There's a high chance aliens are out there, so why haven’t we met them yet?

Hello? Is there anybody out there? If any advanced alien civilisations are reading this, could they please get in touch? Text us, tweet us, send us a selfie – anything that might let us know you're out there.

We have been searching for intelligent life outside our solar system for more than half a century now, at a cost of gazillions, and so far we have turned up zip. Not a single alien lifeform, not even a blob. Our TVs and movie screens are teeming with extraterrestrial life, from Wookiees to Daleks, but in real life there's no sign of any Klingons on the starboard bow.

We’ve been listening for communications from deep space, and we’ve pinged out radio signals into the ether to try and contact other civilisations, but no one has called back. We’ve sent probes out into space to seek out habitable exoplanets, but despite collecting huge amounts of data, they haven’t found evidence of alien intelligence out there.

Heck, we've even tried to communicate in song, via the Carpenters' 1977 hit Calling Occupants of Interplanetary Craft. The song was inspired by World Contact Day, set up in 1953 by an organisation called the International Flying Saucer Bureau. Every year on March 15th, the bureau's members would send out a "telepathic signal" that they hoped would be picked up by passing craft. But no one has acknowledged receipt of this annual thought telegram, and the dulcet tones of Karen Carpenter appear to have fallen on deaf pointy ears.

Humans have long thought about making contact with alien civilisations. At the end of the 19th century, Nikola Tesla was convinced he could dial up Mars on his wireless electrical transmission system. In 1924, when Mars came its closest to Earth in a century, Americans were asked to turn off their radios so the US navy could listen for a message from the red planet. And in 1960, astronomer Frank Drake kicked off the official search for extraterrestrial intelligence when he used a radio telescope to listen for signals from distant stars Tau Ceti and Epsilon Eridani. He heard nada.

We have come tantalisingly close. In 1977, Ohio State University’s telescope Big Ear picked up a strong narrowband radio signal that seemed to come from the constellation Sagittarius. Reviewing the data a few days later, astronomer Jerry R Ehman spotted it and wrote “Wow!” on the computer printout. It has since come to be known as the Wow! Signal, but that was the last we heard from Sagittarius. Maybe aliens don’t like the Carpenters.

With up to 400 billion stars in our galaxy alone, and billions of them similar to the sun, the odds on intelligent civilisations having evolved near another star should be high, and some should even be capable of interstellar travel. So why haven’t we met them yet? That’s the Fermi paradox: the dichotomy between the high probability of intelligent life existing outside our solar system, and the complete lack of such evidence.

The paradox is named after physicist Enrico Fermi, who, in an informal conversation about the probable existence of alien civilisations, famously asked “where is everybody?” Could it be that we’ve been looking for ET in all the wrong places?

We tend to look to stars similar to our own sun as the most likely places to find habitable, earthlike planets, but a new study from scientists at Harvard University suggests that low-mass stars known as red dwarfs may be ideal incubators for alien civilisations. The star closest to our sun, Proxima Centauri is a red dwarf and is orbited by a potentially habitable planet. So aliens could be our next-door neighbours just 4.24 light years down the road.

Because Proxima Centauri is cooler than our sun, the planet would have to be orbiting closer to it in order to create the right conditions for life to evolve. But how do scientists know from that distance whether it’s a habitable planet?

"It's actually amazing, really, because they find out just from the mass and volume of it what the density is. So they can tell if it's a rocky planet rather than a gas planet. So that's the first step," says Deirdre Coffey, a Lecturer of Astronomy and Space Science at UCD School of Physics. "And then from the distance to the star they can find out if there's water. And these two things are very critical for life like our own.

Habitable planets

It would be nice to find evidence of alien civilisations – as long as they’re friendly – but that’s not the prime motivation behind the search for habitable planets beyond our solar system. There are plenty of other reasons to get excited about the discovery of a potentially habitable planet orbiting our nearest star.

“Overall, we want to find out where we come from, so if we look at other solar systems, it can help us understand our own solar system,” says Deirdre Coffey. “So to find something that close to the star, and the fact that it’s so close to us means we can study it in much more detail than anything we’ve ever studied before, so hopefully it will hold the keys to a few questions.”

It could also be crucial to our future as a species – might be handy having another habitable planet nearby to move to after we’ve rendered our own planet uninhabitable.

“It’s about trying to find out how our own solar system could have formed, and whether it’s the only way a solar system can form, or if there are other ways. Are habitable planets the same around every star, and can we colonise them and can we use their resources as well?”

No need to start packing – it will be a long time before we can even think of setting off for Proxima Centauri.

“It sounds a long way off, but the pace of technological development is phenomenal. If you think that just over 100 years ago, we had horses and carts. And now we’re talking about going to Mars. So that timescale is quite short for that level of technological development. So it does sounds very science fiction, but who knows? There’s a lot of progress being made. It’s the century of planetary discovery.”