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We have always been fascinated by the thought of alien life elsewhere in the universe. The idea has provided the basis for a huge wealth of science fiction stories that have been limited only by our imaginations. But can other creatures exist in the vast reaches of space or on other planets or moons? And are there other intelligent forms of life out there—or are we more likely to find something much simpler?

Where are all the aliens?

Our Sun is just one star among billions in our galaxy. In the last few years, scientists have detected thousands of planets around other stars and it seems that most stars have planetary systems. It’s therefore likely that there will be large numbers of habitable planets in the Milky Way galaxy and beyond that are capable of supporting intelligent life. Some of these intelligent civilisations, if they’re out there, may have even developed interstellar travel.

But Earth hasn’t been visited by any intelligent aliens (yet?). This apparent high probability of life, combined with a lack of evidence for its existence, is called the Fermi Paradox, named for the physicist Enrico Fermi who first outlined the argument back in 1950. This begs the question: where is everybody?

Back in 1961, astronomer Francis Drake tried to rationalise this question by developing that takes into account all the factors relevant to finding alien civilisations and gives an estimate of the number of civilisations out there in the galaxy that should be able to communicate with us. It considers factors such as the rate of new star formation, how many planets around those new and existing stars might be able to support life, the number of planets supporting intelligent life, how many of those civilisations might have technology we can detect, whether they’re likely to communicate with us here on Earth, and so on.

Several of the numbers we plug into the equation are more speculation than solid fact, so the answer we get is at best a guess. Rather, the equation is more of a tool for engaging interest and sparking discussion about the issue.

The search for extraterrestrial intelligence

Scientists and radio astronomers have started the search for extraterrestrial intelligence (SETI) in a systematic manner. Several international organisations, including the  and the SETI League, are using radio telescopes to detect signals that might have been produced by intelligent life.

In 1995, the SETI Institute started Project Phoenix, which used three of the most powerful radio telescopes in the world: the Green Bank radio telescope in West Virginia, USA; the Arecibo telescope in Puerto Rico; and the Parkes radio telescope in NSW, Australia. During its initial phase, Project Phoenix used the Parkes telescope to search for signals coming from 202 Sun-like stars as distant as 155 light years away. By the end of its operations, Project Phoenix had scanned a total of 800 ‘nearby’ (up to 240 light years away) stars for signs of life. The project detected some cosmic noises, but none that could be attributed to aliens.

While there’s currently excitement about sending human crews to Mars, missions beyond the Red Planet are at this stage pretty much not feasible: the distances and travel times involved are simply too great. Basically, all exploration for life beyond Earth will need to be done using robotic space probes and landing rovers. These instruments can provide a huge wealth of information and are capable of exploring as far away as Pluto, . But as for life beyond the solar system, the nearest stars are several light years away, and even communications by electromagnetic waves (which all travel at the speed of light) are essentially going to be a one-way message.

While we probably won’t find intelligent life too close to home, there’s a chance we may still find much simpler life forms. Do we have neighbours beyond Earth? Time will tell—and the search continues.