Planet Earth is under attack. Disc-like UFOs litter the night sky, radiating vertical beams through which silhouetted creatures descend. You clench onto your seat. It’s the movie’s climax: a fiery scene of human destruction and alien triumph. The orchestral music reaches a roaring crescendo and all too soon the film is over. But as realistic as CGI effects might be nowadays, there’s a reason they call it ‘science fiction’.
We often turn to sci-fi movies as a means of escaping from our comparatively mundane lives. But the question of extraterrestrial life is, to me, the greatest unsolved problem of science. Could it really be that, within the seemingly infinite expanse of the universe, our home is the only planet with the ingredients for life? If technologically advanced beings are ‘out there’, then why have we not found them?
Our search began 400 years ago with Galileo’s invention: the telescope. Gazing into our neighbouring worlds for signs of life yielded little success. In 1950, Italian physicist Enrico Fermi brought the topic to light when he remarked that, given the immense size and age of the universe, sapient alien civilizations ought to exist. Yet, this hypothesis, termed the ‘Fermi Paradox’, contradicted empirical evidence and conventional wisdom. Back then, most scientists held the conservative view that life was the result of a cosmic chemical lottery so unlikely that we should consider ourselves alone. However, this view has changed radically over the past thirty years and as Nobel Laureate chemist Harold Urey aptly put it, “Life is not a miracle, it is a natural phenomenon.”
As scientific understanding and space technology advanced, so did the quest for Alien species. In 1961, American astronomer Frank Drake attempted to quantify the probability of extraterrestrial life in the Milky Way with a formula that became known as the ‘Drake Equation’. This took into account cosmological factors such as the rate of star formation and the proportion of these stars which could have planets supporting intelligent life. Together with fellow scientist Carl Sagan, Drake designed the pioneer plaque in 1972: an aluminium plate that blasted into space onboard the Pioneer 10 space probe, becoming the first physical message to be sent into space in the hope of attracting extraterrestrial interception. The plaques depicted nude male and female figures as well as information about the location of Earth in the solar system. But, still empty handed in 2003, scientists lost communication with Pioneer 10, which broke from Sun’s gravitational pull and drifted into oblivion.
Today, as scientists look into the depths of space, they seek the three core ingredients of life. First, the correct chemistry set: although we are made of 40 elements, 96% of our chemical composition is simply Carbon, Oxygen, Hydrogen and Nitrogen; second, a battery source such as our Sun to produce a flow of electrons that powers the processes of life; and third, a medium in which life can play itself out – predominantly liquid water.
The cardinal link between water and life is driving our search for life in the Milky Way. Earth is the only planet in our solar system with liquid water on its surface. But evidence of outflow channels on the surface of Mars – straight wide canyons identical to the scablands we find on our own planet – suggests that immense floods once charged across its surface. More compellingly, detection of mineral deposits by NASA’s Opportunity Rover indicates past existence of large areas of standing water. Recent satellite images of Mars even hint at subterranean deposits of water, and infrared spectroscopy has detected methane in Mars’ tenuous atmosphere. This methane may be coming from a biological source such as Archaea – a single-celled prokaryote microorganism resistant to extreme conditions, which is the most common organism beneath the surface of our own planet.
However, Mars is not the only body in our solar system under scrutiny. Jupiter’s icy moon Europa, similar in size to our own Moon and the smoothest body in the solar system, is inscribed with a peculiar network of deep cracks. At -160 degrees Celsius, it would seem an incredibly unlikely place for life, but the position of these cracks suggests that water beneath the surface is causing the outer shell to shift. Measurements of Europa’s magnetic field confirm that its icy surface sits atop a salty ocean, which may be an astonishing 100 kilometres deep. This would give the tiny moon more than double the volume of life-harvesting water than our own planet – a discovery that has made it the most important alien world we know and our best hope of finding extraterrestrial life.
Whether or not we are the lone inhabitants of our universe remains a mystery. But what we are left to appreciate is how valuable, rare and precious our home is, to allow life to evolve and flourish into such magnificent complexity. The prospect of being part of a wider cosmic community is tantalising, but we must remain grounded. The Fermi Paradox has yet to be cracked. As it stands, “the only truly alien planet is Earth.”