Were we contacted by aliens in 1977?

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1. The search for aliens

Are we the only island of life in the vastness of space, or is our Galaxy teeming with other civilisations?

We've long been intrigued by the idea that we are not alone. The search for extra-terrestrial intelligence (SETI) - was born from this curiosity - the use of sophisticated scientific methods to try to detect a signal coming from life elsewhere in the Galaxy.

In the 1960s, radio astronomy was put to work in the search. Radio telescopes surveyed the sky, searching for something that might come from an alien civilisation. For years they heard nothing except the background hum of space. Then one day in 1977, a radio telescope in the US received a signal...

2. A signal from space

Watch Brian tell the story of the signal picked up by the Big Ear radio telescope in 1977.

Clip from Human Universe (BBC Two)

The Wow! signal fitted the profile of an alien transmission. Other explanations have been ruled out. Transmitters on Earth can’t use the same frequency, and the signal was too narrow to come from natural sources. Interstellar scintillation, the audio equivalent of a star twinkle, has also been dismissed.

3. Never to be heard again

Scientists immediately searched for a repeat of the Wow! signal.

They scanned the sky in the direction of the constellation Sagittarius, where the signal had come from. And as technology improved, more sensitive telescopes were put on the case, along with software that was designed to find signals among the background noise.

But despite several decades of searching, the signal has never been seen again. It remains a mystery.

So how likely are we to ever find intelligent life, somewhere in our Galaxy?

4. The Drake Equation

In 1961, astronomer Frank Drake devised an equation to estimate the number of detectable civilisations in our Galaxy.

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Drake estimated there should be between 1,000 and 100,000,000 detectable civilisations. Scientists have continually revised his figures, based on a better understanding of some of the variables in the equation. A conservative estimate today is that there are between two and 50,000. But the real value of the Drake equation is to highlight the areas we must focus on in the search for alien life.

5. The best candidates for life

In the last few years, scientists have found 21 potentially habitable planets outside our solar system. Here are five of the leading candidates.

1/5: HD 40307g (pictured above) orbits in its star's habitable zone, meaning that it is the right distance from the star for water to exist as a liquid. All life needs water to survive, so only planets in a habitable zone are likely to support it.

Image: PHL @ UPR Arecibo

2/5: Gliese 832c is in a habitable zone, but its orbit grazes the zone's inner and outer edges. This means it’s likely to have very hot and cold seasons. A less stable environment reduces the chances of life.

Image: PHL @ UPR Arecibo

3/5: Kepler 186 f (left) is a similar size to Earth (right). Being Earth-sized is a good sign. Too large, and planets could be uninhabitable gas giants, like Jupiter. Too small, and planets may have lost their atmosphere because of weak gravity.

Image: PHL @ UPR Arecibo

4/5: Kepler 62f is a large planet, but scientists think it is rocky. Large and rocky is good; Earth is only just big enough to have plate tectonics (surface movement of the crust, which recycles carbon dioxide and stabilises temperatures for life).

Image: NASA Ames/JPL-Caltech

5/5: Only 13 light years away from Earth, Kapteyn b is around 11.5 billion years old. This makes it the oldest potentially habitable planet ever discovered - and being twice as old as Earth, it has had plenty of time to develop life as we know it.

Image: PHL @ UPR Arecibo

6. “Where is everybody?”

If there are tens of thousands of detectable civilisations in our Galaxy, then why haven’t we picked up any obvious signs of life?

This contradiction is known as the Fermi Paradox, after physicist Enrico Fermi. He argued that if a large number of alien civilisations existed, some would have been around much longer than us and would have developed technologies to colonise other planets.

Some of these civilisations would have had more than enough time to spread across the Galaxy. Yet he hadn't had any indication that they exist. Nor have we since – apart from, perhaps the Wow! signal in 1977.

The idea is best expressed in Fermi’s simple question “Where is everybody?” There are several possible answers...

7. What's the reason for the big silence?

There are several explanations as to why we haven’t we picked up any signals from space.

Space is too vast

Alien civilisations exist too far away

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Space is too vast

Image: NASA, ESA, ESO

We can't survey every part of the night sky, and it's possible that radio signals will become too weak as they cross our galaxy to be detected.

Civilisations don't last

Intelligent species never survive long enough to overlap

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Civilisations don’t last

Image: Don Davis, NASA

Advanced civilisations may tend to destroy themselves, through war or environmental catastrophe. If not, a disaster like an asteroid strike might wipe them out.

Our planet is unique

Earth is the only planet with complex life

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Our planet is unique

Image: Getty Images

According to the Rare Earth theory, even if simple life has arisen elsewhere, the chance events that led to intelligent life on our planet is highly improbable.