But let’s say, just for the sake of argument, that there are one or more star-faring alien civilizations in the Milky Way. We’re all familiar with Hollywood’s darker depictions of what aliens might do when they come to the Earth: zapping the White House, harvesting humanity for food like a herd of cattle, or sucking our oceans dry. These scenarios make great films, but don’t really stand up to rational scrutiny. So let’s run through a thought experiment on what reasons aliens might possibly have to visit the Earth, not because I reckon we need to ready our defenses or assemble a welcoming party, but because I think considering these possibilities is a great way of exploring many of the core themes of the science of astrobiology.

I. Aliens come to Earth to enslave humanity or for breeding partners

Alien races enslaving each other is a common trope of many science fiction universes. While enslavement of defeated enemies or other vulnerable populations has regrettably been a common feature of our history on Earth, it’s hard to see why a species with the capability of voyaging between the stars, and therefore having already demonstrated the mastery of a highly advanced level of machinery and of marshaling energy resources, would have any need for slaves. Constructing robots, or other forms of automation or mechanization, would be a far more effective solution for labor—people are feeble in comparison, harder to fix, and need to be fed.

Likewise, the idea of an alien species needing humans for breeding doesn’t really stand up to scrutiny. The act of sexual reproduction, on a genetic level, involves the combination of  DNA from two individuals. So on the most fundamental level, for an alien race to be compatible with us, they would need not only to use the same polymer, deoxyribonucleic acid, as the storage molecule for their genetic information, but also to use the same four “letters” for their genetic alphabet (and not other purine and pyrimidine bases that exist in chemistry), and the same coding system for translating those sequences of genetic letters into proteins, and the same organizational structure of the DNA strands into chromosomes, and so on. There is a lot of ongoing research on whether extraterrestrial life is likely to use DNA, or what molecular alternatives there might be, but it is a huge stretch to expect alien life to be that similar to human genetics. Humans can’t even interbreed with our closest evolutionary relatives on Earth, the chimpanzees (indeed, this is the basis of the definition for different species—two organisms which are not able to reproduce fertile offspring), and so it is overwhelmingly improbable that an alien life form from a completely different evolutionary lineage would be compatible.

II. Aliens come to Earth to harvest us for food

If aliens wouldn’t be bothered about enslaving or breeding with us, might they simply be coming to Earth for a drive-by meal? The question of whether an alien biochemistry would be able to digest us as food actually comes down to some very fundamental features of the molecules of life. Our cells are made up of various organic molecules: proteins (polymers of amino acids), nucleic acids DNA and RNA (polymers of bases and sugars), and membranes of phospholipids. And so for making more cells for reproduction, growth and repair of our bodies we need a source of these simple building blocks. We eat other animals or plants and our digestive system breaks them down into their component amino acids, sugars, and fatty acids, which we then use as the building blocks for ourselves. So in order to derive any useful nutrition from eating a human, an alien monster would need to be based on very similar biochemistry, and thus have the enzymes needed for processing the molecules we are built from.

A whole variety of amino acids, sugars and fatty molecules are actually found in certain meteorites, having been produced by astrochemistry in outer space, and so maybe extraterrestrial life would be based on the same basic building blocks as us. But there’s another, very interesting subtlety here. Simple organic molecules like amino acids and sugars can exist in two different forms, mirror images of each other (in the same way your two hands are similar shapes but can’t be placed exactly one on top of the other). These two versions are known as enantiomers, and it turns out that all life on Earth uses only left-handed amino acids and right-handed sugars, whereas non-living chemistry produces even mixtures of both kinds.

So if we do find traces of amino acids on Mars, one very good way of telling whether these organic molecules are the relics of ancient Martian life or are just the product of astrochemistry would be to check if they are mostly left- or right-handed forms, or just an even mixture. The most exciting discovery would be to detect traces of ancient bacteria on Mars and to find that they employ the opposite forms of organic molecules to us: right-handed amino acids or left-handed sugars, because then we would know for sure that this life was definitely extraterrestrial and not merely contamination from Earth. So here’s a fascinating thought: alien invaders could be based on exactly the same organic molecules (amino acids, sugars, etc.), but they still wouldn’t gain any nutrition from eating us as the origins of life on their own planet settled on the opposite enantiomers. We’d be mirror images of each other, on a molecular level.

III. Aliens come to Earth to suck our oceans dry

If alien marauders would need to have an essentially identical biochemistry to bother culling us for food, maybe they come to Earth to harvest some other vital substance. All life on Earth is water-based; H2O is astonishingly versatile as a solvent and participant in biochemistry and so it seems likely that extraterrestrial life would also be based on this compound. Perhaps, then, aliens may be drawn to the Earth for our wonderfully wet oceans and seas and rivers and lakes—to siphon off our hydrological cycle.

