Take Me to Your Leader: Perspectives on Your First Alien Encounter by Neil deGrasse Tyson - 2

If an Alien approaches you with multiple appendages, one of which looks like an extended hand, resist grabbing and shaking it. You don’t know in advance what part of the Alien’s anatomy you just touched, and you probably don’t want to find out. Furthermore, the handshake is not even a worldwide custom, so it’s surely not universal. Nor do you know if raising your hand in a “Hi!” gesture can be construed as an act of kindness or aggression. And what if the Alien chooses to greet you the way Earth dogs greet each other, and it simply wants to come around and sniff your butt? You don’t know anything about your new friend’s customs, so on a first encounter, it’s probably a good idea to leave all your habits at home, until you learn a thing or two about theirs.

We’re taught in biology class that humans share 99 percent of our DNA with chimpanzees, accounting for our common physical features and serving as evidence of a relatively recent common ancestor, living a mere 6 million years ago. 1 Because all life on Earth has a common origin, that means any two species will have a common ancestor somewhere in the tree of life. The more different you are from each other, the further back in time you need to look. For example, the common ancestor between humans and bananas lived approximately one and a half billion years ago. A time in Earth’s tree of life that we could justifiably call the “banana split.” 2 As long ago as that was, humans nonetheless share 25 percent identical genes with bananas. 3

Don’t be upset. We evolved on the same planet.

But if a space Alien arrives from across the Galaxy and has no genes in common with humans—or no genes at all—then we might expect it to look at least as different from all life on Earth as humans and bananas look from each other. Yet most film renderings don’t even try, with Aliens looking identically human—on the outside. If they’re also identical on the inside, then we can ask what it means to think of them as Alien at all. If, however, they’re different on the inside—organs, physiology, biochemistry—a medical professional equipped with no more than a stethoscope would know immediately. If visiting Aliens never got ill or if they never required medical attention, then they could easily be living and working among us, hiding in plain sight. Otherwise, to blend in safely they’d need to hang out at cosplay festivals where dressing like Aliens is common, such as Galacticons and Comic-Cons around the world.

Without good evidence of what actual Aliens look like, we’re stuck imagining them. And imagine them we do. The Internet Movie Database 4 lists no fewer than three thousand films, TV shows, video games, and documentaries with “Alien” in their title or description—both friendly and evil. Mostly evil. Many carry irresistible titles, such as: My Stepmother Is An Alien (1988), Sex and the Single Alien (1993), Cowboys & Aliens (2011), Alien Surf Girls (2012), and Aliens, Clowns & Geeks (2019). In all these portrayals, the humanoid form persists. And none of them look like bananas. Add to these Aliens the mythical-magical places on Earth that creative writers have conjured, such as Brigadoon, Narnia, Gondor, Hogwarts, Arendelle, and of course the Emerald City of Oz, and you’ve encountered Aliens while never leaving the planet. Aliens may have reached a pinnacle of pop culture in the 1996 animated film Space Jam , when a team of five humanoid, trash-talking monsters—one each in the colors blue, green, purple, orange, and red, collectively named the “Monstars”—arrive on Earth to play basketball with live-action NBA great Michael Jordan, along with Bugs Bunny and every other Warner Bros. Looney Tunes character that old-timers will remember, from Daffy Duck to Porky Pig. I

Yes, the Aliens are among us, and they’re here to stay.

In spite of the humanoid bias, Hollywood offers important starting points, however flawed, for us all to think about our first (or next) encounter with actual Aliens. On the matter of size, movie Aliens don’t tend to be extremely small, nor extremely large. In the 1987 film Predator , the ugly humanoid Alien stands about a foot taller than his human adversary, played by Arnold Schwarzenegger. Pegging Arnold at five foot ten II puts the Predator around six foot ten. One in six NBA players is at least that tall, 5 so the Predator’s human proportions are not entirely off scale. And it bleeds. Perhaps they’re scarier when they can look straight in your eyes, rendering the human-sized velociraptor equally terrifying as Tyrannosaurus rex in the 1993 film Jurassic Park .

One might also wonder whether an entire race of Aliens can be no larger than an insect. The question is not whether Alien life could be that small, it’s whether Alien life could be that small and build spaceships that cross the Galaxy. While there’s no law of physics that prevents it, the forces of Nature manifest differently on small scales. It’s the realm of biophysics, and it’s why insects can walk up vertical walls and you can’t. It’s why fleas can jump 150 times their own height 6 and you can’t. It’s why ants can carry up to fifty times their own body weight and you can’t. It’s also why spiders have spindly legs, but hippos, rhinoceroses, and elephants have thick, chunky legs.

