Animation by Russell Birkett, narrated by Clive Cookson, produced by Maija Palmer
Why haven't we found signs of extraterrestrial life? Astronomers say there are more than 100 billion planets in our galaxy, the Milky Way. Potentially some of them could be habitable by living creatures. The laws of probability suggest that aliens should be plentiful. But we haven't found signs of any of them. As the great Italian physicist Enrico Fermi asked his colleagues over lunch in 1950, "Where is everyone?"
Fermi's paradox is more troubling today than ever. We've been looking systematically for almost 70 years. But we still have no evidence of creatures great or small, intelligent or otherwise, beyond Earth. Despite this, the search for extraterrestrial life is gaining momentum.
In 2015, a $100 million astronomy programme called Breakthrough Listen began a project to detect artificial signals from space. It will survey a million of the stars closest to Earth and 100 of our nearest galaxies in both optical and radio frequencies. Astronomers are also using new techniques to search for life, looking for stars showing strange energy and light patterns that might indicate megastructures built by advanced civilisations. A particular object of fascination has been a star 1,300 light years away called Tabby's Star, which shows unexplained changes in brightness.
At the same time, astrobiologists are searching for simple extraterrestrial life forms closer to home within our solar system. Microorganisms might thrive beneath the arid surface of Mars or further out on the moons of Jupiter or Saturn, in the salty oceans of liquid water that have been discovered beneath their icy crusts. Saturn's largest moon, Titan, has no liquid water, but it has lakes, seas, and rivers running with liquid methane and ethane. Could some form of cold-climate, hydrocarbon-based life have evolved there?
For now, Fermi's paradox remains unresolved. But technological advances and new discoveries in everything from biology to chemistry mean we could be getting closer. It would be the greatest scientific discovery of the century.