The problem with this supposition is that there are loads of far better sources of water in space. In fact, we think that when Earth first formed from the swirling disc of gas and dust around the proto-Sun it was actually a pretty dry planet; the water to fill our oceans was delivered later by a barrage of comets and asteroids from the colder, outer regions of the solar system. In fact, Europa, one of the moons orbiting Jupiter, contains more liquid water in the global ocean beneath its frozen surface than our entire planet—Europa, and not Earth, is the Waterworld of our solar system. So if you were an alien voyaging between star systems in need of a drink, you’d have access to a far greater amount of water in the icy moons and cometary halo of the outer solar system. You’d also find it much more practical to operate in deep space, rather than trying to suck up the oceans against the gravitational pull of the planet Earth.

IV. Aliens come to Earth for some other raw material

If not water, then maybe there’s some other natural resource that aliens might invade the Earth to exploit. Perhaps they intend to wipe away our cities and begin strip-mining the crust of the planet for ores to extract metals and build more vast spaceships. But in fact, because the Earth formed from a molten state with iron sinking down to the core, our planet’s crust is actually pretty depleted of useful metals like iron, nickel, platinum, tungsten and gold. And as with the water, it’s hard to see why aliens would bother extracting material against the gravity of the Earth when the asteroids are composed of the same basic rocky stuff. In fact, some asteroids are believed to be essentially pure lumps of metal—they were once the cores of protoplanets that were smashed apart again by the colossal collisions in the early history of the solar system. Several companies are already proposing to launch asteroid mining operations to exploit these exceedingly valuable resources.

Perhaps, though, there might be a reason that our hypothetical aliens would come to mine the Earth. While it’s true that the asteroids and Earth, and other terrestrial planets, are made up of essentially the same rocky material, the Earth isn’t simply an inert lump; it’s a very active, dynamic place. In particular, the thin crust of the Earth is fractured into separate shards that are continually sliding around on top of the hot gooey mantle, rubbing alongside each other, crunching head-on, subducting one beneath another, or pulling apart to create fresh crust. This is the churning process of plate tectonics.

So far, astronomers have already found over four and a half thousand extrasolar planets—worlds orbiting other stars—and the expectation now is that there are billions of rocky planets in our Galaxy. But here’s a thought right on the forefront of current planetary science and astrobiology. Perhaps terrestrial planets are common, but terrestrial planets with plate tectonics are rare. Plate tectonics is thought to be vital for keeping the Earth’s climate stable over billions of years to allow complex life like ours to evolve, and it also acts to concentrate certain metals into rich ores. It seems likely that only a small proportion of terrestrial planets undergo plate tectonics (neither Mars nor Venus does). So perhaps an alien civilization would come to the Earth for our exceptional plate tectonics and concentration of particular metals, and the fact that the same tectonics had also enabled a rich biosphere to develop would be merely an inconvenience.

V. Aliens come to Earth looking for a new home

There is a considerable amount of rocky real estate in the galaxy for aliens to consider moving home to, but a terrestrial planet might need to offer more than just a habitable zone locale to be able to support complex life. Communities of hardy microbial cells thriving off inorganic energy deep underground might be able to survive pretty much anywhere, but complex life requires much narrower environmental conditions on the surface. Various features of the Earth beyond our warm oceans are thought to be crucial to maintaining a stable surface environment for geological time periods. These include plate tectonics acting to regulate the climate, a large moon preventing the spin axis of the planet from wobbling too much, and a global magnetic field for deflecting aside the solar wind and preventing the atmosphere being blown away into space. For these reasons, maybe planets like the Earth are something of a rarity, and so present particularly desirable targets for alien colonization.

But while it’s true that such worlds may well be required for complex life to evolve in the first place, once an intelligent species becomes technologically advanced enough to travel between the stars it’s also likely to be able to artificially manage a planet’s environment. For example, many people are already starting to talk seriously about “mega-engineering” or “geoengineering” projects to avoid the worst effects of global warming on Earth, and we’ve worked out, at least in broad terms so far, how further in the future we could “terraform” Mars to create a habitable environment for humans to live on the surface without needing spacesuits. Indeed, the very fact that Earth is already teeming with its own life (most of which is tenacious microbes that affect the chemistry of the atmosphere and oceans) may well be a hindrance to an alien species, with its own quirky biochemistry, looking for somewhere to colonize. It may well be easier to find a terrestrial world that hasn’t already developed life of its own, and install its own biosphere on an empty planet.

VI. Aliens come to the Earth for the Earthlings

To my mind, then, the enormous amounts of time and energy that are likely to be necessary for traveling between the stars in a galaxy, and the fact that raw materials can be sought elsewhere more practically, would rule out aliens coming to the Earth simply to take something we have. I think we can safely rest assured that even if intelligent alien species do exist in our galaxy, they are not about to appear in our skies with an invasion fleet to subjugate humanity and begin stripping our world. Perhaps the thing that may attract only extraterrestrials to Earth is us. I suspect that if aliens did come to Earth, it would be as researchers: biologists, anthropologists, linguists, keen to understand the peculiar workings of life on Earth, to meet humanity and learn of our art, music, culture, languages, philosophies and religions.