The culprit here is that your weight is proportional to the cube of your size, while the strength of your bones is proportional only to the square of your size. So, if you make an animal twice as big, it will weigh 2 x 2 x 2 = 2 3 = 8 times as much as before. But the animal’s capacity to carry that weight increased by only a factor of 2 x 2 = 2 2 = 4 times as much as before. This places a practical upper limit on the size of living beings, lest they collapse under their own body weight. Note that the weight of whales is commonly reported as though they live and work on land. By that measure, the blue whale checks in at 150 tons. But, of course, whales don’t hang out on land. They live in the ocean, where the buoyant force of salt water on their bodies renders them functionally weightless. That’s why they can grow so large with no stress to their physiology and why a beached whale is as good as a dead whale. When life is small, strength wins relative to body weight, granting insects and other small critters their record-setting properties—in their own world.

In our world, however, insects can’t do much. You’d need the weight of 100,000 ants to bend a paper clip. III And that’s only if they all piled on, like college cheerleaders, in just the right configuration. So, how would they manipulate raw materials to fashion a space-worthy launch vehicle? The laws of physics greatly restrict the likelihood of Earth being visited by, much less invaded by, tiny Aliens.

The biggest-ever land animals on Earth come from the age of dinosaurs. Not quite as large as Godzilla, but they weighed upward of a hundred tons. Jumbo Aliens can surely bend our paper clips, and nothing prevents them from visiting us in outsized ships of their own design. But really big, mega-Aliens pose a problem. How about one the size of our solar system? A beam of light requires eight hours to span the diameter of Neptune’s orbit. Life-forms of that size would be neither nimble nor responsive to stimulus. If the Alien’s proverbial bald head itched, the signal would take eight hours or more (at the speed of light) to reach its fingernails and trigger a scratch response. This time delay would afflict any and all decisions our jumbo Alien would make. One might also ask on what mega-planet might such a mega-Alien live, if not just free-floating in space, and how it got to be that size. Not impossible, just unrealistic.

After decades of unimaginative-looking intelligent Aliens in pop culture, the 1977 film Star Wars: Episode IV—A New Hope IV featured a memorable scene in a dive bar, on a planet, in a galaxy, far, far away. Patrons were comprised of impressively varied Alien-creatures, ranging in size from five feet tall to seven and a half feet tall. Each one weirder than the next—drinking or smoking or both. The bar also featured an Alien jazz band with each performer playing a different wind instrument. This scene broke many molds for what Aliens could look like in films and paved the way for even more creative Aliens in the film’s sequel, and beyond. Yet nearly all Aliens depicted had a head, two eyes, a mouth, shoulders, two arms, two hands, and ten fingers.

I suppose we shouldn’t be surprised that most Hollywood Aliens are humanoid, with faces and bodies that resemble our own—an enduring feature of land vertebrates in Earth’s animal kingdom and of human actors paid to don Alien costumes. Even the xenomorph V creature, birthed in the 1979 film Alien, VI starring Sigourney Weaver, sports an (oblong) head, eyes, mouth, teeth, neck, shoulders, arms, torso, legs, fingers and toes, and a tail. But vertebrates occupy just one trunk in the tree of life’s foliage. What of other life-forms? How about spineless invertebrates? Ubiquitous fungus? Photosynthesizing plants? If Earth life is what your Alien models must mimic, then why limit creativity to humanoid vertebrates?

A good place to see what’s biologically possible on Earth is the fossil fields of the Burgess Shale in the Canadian Rockies of British Columbia, Canada. Most animal fossils of the world are the mineralized remnants of animal bones and exoskeletons. The unique composition of the Burgess Shale enabled it to preserve records of soft tissue in countless aquatic animal species from 500 million years ago, just after the Cambrian Explosion of life. This special place on Earth was the focus of the 1989 bestselling book Wonderful Life , written by evolutionary biologist Stephen Jay Gould. A sampling of the Shale’s mostly extinct creatures reveals all manner of squiggly appendages, spikes, random eyeballs, legs, fins, other means of locomotion, and tentacles. Any one of them would make a more inventive Alien than what Hollywood tends to produce.

Once, just once, I want to see a gray Alien portrayed with a full head of coiffed hair. The 1996 farcical sci-fi film Mars Attacks! , with an all-star cast, had a curvaceous femme Alien with big hair, but she was costumed by the invading Martians to look entirely human. So that doesn’t count. What they’ve gotten right from the beginning, however, is flying saucers with ramps that extend down to the ground. When not otherwise beaming you or themselves in or out of their craft, their use of ramps means that Hollywood flying saucers were wheelchair-accessible long before such accommodations were legally mandated in the United States.

