Life out there
The astronomer royal on the—increasingly serious— investigation into extraterrestrial intelligence
We are used to thinking of extraterrestrial life and intelligence as topics on the speculative fringe. But recent advances on several fronts are rendering them almost mainstream. The transition from complex chemistry to the first entities that could be described as “living” poses one of the most crucial problems in science. Now this problem is being taken seriously. And as it is grappled with, we can begin to think rigorously about whether some of the hundreds of billions of Earthlike planets in our galaxy might also have a biosphere.
Speculations on “the plurality of inhabited worlds” may be newly respectable, but the speculations themselves are anything but new. Indeed, in 200 AD, Lucian of Samosata wrote what we would now call a fantasy novel, about a trip to the Moon where exotic monsters roamed. From the 17th to the 19th century, it was widely suspected that the other planets of our solar system were inhabited. The astronomer William Herschel thought that even the Sun might host life.
The arguments were o en more theological than scientific. Eminent 19th-century thinkers argued that life must pervade the cosmos, because otherwise such vast domains of space would represent a waste of the creator’s efforts. The physicist David Brewster (1781-1868), for example, conjectured on such grounds that the Moon must be inhabited. Had it been “destined to be merely a lamp to our Earth,” he wrote, “there was no occasion to variegate its surface with lo y mountains and extinct volcanoes, and cover it with large patches of matter that reflect different quantities of light and give its surface the appearance of continents and seas. It would have been a better lamp had it been a smooth piece of lime or of chalk.”
Natural selection theory was an intellectual revolution that drew a line under such teleological ways of thinking,
most obviously in biology: things were as they were until they evolved in response to competition with other species or environmental changes; there was no longer any need to appeal to the designs of a creator or any other form of plan. Tellingly, an amusing critique of ideas such as Brewster’s is given in the 1904 book Man’s Place in the Universe by Alfred Russel Wallace, the man who—in parallel with Darwin—came up with the theory of evolution.
“The origin of life used to be one of those problems condemned to remain in the ‘too difficult’ box”
But a less “designed” universe didn’t have to mean an emptier one. By the end of the 19th century, so convinced were many astronomers that life existed on other planets in our solar system that the Académie des sciences in Paris offered a prize of 100,000 francs to the first person to make contact with them. And the prize specifically excluded contact with Martians—that was considered far too easy. The erroneous claim that Mars was criss-crossed by canals had been taken as positive proof of intelligent life on the red planet.
Ironically, what really closed the scientific speculation down was the initial opening of the final frontier. The Space Age of the 20th century brought sobering news. Venus, a cloudy planet that promised a lush tropical swampworld, turned out to be a crushing, caustic hellhole. Mercury was a pockmarked blistering rock. And probes such as Nasa’s Curiosity showed that Mars, though seemingly the most Earth-like body in the solar system, was actually a frigid desert with a very thin atmosphere. There may, conceivably, be some form of life swimming under the ice of Jupiter’s moon Europa or Saturn’s moon Enceladus, but nobody can be optimistic. Certainly, there’s no expectation of advanced life anywhere else in the solar system.
But the prospects brighten enormously when we extend our gaze beyond our solar system—and beyond the reach of any probe that we can devise today. What has transformed and energised the field is the realisation that most stars are orbited by retinues of planets. Giordano Bruno had speculated about this way back in the 16th century, but it was as recently as the 1990s that the hard evidence started to emerge. These “exoplanets” are hard to detect directly but are inferred from precise observations of their parent star. If a planet transits in front of its star during orbit, then it will block out some of the star’s light—so the signature of the planet is a periodic dimming of the star that it is orbiting. The interval between successive dips tells us the planet’s “year”; the extent of the dimming tells us how big it is.
There’s huge variety among these exoplanets, but there are probably around a billion planets in the Milky Way that are “Earth-like,” in the sense that they are about the same size and at a distance from their parent star such that water can exist, neither boiling away nor staying permanently frozen.
These planets could potentially be “habitable.” But of course that doesn’t mean that they are inhabited; indeed, we can’t yet exclude the possibility that life’s origin involved a fluke so rare that it happened only once in the entire galaxy. On the other hand, that all-important transition from complex chemistry to “life” might have been almost inevitable given the right environment. We just don’t know—not least because we don’t even know if the particular DNA/RNA chemistry of our own terrestrial life is the only possibility, or whether it is just one chemical basis among many options that could be realised elsewhere.
The origin of life used to be one of those problems (consciousness, for instance, is still in this category) which, though manifestly important, was intractable, and condemned to remain in the “too difficult” box. And despite all we have learnt about Darwinian evolution, life’s origin—the transition from complex chemistry to the first metabolising and reproducing entities that we would call “alive”—remains a mystery. But it’s now attracting serious attention, because the tantalising prospect of alien life has combined with progress in astronomy and biochemical science.
