ÎçÒ¹¸£Àû1000¼¯ºÏ

Read more: “Instant Expert: Astrobiology“

Is there other intelligent life out there? To some it seems inevitable that there must be – it’s a very big universe. But it is also possible that we and our home world are a rarity, a Goldilocks event in time, space and the roulette wheel of evolutionary history.

Finding the answer is going to be tough. We think there are a few hundred sextillion stars (about 10²³) in the entire observable universe. Suppose 1 per cent of these have small rocky planets orbiting them and suppose that 1 per cent of such planets are habitable. Then suppose that 1 per cent of these evolve multicellular life. That would mean there were about 100 quadrillion (about 10¹⁷) instances of complex, potentially intelligent, life in the universe.

This sounds promising. Yet if we spread these out across the cosmos, on average such worlds will be separated by almost a million light years. Of course most stars are found in galaxies, so there could be a million instances of complex life in the Milky Way, separated by an average of only 300 light years. The problem is we have no idea what the true occurrence of genuinely habitable planets is, or the likelihood of complex life, or how often it might become recognisably intelligent and then survive long enough to be noticed. Speculation is not going to get the answer. We have to go looking.

One approach is to listen for signs of life broadcasting information. SETI, the search for extraterrestrial intelligence, has been doing this for decades. Modern, purpose-built experiments like the Allen Telescope Array in California carry on the search with state-of-the-art technology. Alternatively, if we find exoplanets with signs of biospheres then there may be clever ways to infer the presence of more than microbial life. Humans actively alter Earth. Our civilisation dumps chemicals into the atmosphere, manipulates the planetary surface, and even places thousands of artificial satellites around our world. There is no reason why intelligent life elsewhere would have to do the same, but if our neighbours do have similar dirty habits, this might lead us to them.

Finding irrefutable signs of any life beyond Earth, whether in our solar system or around nearby stars, would surely be one of the most revolutionary discoveries for human science and society. Optimistically, it could help us recognise ourselves as the single species we are, not the fragmented groups we often see ourselves as. The opportunity to finally study the incredible phenomenon of which we are a part – in its proper context, next to another example – would be extraordinarily compelling. The only remaining question would be whether we could find the ambition to go touch, smell, hear and perhaps even talk directly to this other life.