Some of the greatest human minds are scanning the universe, with a range of increasingly powerful telescopes, in the search for extraterrestrial life. The universe is unimaginably vast—how could we be the only ones?
Are we alone in the universe or do we share it with creatures we might one day meet? It’s one of the most fascinating and puzzling questions out there. Either way, it’s a staggering thought.
Lord Martin Rees, leading astrophysicist and the UK’s Astronomer Royal, says, “In the last 20 years (and especially the last five) the night sky has become far more interesting. Astronomers have discovered that many stars—perhaps even most—are orbited by retinues of planets, just like the Sun is.”
He continues, “There’s special interest in possible ‘twins’ of our Earth—planets the same size as ours, orbiting other Sun-like stars, on orbits with temperatures such that water neither boils nor stays frozen. The Kepler spacecraft has identified many of these, and we can confidently infer that there are billions in our galaxy.
"The galaxy might contain 100 billion habitable, Earth-like planets"
“Within 20 years the next generation of telescopes will image the nearest of these planets. Will there be life on them?”
Of course, it’s intelligent life that we’re most fervently searching for. As Dr Stephen Webb, physicist at the University of Portsmouth, puts it, “One recent estimate from an international team of astronomers suggested that the galaxy might contain as many as 100 billion habitable, Earth-like planets. There are about 500 billion galaxies in the universe, and so there might be as many as 50 sextillion potential homes for life. That’s a five followed by 22 zeros. Surely we can’t be the only intelligent species?”
In his book If the Universe Is Teeming with Aliens...Where Is Everybody?, Webb looks for an explanation for this paradox. He first considers whether intelligent aliens could already be here in our solar system.
Could E.T. already be home?
One theory suggests aliens are observing us
It’s a popular idea. Forty-five per cent of Americans believe in alien spacecraft, according to a 2015 opinion poll. But if they’re already here, why haven’t we seen them?
Well, our solar system is a big place. It’s nearly 3 billion miles from Earth to Neptune, our outer planet. So our chances of finding a small alien artefact by accident in billions and billions of cubic miles of space are essentially zero.
And it’s not as if we haven’t been actively looking. Webb explains, “One can reasonably argue that an extraterrestrial civilisation (ETC) wishing to explore our solar system would send small unmanned (unaliened?) probes rather than a fleet of crewed spacecraft. We know of several places suitable for parking an observational probe and at least one dedicated search has been made—but no proof has been found.”
Mind-boggling theories include “The Zoo Scenario”, proposed by Massachusetts Institute of Technology astronomer John Ball back in 1973. Could ETCs have set aside planet Earth as a cosmic zoo and be watching us from afar, while choosing not to interact with us?
"Radio signals may have been ditched long ago by alien civilisations"
In 2001, science fiction writer Stephen Baxter conceived “The Planetarium Hypothesis”—that we’re living in a computer simulation designed by beings far in advance of ourselves who want us to believe that the universe is devoid of intelligent life. Webb says, “With each breakthrough in the study of genetics, it becomes increasingly apparent that all life on this planet is deeply related. We can’t discount the possibility that every species came from the same extraterrestrial source.” Some take this idea further and suggest that an ancient ETC sent ready-made spores towards planets such as ours with conditions favourable to the survival of life. In which case, we would all be aliens.
Putting aside our own possible off-world origins, he concludes, “The evidence that extraterrestrials are currently visiting Earth is non-existent,” and moves onto a second theory...
They’re out there, despite the silence
Voyager 1 launched in 1977, and has been travelling through space ever since
The next possibility is that intelligent aliens do exist out somewhere in the galaxy, but we’re yet to hear from them.
“Perhaps the most straightforward explanation is that the distances between stars are too great to permit interstellar travel,” Webb suggests.
As an example, space probe Voyager 1, launched in September 1977, was four times farther out than the solar systems’s outermost planet by 2014. The first star it will encounter is called AC+793888— but it’s going to take 40,000 years to get there!
In more than 50 years of searching we’ve heard nothing. But maybe intelligent ETCs are signalling to us: we just don’t know how to listen. He explains, “Radio signals may have been ditched long ago by alien civilisations. Who knows what communication channels might be available to technical cultures that are millions of years in advance of our own.”
Similarly, he asks, “Who knows where a civilisation much older than ours would choose to live? Perhaps they do colonise space but they don’t colonise our particular piece of real estate,” he says.
French philosopher Clement Vidal has argued that high-tech aliens could be living inside black holes, where they can harness huge amounts of energy. Trouble is, the nearest black hole is believed to be 3,000 light years away.
Or it could be that we need to completely change our mindset. Serbian astronomer Milan Cirkovic´ argued that biologically based intelligent life will arise at various points in the galaxy, but inevitably pursue a trajectory that leads to post-biological evolution.
Webb explains, “Given enough time, biologically based intelligence will create artificial intelligence. Post-biologicals, freed from the shackles of a corporeal existence, need not remain planet-bound and huge differences between our minds and theirs might make communication impossible.” Effectively, we would be alone.
They don’t exist— we’re home alone
The Earth-Moon system may be essential for life
Some scientists believe that our home planet is a one-off. So perhaps the uniqueness of our beautiful planet explains why we are alone?
For a start, all the planets in our solar system have natural satellites, but our moon is unusual in being so large compared to the Earth. It’s the moon’s steady pull that stabilises Earth’s rotation and tilt, keeping our climate relatively stable. It also raises ocean tides (a possible factor in getting life started) and its gravity-amplified volcanic activity would have helped form our atmosphere. “Perhaps double planets such as our Earth-Moon system are necessary for life,” adds Webb.
David Waltham, British geophysicist and author of Lucky Planet, argues that Earth’s unusual 4 billion years of climate stability has allowed complex, multicellular life to develop.
Despite this relative calm, life on Earth has had to contend with meteor impacts, super volcanoes, extreme glaciation (ie, Snowball Earth events) and mass extinctions. Dire as they sound, perhaps even these were necessary for the development of intelligent life.
“According to some scientists, neither higher plants nor animals would exist today if it weren’t for past Snowball Earth events,” says Webb. Researchers suggest that a surge of nutrients into the sea after the big melt could have spurred the evolution of more complex animals.
Another explanation might be that life itself is a rare phenomenon. Webb explains, “Perhaps the emergence of life from non-living material is an almost miraculous fluke. Perhaps only Earth experienced the right sequence of biological and environmental events that made possible the evolution of animal life?”
Or we may be alone because intelligence at the human level is unique. After all, we’re the only species (out of 50 billion or so) in the history of Earth to have developed language. And our extraordinarily dextrous hands have allowed us to make tools and begin our journey to high technology. “What is the chance that an extraterrestrial species will follow the same sort of evolutionary path as humans?” questions Webb.
He concludes, “I would like to think that we share the galaxy with weird and wonderful strange creatures with whom we could communicate about philosophy, religion and science. But I don’t buy this ‘high intelligence evolved on Earth so it must eventually evolve on other planets’ argument.
“I think we might one day visit distant planets and find oceans teeming with strange, microscopic organisms—lots of life, but not life with which we can communicate. And why should we expect high intelligence to be widespread? Millions of species on Earth manage to survive in a spectacular number of ways without it.
“We are searching for intelligent, conscious, tool-making beings that have developed a language we’re capable of understanding and the tools of science and mathematics. We’re searching for ourselves.”
If the Universe Is Teeming with Aliens…Where Is Everybody? by Dr Stephen Webb (Springer International Publishing, £22) is out now.