Back in the 1500s, as well as postulating the existence of exoplanets, Bruno conjectured that some might host not merely flickers of life but other creatures “as magnificent as those upon our human Earth.” If simple life exists, what are the odds that it has evolved into something that we would recognise as intelligent? Even if primitive life were common, the emergence of “advanced” life may not be—it may depend on many more contingencies. The course of evolution on Earth was influenced by phases of glaciation, the Earth’s tectonic history, asteroid impacts and so forth. Several authors have speculated about possible “bottlenecks”—key stages in evolution that are hard to transit. Perhaps the original transition from mono- to multi-cellular life is itself one of these. Certainly, the fact that simple life on Earth seems to have emerged quite quickly, whereas
even the most basic multi-cellular organisms took more than two billion years to appear, suggests there are special barriers to complex life.
Or the “bottleneck” could come later. Even in a complex biosphere, the emergence of intelligence isn’t guaranteed; some evolutionary scientists regard it as unlikely. If, for instance, the dinosaurs hadn’t been wiped out, the chain of warmblooded mammalian evolution that led to humans may have been foreclosed. Without the five disruptive “mass extinctions” during Earth’s history, evolution could have been slower—giant but intellectually dim creatures could have remained comfortably entrenched at the top of the food chain for far longer. We just don’t know whether another species would then have taken our role.
In considering the possibility of life “out there,” we should surely be open-minded about where it might emerge and what forms it could take—and devote some thought to non-Earthlike life in non-Earth-like locations. The entities that extraterrestrial searches could reveal may not be “organic” or biological in the usual sense. One thing astronomers know is that the Sun is less than halfway through its life—it will last another six billion years, and the Milky Way has longer to run. We humans are surely not the culmination of evolution. Some stars are much older than the Sun, so life on a planet orbiting one of them could have had a head start of a billion years, and perhaps evolved into some digital form, where “secular intelligent design” takes over from Darwinian selection. Another twist could be that it leaves the planet where its biological precursors had lived—interstellar voyages are less daunting for such entities.
But it plainly makes sense to start with what we know: proceeding with the “searching under the streetlight” strategy by focusing our hunt, at least initially, on the places that can be illuminated by our established understanding. Distance must be a criterion for prioritising our efforts. The unimaginatively named “Extremely Large Telescope” will have a mirror 39 metres across, and from its home in Chile will gather enough light for us to infer whether any nearby exoplanets have biospheres or vegetation. We may get clues sooner than that from the James Webb Space Telescope, due for launch shortly as a successor to the Hubble Telescope which orbits the Earth. But even these next-generation telescopes will have a hard job separating out the spectrum of the exoplanet’s atmosphere from the spectrum of the far brighter central star. To optimise the prospects, we therefore need beforehand to have scanned the whole sky to identify the nearest planets that appear to be Earth-like in those basic respects of temperature and gravity.
In time, however, we might be able to push at the frontiers with fewer restrictions. One day we will be able to deploy robotic fabricators which can construct and assemble large, lightweight structures in space or on the Moon—perhaps using materials harvested from asteroids. So rather than constructing telescopes for orbit and launching them into space, they will begin life there. And with huge gossamer-thin mirrors assembled under zero gravity, these tools will further expand our vision of the stars, galaxies and the wider cosmos.
Meanwhile our entire solar system—planets, moons and asteroids—will soon be explored by flotillas of tiny robotic cra . In a quest to discover life “out there,” the practical case for risky and costly manned spaceflight gets ever weaker. Techniques are advancing fast, allowing more sophisticated unmanned probes. Our scientific priority should be to use these to search for the most basic forms of life under the ice of Enceladus or Europa. Such a discovery would tell us that the origin of life wasn’t a rare fluke: if it had emerged twice in a single planetary system (such as our own solar system), we could draw the momentous conclusion that it exists in billions of locations in our Milky Way (which contains a hundred billion stars).
If some people will one day walk on Mars (as I hope they will), it will be as a personal adventure. The huge gulf in cost, and the possibility of one-way tickets, makes machines the more fruitful instrument of discovery.
Life may well be out there, but it’s important to think about whether it is life as we would recognise it. That is a philosophical question, but it is suggestive of potential practical problems. We may not be “alone” in the universe—and our best chance of discovering something is by accepting that it is not a challenge for humans alone. Instead, we should grab all the automated help we can get.
Personally, I think there’s a reasonable chance of our finding some form of extraterrestrial life— but I’m not holding my breath for the kind of intelligent aliens familiar from science fiction.
Martin Rees is the astronomer royal. “The End of Astronauts,” co-authored with Donald Goldsmith, is out in March 2022
“Alien life could have had an evolutionary headstart and changed into digital form”