I want to end this book with a good old-fashioned romance. Maybe even a tear-jerker. You might wonder what romance is doing in a science book, but this is a story that plays directly to our emotions, and our sense of who we are. Like all romances, it has protagonists. And, in this case, the protagonists are us and them. Us because we’re human, and them – whoever or whatever they are – because we can’t stop thinking about them.
It was back in the time of Galileo and Kepler that people first thought seriously about intelligent life in space, and we haven’t stopped since. Johannes Kepler was the German mathematician and astronomer who worked out the laws of planetary motion in the early 17th century. In 1610, he wrote a long letter to Galileo in praise of his newly published Starry Messenger and the discoveries it reported, particularly in regard to our Moon and the moons of Jupiter. It contains several allusions to the citizens of the Moon (who, he was sure, had created circular embankments to protect themselves from the Sun’s radiation) and the inhabitants of Jupiter.
His logic regarding the Jovians is impeccable:
The conclusion is quite clear. Our Moon exists for us on the Earth, not for the other globes. Those four little moons exist for Jupiter, not for us. Each planet, in turn, together with its occupants, is served by its own satellites. From this line of reason we deduce with the highest degree of probability that Jupiter is inhabited.
QED – but I’m not sure that it stands up terribly well. It’s on a par with Star Wars or the adventures of my childhood space hero, Dan Dare, in Britain’s Eagle comic. At least their exploits were unashamedly fictional – although Dan Dare’s gifted artist, Frank Hampson, threw in a healthy dose of real science, too. Perhaps that’s why I’m still excited by this stuff, 60 years later.
Science fiction has imagined extraterrestrial life-forms in every permutation from the hostile to the benevolent. But as Stephen Hawking noted in 2016, ‘Meeting an advanced civilization could be like Native Americans encountering Columbus. That didn’t turn out so well.’ We in Australia don’t have to look far to see truly devastating parallels in our own history. And is it even possible that the marauding extraterrestrials might only be interested in trying out a scrumptious new protein source, oven-ready and tastefully dehaired? Yes, it probably is.
That said, there’s really no point in trying to hide from them. While we haven’t made a habit of aiming radio signals at likely-looking solar systems (apart from a couple of early experiments), our planet has been radio-loud for over 80 years, emitting broadcasts and communications to the Universe at large. Astronomer Seth Shostak of the SETI Institute speaks of ‘leakage wafting sky-wards’, which any extraterrestrial society capable of threatening us would be able to detect. And there are five NASA spacecraft leaving the Solar System altogether, to wander through interstellar space for perhaps billions of years. They are the two Pioneers (launched in 1972 and 1973), the two Voyagers (both launched in 1977) and New Horizons (launched in 2006). Each carries tokens of humanity, including directions on how to find us and, in one instance, depictions of how tasty we look. On the issue of beaming signals to advertise our presence to interstellar targets, however, the Breakthrough Initiatives founded in 2015 by Russian entrepreneur Yuri Milner include something called ‘Breakthrough Message’. Its declared aim is ‘To encourage global discussion on the ethical and philosophical issues of sending messages into space.’
SO, WHERE ARE WE IN THE QUEST TO FIND EXTRATERREStrial intelligence? The science of astrobiology – the study of the origin, evolution and distribution of life throughout the Universe – is thriving. It is teaching us a lot about ourselves and our fellow earthly species, as well as giving us insights into the possibilities of life elsewhere in the cosmos. We’re encouraged by the fact that on Earth, microbial organisms occupy every possible niche, from mountain-tops to deep oceans – not to mention in the planet’s crust and atmosphere. And extremophiles such as tardigrades are great examples of the tenacity of life.
It does raise the question, though, of how life is actually defined, and in a cosmic sense, that’s not an easy one to answer. It’s no good looking for something like DNA and hoping for the best. Less Earth-specific definitions are required. One I quite like defines a living organism as a self-sustaining, self-replicating entity that is capable of Darwinian evolution. You might think that’s not specific enough, though, because it’s possible to envisage machines that exhibit those characteristics.
My astrobiology colleagues, Paul Davies (Arizona State University) and Charley Lineweaver (Australian National University), want to espouse even broader definitions of life, however – definitions inspired by some unexpected work carried out by the physicist Erwin Schrödinger in the 1940s. You might remember him from his famous quantum cat, which is simultaneously alive and dead until you have a look in its box, whereupon it is decidedly one or the other.
