know 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.

Others, including Tim Mulgan, argue that, 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?


  1. On two sides of a piece of paper, write a message to an alien explaining what life is like on Earth.

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 asked 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 suggests Kantian Wontianism, based on philosopher 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 who has given the Universe its morality. If that is so, they may have reached “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.
Belief; a thing that is accepted as true or as certain to happen, without proof.
Astrobiology is the study of life in the Universe, including how it originated and evolved, and whether aliens exist.
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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