Derived from eyewitness accounts, often under stress, including hypnotic revelations, Aliens tend to sort into twelve archetypes: 7

Does the universe offer only a dozen kinds of Alien types, or is that the limit of our capacity to imagine?

The current winner of the most creatively rendered Aliens contest, with no contenders, goes to the adult animated series Rick and Morty , on Cartoon Network. Rick is a crazed sci-tech genius who, through his various inventions, can open wormholes at will, granting him access to all of space, including other universes. Rick drags Morty, his pubescent grandson, on these adventures as they encounter Alien life-forms that would make the Burgess Shale proud. We see not only morphological diversity, but biological and behavioral diversity as well. The writers clearly enjoyed all their science classes in high school and college. Four examples from countless Rick and Morty Aliens include (1) Eye Worms, which are blue worms with a single large eyeball mounted up front. (2) Fart, which is a multidimensional gaseous entity that can convert one element into another, wielding the power of alchemy. (3) Testicle Monster, which is an amorphous blob with elongated tentacles and bulbous legs. It also wields a dozen different-sized eyes. And (4) Memory Parasites, which infect your mind with fake memories that are all pleasing and happy. The internet is persistently abuzz with rankings of the show’s Aliens, but beyond the storytelling, Rick and Morty is an exercise in imagining all the ways of being alive that our universe, and others, might harbor. It appears, however, that this exercise in human imagination long predates Hollywood.

Thousands of years of ancient art—from cave paintings to stone and wood carvings to papyrus etchings—have showcased fantastical renderings of human beings or other life-forms. There’s no end of examples from the dawn of civilization. Common tropes include bubble heads, peculiar appendages, ray beams emanating from hands or eyes, and levitation. The subject is so fertile that the show Ancient Aliens , VII one of the History Channel’s longest-running franchises, persistently tries to convince the audience that all unexplained imagery derives from Aliens meddling in our earthly affairs. The program is now in its third decade and still going strong.

Their formula works—if you ignore these alternative explanations…

… leaving the viewer ripe for thinking that prescientific ancient peoples were eyewitnesses to visiting space Aliens.

In 1974, during the summer after tenth grade, I participated as a student researcher on an expedition to explore uncharted prehistoric menhirs, like Stonehenge, that dot the British Isles. I was especially intrigued by the accompanying geometric stone etchings of either these ancient peoples or their visiting Aliens. So I thought I’d try it myself. With a fist-sized stone I started rubbing circles into a larger stone. To my astonishment, I made similar markings as my ancient predecessors with no more than ten to fifteen minutes of invested time, which left me thoroughly demystified.

By invoking Aliens to explain all of history’s mysteries, we deny our ancestors the power of imagination, which today we cherish and celebrate in our fantasy-fueled storytelling across genres. Five of the top ten highest-grossing films of all time, led by James Cameron’s 2009 Avatar , involve imagined, nonexistent creatures—stuff we just conjured in our heads, with or without hallucinogens. If we add superheroes to that list, the count rises to eight out of the top ten. Consider how many exotic creatures exist in ancient Greek, Roman, and Norse religions (all European), where nobody is crediting ancient Aliens, or Aliens of any kind. The list is long and impressive: from Minotaurs (half bull/half human) to Centaurs (half horse/half human) to Gorgons (including snakes-for-hair Medusa) to the flying Valkyries. Not to mention Apollo’s and Mars’s sky chariots, griffins, and cherubs with aerodynamically useless wings.

The original peddler of this brand of thought is Danish author Erich von Däniken and his 1968 mega-bestseller, Chariots of the Gods . His assertions, however, didn’t deny creativity to ancient peoples. Instead, he denies them intelligence. For example, in the Nazca plains of Peru—the roaming grounds of a civilization from two thousand years ago—appear outsized illustrations etched in the desert floor. Using AI-assisted analysis of satellite imagery, the number of identified and discrete geoglyphs has nearly doubled from previous estimates. 8 The patterns vary. Some are crisscrossed lines. Others resemble wild animals. Some are humanoid. Still others are geometric. The recognizable patterns are so large that they make sense to a viewer only from a high elevation above the plains. Since von Däniken can’t figure out how ancient peoples would possibly know how to create such huge illustrations, and in his own mind he’s clearly smarter than them all, he presumes they were guided by Aliens floating above the plains. Of course, the crisscrossed lines were runways for Alien crafts, even though anything that can hover does not require a runway. But it’s not hard to imagine a way to accomplish all this without appealing to extraterrestrial landscape architects. Create a small grid of squares and draw a picture on it, then make that same grid as large as you choose. All you need to do is draw the same lines within the corresponding boxes of the larger grid. This effectively magnifies your illustration to arbitrarily large sizes, removing the need for Aliens.