Schrödinger made a valiant attempt to reduce living organisms to applied physics, but came to the conclusion there must be something else going on, too, because life seems to defy the fundamental laws of thermodynamics. Davies proposes that the ‘something else’ is information, whether it’s encoded in DNA or some more amorphous construct such as networks of chemical reactions. Lineweaver goes further, and looks for what he calls ‘far from equilibrium dissipative systems’ – things that are out of balance with their surroundings in a chemical or physical sense. The trouble with that definition is that it includes entities we wouldn’t normally think of as living, like the turbulent atmospheres of planets and stars. Charley Lineweaver is unfazed by that, citing the fact that it makes people think more carefully about such matters.
I THINK IT’S FAIR TO SAY THAT MOST ASTROBIOLOGISTS have tended to avoid these issues by focusing on life as we know it on Earth – bog-standard carbon-containing water-based life. And, in that regard, the results coming from astronomy and planetary science are wholly encouraging. Water is everywhere – it’s the most abundant two-element molecule in the Universe. And it’s in plentiful supply in the Solar System. Admittedly, much of it is frozen, in the subsurface soil of Mars, the ice-shells of moons like Enceladus, and the nuclei of comets. Even in liquid form, it’s more abundant than you might expect. Jupiter’s moon Europa, for example, harbours perhaps twice as much water under its icy crust as there is in Earth’s oceans, and Saturn’s Titan is thought to have even more. And then there’s the carbon, which is found all over the place, often locked up in complex organic molecules.
When you broaden your horizons beyond the Solar System to our Milky Way Galaxy, the picture becomes even more promising. Since the only life we are aware of has evolved on a planet, other planets would seem like a good place to start, and the exoplanet community has been doing a grand job of finding them. Our tally of confirmed exoplanets (planets outside the Solar System) passed 4000 in March 2019, and will eventually be joined by another 2870 candidate planets currently awaiting confirmation. And who knows how many more beyond that? The bottom line is that planets are commonplace – something we didn’t know in 1995, when the first exoplanet orbiting a normal star was discovered. Statistically, every star in the Galaxy must have at least one planet.
Something else we didn’t know about in 1995 was the enormous variety of planetary systems out there – from hot Jupiters that almost skim the surface of their parent stars to remote objects whose orbits take thousands of years to traverse. They range in size from planets many times larger than Jupiter to worlds little bigger than the Moon, and exhibit an equally spectacular range of environments, from frigid ice-worlds to planets so hot that iron drizzles out of the clouds.
Somewhere in the middle of this glittering variety of planet-hood are those orbiting within the habitable zone of their parent star – the region where water can exist in liquid form. Comfortingly known as the ‘Goldilocks zone’, it’s where the temperature is not too hot and not too cold, but just right. And Goldilocks-zone objects about the size of Earth have a particular appeal to astrobiologists. The detailed study of such worlds is difficult with the current generation of large optical telescopes, but the new generation of ‘extremely large telescopes’ with mirrors more than 20 metres in diameter will come online in the mid-2020s, providing new capabilities. In particular, the spectroscopic analysis of exoplanet atmospheres will become routine, allowing us to search for biomarkers – the signatures of chemicals associated with living organisms. If those chemicals also include industrial pollutants that could never be created by natural processes, that would be the discovery of the millennium.
Finally, astrobiologists’ optimism comes from the sheer numbers that emerge when you look beyond our Galaxy to the wider Universe. The most recent estimate of the number of galaxies that are observable from Earth is two trillion. Typically we’d expect each to contain 100 billion stars or so, resulting in an estimate for the total number of stars in the observable Universe of 2 × 1023. You can probably guess what’s coming next: how does that stack up against Carl Sagan’s famous statement that there are more stars in the Universe than grains of sand on all the beaches of the Earth? A long time ago, I checked his calculation, and he was right. In fact, if you throw in that latest estimate of the number of galaxies, he was more right than he could have known. The stars in the Universe outnumber the grains of sand on all the beaches of not one, but two hundred Earths.
SO, IS LIFE ABUNDANT THROUGHOUT THE UNIVERSE? IT may well be. Simple life, at least – single-celled organisms, or microbes. Green slime, perhaps, or its interplanetary equivalent. But what are the odds of those single-celled micro-organisms evolving into complex life-forms, and ultimately into intelligent life? Once again, we immediately run into the problem of definition. What is intelligent life?
In a 1995 article, Carl Sagan defined intelligent organisms as being the functional equivalent of humans. And astrobiologist Charley Lineweaver points to two possible routes for achieving that. One, known in evolutionary biology as convergent evolution, says that the same capability-enhancing traits can be independently acquired by unrelated species with completely different lineages. The evolution of flight is a good example. But, as Lineweaver points out, several environments on Earth have been isolated from each other by the drift of continental plates for far longer than it took the human brain to achieve its present complexity in Africa – but have not yielded any independent equivalent. So he adopts the opposite view, in common with a number of evolutionary biologists. This is that the evolution of intelligence is a quirk of nature resulting from a rare and probably unrepeatable sequence of events.
