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This post is about the zoo hypothesis, a term coined in 1973 by the astronomer John Ball, but the idea had been in existence for decades before then.  The zoo hypothesis states that there are many advanced and intelligent alien civilisations out there, but they hide their existence from us so that they they do not interfere with our development. Human beings are effectively in a cosmic zoo being observed by more advanced aliens.

As I’ll talk about later, the zoo hypothesis has formed the basis for many science fiction stories.

Origins of the zoo hypothesis

As discussed in a previous post, the Sun is one of 400 billion stars in our Milky Way galaxy and there are hundreds of billions of galaxies in the observable Universe. In 1961 Frank Drake invented an equation to estimate the number of intelligent civilisations within the Milky Way with whom we could potentially communicate, to which he gave the symbol N.  Drakes’s equation consists of seven numbers multiplied together:

N=  R*  x  FP  x  NE x FL x FI x FC x L


  • R* is the average number of stars formed per year in our galaxy
  • FP is the fraction of the stars within our galaxy which have a planetary system with one or more planets, expressed on a scale of 0 to 1
  • NE is for the average number of bodies, either planets or moons of planets, with the right conditions to support life
  • FL is the fraction of bodies with the right conditions to support life, on which life actually evolves, expressed on a scale of 0 to 1
  • FI is the fraction of bodies having life, on which life has evolved into intelligent civilisations, expressed on a scale of 0 to 1
  • FC is the fraction of bodies with intelligent life which develop a technology that releases signs of their existence into space
  • L is the average lifetime of a civilisation in years.

However, because there is no consensus among astronomers what these value are, the Drake equation cannot tell us with any degree of certainty how many alien civilisations there are. If the values of one or more of these numbers are very low, then the value of N will be very low and we could be the only intelligent life form in the galaxy.

What our Milky Way galaxy would look like from outside – Image from NASA

On the other hand, if most of the values in the Drake equation are very high, intelligent life would be very common and there could be as many as 80 million intelligent communicating civilisations in the galaxy. If this is the case, many of these civilisations will have been around for a long time.

On Earth civilisation is only a few thousands of years old and in the last 100 years,which is an incredibly short time compared to the lifetime of the galaxy, we have discovered nuclear energy, made huge advances in microelectronic and computing, sent space probes to explore the entire solar system and sent astronauts to the Moon. Within the next 30 years humans may walk on the surface on Mars. If a civilisation were millions of years old it is likely that it would have colonised the neighbourhood where it first emerged and would have spread throughout the galaxy, perhaps in cooperation with other advanced civilisations.

It might even be the case that in certain civilisations intelligent machines have become so advanced that they have taken over from the creatures who originally built those machines. It would be easier for aliens to disperse themselves outside their own solar system if they had evolved into machines which are more robust and longer lasting than carbon-based lifeforms.

Assuming such civilisations exist and are widespread throughout the galaxy, they would almost certainly have visited the Earth or at the very least be watching us and monitoring our development. For nearly sixty years astronomers such as Frank Drake have been searching for radio signals with sensitive radio telescopes.  However, no transmission has ever been detected from any alien intelligence and there is no scientifically verified evidence that aliens have ever visited Earth (see notes).

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Frank Drake – image from Wikimedia Common 

If aliens were to contact us it would be the greatest event in human history. If they chose to do so, aliens could give us the huge advances in technology which we had not yet discovered for ourselves. This would interfere with our development, which would not then follow its natural course. So, in the zoo hypothesis, aliens choose to remain hidden and place the Earth ‘out of bounds’.

In some versions of the zoo hypothesis the aliens watching us will reveal themselves when civilisation on the Earth has advanced to a certain level, for example when humanity has been able to achieve interstellar travel.

The zoo hypothesis in science fiction

Unsurprisingly, the zoo hypothesis has been a popular topic for many science fiction writers.

In one of my favourite Arthur C Clarke novels, Childhood’s End (published in 1953), an alien civilisation known as the Overlords have been observing the Earth’s evolution and human history for thousands of years.

At the beginning of the book, when mankind is about to achieve spaceflight, the Overlords reveal themselves. They then supervise humanity’s development to ensure that we do make any mistakes such as destroying ourselves in a nuclear war.

Those of you who follow Star Trek may recall a rule of the Federation called the ‘Prime Directive’. This prohibits Starfleet personnel from interfering with the internal development of alien civilisations which are below a certain threshold of technological, scientific and cultural development.

In ‘The State of the Art’ a novel by the Scottish science fiction writer Iain M Banks (1954-2013), an advanced interstellar civilisation called ‘The Culture’ secretly visit the Earth. They decide to leave the Earth without revealing themselves to its inhabitants so that they can watch its development as if it were a control group in an experiment.

In ‘Cancelled’, an episode of the adult cartoon series South Park, the whole of Earth is the subject of a reality show watched by an advanced alien intelligence. When the aliens realise that humans have discovered that they are participants in a show, the aliens consider whether or not to cancel the show. This is because, now that humanity realises what’s happening, the show’s quality will be reduced. Unfortunately, cancelling the show means destroying the Earth. In the end a deal is done whereby the show is allowed to continue but everyone on Earth has their memory wiped of the knowledge that they are participants.

These are just four examples, but there are many other works of fiction which are based around the zoo hypothesis.  Feel free to mention your favourites in the comments!

Problems with the zoo hypothesis

Although it has provided an interesting basis for many science fiction writers, only a very small minority of astronomers believe in the zoo hypothesis. One problem with it is that if intelligent alien life is commonplace within our galaxy, then it will have arisen independently in many places. Each of these civilisations is likely to have very different rules and values. It is difficult to see how all of them would adhere to something akin to the prime directive. It would only take one of these alien civilisations to try to contact us for the zoo hypothesis to break down.

I am convinced by the arguments made by scientists (such as the British theoretical physicist and popular science writer John Barrow) who believe that some of the values in the Drake equation should be set very low. There are a large number of steps which occurred between the emergence of the first primitive single-celled life forms and the evolution of man. Each of the individual steps may have a very low probability. So FI – the probability of life evolving into intelligent civilisations – would be extremely small and mankind is probably the only intelligent civilisation not only in our galaxy but in the thousands of neighbouring galaxies as well.  This is sometimes called the rare Earth hypothesis and is such an interesting topic that I’ll talk about it in a future post.


This is sometimes called the Fermi Paradox,  named after the Italian-American physicist Enrico Fermi (1901-1954) who first made this point in 1950.

Image from Wikimedia Commons

Fermi’s argument is as follows:

  • There are billions of stars in the galaxy that are similar to the Sun, and many of these stars are billions of years older than Earth.
  • It is highly probably that some of these stars will have Earth-like planets, and some might develop intelligent life.
  • Some of these civilisations might develop interstellar travel.
  • When a civilisation develops interstellar travel, it may visit vast numbers of place in the Milky Way within a few million years.
  • Therefore, the Earth should have already been visited by aliens.
  • So where is everybody?