That’s a case of “I’m, clueless, so Aliens must have done it.”

Perhaps the most celebrated wonders on Earth that some people are sure were either built or orchestrated by Aliens are the Egyptian pyramids of Giza in northeast Africa, which date from five thousand years ago. For context, the next-tallest stable structure to be built on Earth was the Eiffel Tower in 1889. We should indeed be impressed by this Egyptian architecture from so long ago. But some people are in denial that the ancient Egyptians had anything to do with the achievement. Holding aside the racist undertone that Africa could not possibly spawn an advanced civilization before Europe did, or at all, the premise of the otherwise-entertaining 1994 film Stargate , starring Kurt Russell, was that technologically advanced Aliens portaled from deep space to the Nile valley and built the pyramids. A premise so fertile that it spawned a multi-season, multi-spinoff television franchise.

That’s a case of “I don’t believe it, so Aliens did it.”

For decades, mysterious, large-scale, intricate, and beautiful illustrations would appear overnight in the wheat fields and other grain crops of farmers. Felled and trammeled stalks made excellent, highly visible geometric patterns, best viewed from high above. They came to be known as “crop circles” and were widely presumed to be designed and drawn by Aliens. Most appeared in southern England. The rest in France, Germany, Australia, and of course the United States—all countries, by the way, with no food shortages. Most crop circles would ultimately be revealed as hoaxes. But most is not all. Some have no explanation. Same with reports of mutilated cattle from the 1960s through the 1980s, popular in the United States, especially in Kansas, Nebraska, Colorado, and New Mexico. Some have natural explanations, such as the leftover carnage from a local predator. The rest, a mystery. But what they all have in common is that nobody saw it happen. So, if Aliens are responsible, it means they worked fast, under the cover of night. It also means they like geometric farm art as well as the internal organs of domesticated farm animals. And above all else, they are shy.

A case of “Nobody saw anybody do it, so Aliens did it.”

None of these Aliens, if they existed as imagined, were particularly maleficent. Somebody had to think that up and write stories about it. Between 1895 and 1897, H. G. Wells birthed the evil Alien genre with the serialized novel The War of the Worlds . In keeping with the mythological canon, these non-humanoid Aliens came from planet Mars to kill all humans. Mars, of course, is named for the Roman god of war. The Aliens from Mars were eventually weakened and defeated, not by our most advanced weapons or military tactics, but by Earth-based germs, VIII to which all humans had become immune through natural selection. The freshly arrived Martians, of course, had no such evolutionary immunity.

Some, if not most, of the early popularity of that story can be credited to the wealthy, turn-of-the-century, Mars-crazed astronomer Percival Lowell. He built one of the world’s best observatories in the desert mountains of Arizona. From there, in the 1890s, he would observe Mars and declare that, through his telescope, he could see “non-natural” features, especially canals crisscrossing its surface. He hypothesized that Martian cities were running dry and needed extensive waterways to melt and draw water from the famous Martian ice caps for distribution to the rest of the planet’s surface. He would publish three books, titled Mars (1895), Mars and Its Canals (1906), and Mars as the Abode of Life (1908).

Nobody was able to duplicate his observations. Of course not, because Lowell’s telescope was better than theirs. When bigger telescopes came on line with higher resolution, still nobody was able to duplicate his observations. Of course not, because it was all in Lowell’s head—a bias so strong that it influenced what he believed he saw through his telescope. But that didn’t matter. Mars had catapulted to pop culture awareness as never before. Lowell would also achieve fame by launching the search for Planet X, which led to the discovery of Pluto at his observatory by Clyde Tombaugh in 1930.

So scary and compelling was Wells’s story that it inspired an infamous 1938 Halloween Eve radio play by Orson Welles, presented as breaking news, which spooked many listeners into thinking the invasion was real. It also spawned three feature films of the same title, one in 1953, a second in 2005 featuring Tom Cruise, and one in 2025 featuring the rapper-actor Ice Cube. Actually, there’s a fourth film worth mentioning. The 1996 summer blockbuster Independence Day , starring Will Smith. Again, evil Aliens came from Mars to destroy Earth. But in a plot point not entirely acknowledged in the credits, nor in the marketing, the film draws from the H. G. Wells playbook where, once again, the Martians are defeated, only this time by a computer virus, cleverly uploaded to the Alien computers to undermine their defenses.