If that seems depressing, the broader biological picture offers no comfort. We know that the first microbial life appeared on the infant Earth perhaps four billion years ago and a few hundred million years later the first complex organism emerged. There may have been other varieties, but only one survived. How do we know that? Because all complex life on Earth – known as eukaryotic life – can be traced back genetically to that unique progenitor, known as LUCA (the Last Universal Common Ancestor). So far, we have found no trace of an evolutionary false start that could hint at a second genesis of eukaryotic life on Earth.
Once again, thermodynamics enters the picture here, with a few scientists pointing out that eukaryotes are vastly more energy-hungry than their single-celled ancestors, and perhaps that is why their emergence was a one-off. British biochemist Nick Lane has noted that because of the energy demand, ‘there is no inevitable evolutionary trajectory from simple to complex life’. He says complex life is just a fluke. This view is shared by other astrobiologists, who are pessimistic about the development of any multi-celled organisms beyond Earth – let alone higher life-forms and extraterrestrial intelligence.
That pessimism provides a gloomily convincing explanation for the question posed in 1950 by Italian physicist Enrico Fermi. It’s now known as the ‘Fermi Paradox’, and asks, ‘Where is everybody?’ It’s based on the Copernican principle that there’s nothing special about us, so you’d expect intelligent life to be commonplace. Given the hundreds of billions of stars in our Milky Way Galaxy, and its age of around 12 billion years, even if there is only a small probability that intelligent life has evolved elsewhere, its existence should be evident by now. They’d be everywhere. If you can travel close to the speed of light, interstellar distances are no problem. And even if you can’t, there’s always the possibility of interstellar voyages incorporating successive generations of travellers. Then there are leaked radio transmissions of the kind that Earth has been emitting for decades. So, yes, intelligent species should be detectable unless they have evolved in such a way as to make their presence invisible. Or unless they’ve been and gone, and are now all extinct. But those thermodynamically minded biologists think it’s much more likely that they haven’t turned up yet. Except here on Earth.
This view is also supported by a recent Oxford University study that looks in detail at the famous Drake Equation, formulated by American astronomer Frank Drake in 1961. The equation attempts to estimate the number of intelligent civilisations in our Galaxy by looking at a series of factors such as the rate at which suitable stars form, the fraction of those stars with planets, the number of those planets suitable for life and the number on which life actually appears. Then you factor in the fraction of life-bearing planets on which intelligence emerges, the number of those that produce technology capable of emitting signals into space, and the fraction that actually go ahead and do so. Most of these factors are just guesses, although at least we now know that most stars do have planets. But with the very best current estimates, the new study indicates that there are unlikely to be any other civilisations within the observable Universe. So – the Fermi Paradox is no longer a paradox. They just aren’t there.
Should we stop looking for them? No, because of what we might discover on the way – and that’s why I’m an enthusiast of initiatives like SETI and Breakthrough Listen. Our quest for higher life-forms inevitably takes us beyond the confines of the Solar System, and we rely on technology that is the stock-in-trade of astronomical research. That means telescopes – optical and radio – with the associated smart technology that we’ve met elsewhere in this book. In most cases, the technology is the same whether you’re investigating the snacking habits of supermassive black holes or seeking signs of intelligent aliens on distant planets. And astrobiology, like most other branches of astronomy, pushes these technologies to their limits.
I PROMISED YOU ROMANCE AT THE START OF THIS CHAPter. We’re still in love with the idea of beings like ourselves going about their business in a galaxy far away. That remains a possibility, of course – and we’re never going to be able to prove it’s not the case, with all those stars to check out. Until we have evidence for their existence, however, the extraterrestrials will remain in the realm of fantasy. Of course, that does bring its own silver lining. If they don’t exist, they can’t eat us.
I think there’s also a perverse romance in the idea of this vast, incredible Universe that you’ve been reading about containing only one species able to contemplate it. It is strangely disturbing. What’s it all for? Does it suggest that we – as a bizarre and unlikely outcome of the laws of physics and natural selection – don’t actually belong here? And, if we weren’t here, would the Universe still be?
These are profound questions that we may never be able to answer because we’re simply not capable of it. Perhaps the situation was best summed up by the great theoretical physicist and Nobel Laureate, Max Planck. ‘Science cannot solve the ultimate mystery of Nature,’ he once remarked, ‘and it is because in the last analysis, we ourselves are part of the mystery we are trying to solve.’