If aliens do come to pay us a visit, there’s one final way that the movies have probably got it all wrong. The laws of physics (at least as we currently understand them—after all, in 100 years we may have worked out how to build a practical warp drive or stretch stable wormholes through the fabric of space-time) strongly constrain movement across the vast gulfs between stars. To make the journey time from one star system to the next anything less than scores of millennia, you need to accelerate your spaceship to a fair fraction of the speed of light. The greater the mass you need to accelerate, the greater the energy required, so you really want to keep your starships as small and light as possible.

Intelligent life forms like humans are inherently bulky things, particularly when you want to send a team of them along with all the life support machinery and regeneration systems for keeping them alive in space. But a much more plausible alternative presents itself. Perhaps it’s unrealistic to expect ET to go through all the discomfort and bother of actually voyaging in person across the oceans of interstellar space to far-flung worlds, but instead to travel by proxy. To cross the galaxy not by encasing wet, vulnerable biological organisms within complex life-support technology, but as the hardened, durable technology itself. With a more complete understanding of how the human brain works—the neuronal wiring diagram and other interactions that give rise to intelligence and consciousness—it stands to reason that we could not only simulate this perfectly within hardware to construct an AI (artificial intelligence), but also potentially upload the consciousness of a living person into a computer.

Contained within a capsule of miniaturized electronics and systems for self-repair you’d not only be essentially immortal, but also incredibly compact and light and much better suited for inter- stellar travel. In this sense, perhaps most life in the galaxy isn’t carbon-based (organic), but silicon-based. I don’t mean this in the sense of silicoid monsters imagined living inside volcanoes in The X-Files or Star Trek, but as the hardware supporting complex sentient computer programs. Silicon life would be second generation, existing only because it has been designed and created by a precursor organic species, which itself evolved naturally on a habitable world.

For these reasons, it strikes me that if there is intelligent alien life out there in our galaxy, they almost certainly wouldn’t pay us a visit in person in huge city-sized motherships, but by sending their sentient robots as emissaries. But how would they know we’re here in the first place? Humanity has been leaking (or deliberately transmitting) radio waves out into space for roughly a century,

So an alien civilization running a SETI program with sensitive radio telescopes could detect us. But this radio bubble announcing our technological emergence, centered on the Earth and expanding out into space at light speed, is only around 200 light years across. That is a minuscule region of space in the galaxy as a whole, a disc 100,000 light years in diameter, and so even if the galaxy does contain other intelligent life forms, they would likely still be oblivious to our recent appearance. But although humanity has only been detectably civilized for a century, the Earth itself has been conspicuously alive for many hundreds of millions of years, and this links to one of the hottest topics in current astrobiology. Life on Earth, and specifically photosynthetic life such as plants and cyanobacteria that grow by absorbing the energy of sunlight and splitting water, has been releasing oxygen as a waste gas at such a high rate that it has built up in the atmosphere, first to just a few per cent, and today constituting a fifth of the Earth’s air. Oxygen is a very reactive gas, and the only reason it has been able to accumulate in the atmosphere is that it is constantly being replenished by living organisms. In fact, the presence of oxygen in the atmosphere is thought to be so unusual to the geochemistry of a planet that astrobiologists consider it to be a biosignature of life (specifically if oxygen and a reduced gas like methane are both present).

We are currently on the verge of building space-based telescopes that use spectroscopy to read the composition of the atmospheres of terrestrial exoplanets, and so survey the night sky for signs of life. And we’re only relative newcomers on the galactic scene. There’s nothing special about this exact moment in galactic history, and life on another planet could have evolved intelligence millions of years ago and used their own telescopes to look out for planets displaying the telltale sign of an oxygen-rich atmosphere. But apparently, as far as we can tell the fact that the Earth is obviously sporting biology has not prompted anyone to say hello.

This is a very curious observation, and to my mind could be down to two equally intriguing possibilities. The fact that Earth’s oxygen-rich atmosphere has apparently attracted no one’s attention may simply be because life is so rare that there is not a single other civilization in the galaxy with us to have their attention drawn. Or perhaps planets with an oxygen-rich atmosphere are so staggeringly common that the Earth just doesn’t stand out among the masses. In the first possibility we are solitary and lonely intelligent beings in the galaxy; in the second, life is absolutely rife in the cosmos. Both, to me, are equally profound realizations. And the most exciting aspect is that within your and my lifetime we will have launched our atmosphere-reading space telescopes and the science of astrobiology will have been able to tell which one is right.

What a time to be an intelligent life form on Earth!

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From Aliens: The World’s Leading Scientists on the Search for Extraterrestrial Life, ed. Jim Al-Khalili. Used with permission of Picador. Copyright © 2017 by Lewis Dartnell.