If evil Aliens are from Mars and want to kill you, then beautiful Aliens are surely from Venus, a planet named for the Roman goddess of love, and they want to have sex with you. This trope is captured in the 1958 film Queen of Outer Space , where an all-male crew of astronauts lands on Venus, only to be greeted by Zsa Zsa Gabor, a sex symbol of the day, and a colony of aggressively beautiful women, including Marilyn Buferd, a former Miss America. The film was written by men, expressing some kind of Alien sexual wonderland. That male fantasy ran amok in the 1995 film Species, which features a female Alien desperate to mate with human men so she can merge her DNA with ours. How would you accomplish that? Make her irresistibly beautiful, and horny.

Turning the tables, pop-music icon Katy Perry, among her nine number one hits, released a song in 2010 titled “E.T.” for her third studio album, Teenage Dream . That track contains multiple suggestive lines among its fifteen stanzas in which she imagines an Alien encounter. My two favorites are “Stun me with your laser” and “I wanna… be there when you vibrate.” Either it’s all metaphor, or she simply wants to bone the Alien.

Such urges enjoy curious precedent in the form of incubi and succubi—demonic spirits that invade your sleeping body at night. The succubus sexually arouses the male. The incubus arouses the female. Centuries of medieval folklore and religious references to these phenomena offered victims guilt-free accounts of their immoral fantasies and projections. With the arrival of the twentieth century’s space-induced era of flying saucers and reports of abductions, sex-curious demons morphed into sex-curious Aliens. Accounts were rampant of Aliens who poked and probed the abductee’s sexual organs. The victims typically recounted these visitations under hypnosis, because, of course, they were asleep when it happened. 9 The root of this phenomenon may be a condition called “sleep paralysis,” where your body is immovable, but your mind drifts between a state of stupor and awareness, which can impart the memory of a paralytic encounter with a force beyond your control, whether or not sex was involved. 10

As for conscious sex with Aliens, Captain Kirk of the original Star Trek television series was quick to romance comely Aliens who were clearly female, but otherwise green or blue or some other skin color not found among Earth humans. In the 1960s this was entirely acceptable behavior on prime-time television. But when he and his (100 percent human) Nubian Lieutenant Uhura kissed each other on the lips, in a brief scene in episode 10 of season 3, the show received endless angry letters—especially from the American South—objecting to what was the first-ever kiss between a White person and a Black person on television. 11 Sure evidence that race relations on Earth had much further to go. IX

Apparently, being different can be terrifying to others, even if you otherwise look the same. In the creepy 2025 AppleTV series Pluribus , nearly all humans on Earth get infected with Alien DNA, changing their behavior into a hive mind. Everyone otherwise looks exactly the same as before. The OG film in this genre is the 1956 film Invasion of the Body Snatchers . Alien pods arrive on Earth that slowly and systematically replace actual humans—absorbing their looks, their physiology, their talents, their memories. What they lack, however, is human emotion. No monsters. No bloody massacres. No spooky scenes. Yet it’s one of the greatest horror films ever made. Characters in the film simply do not know whether any of their friends or loved ones have been replaced. And neither do you, the viewer. At the time, the United States was only just emerging from the McCarthy-era Red Scare, which stoked a moral panic that led people to suspect their friends and neighbors might secretly be Communists—a political landscape that magnified the film’s storyline.

What if an Alien is behaviorally indistinguishable from you, but is physiologically different? In a 1961 episode 12 of Rod Serling’s The Twilight Zone , titled “Will the Real Martian Please Stand Up?” an Alien craft arrives in the dark of night near a roadside café, but is otherwise concealed by dense woods. Hearing reports of a UFO, state troopers arrive at the scene. Suspicious that some of the diners in the café may be Aliens posing as Earthlings, the troopers interrogate them all to no avail, and leave. Turns out, two of them were actually Aliens, but you couldn’t discern this fact by simply talking to them. One was a (male) scout from Venus, while the other was a scout from Mars—each preparing for an Earth invasion. A fun, mildly terrifying story, but none of these details matter to us here. How were these two Aliens from other planets portrayed? The whole time, the Martian had concealed he had a third arm. The Venutian, under a fez-style hat, reveals a third eye in the middle of his forehead. Those simple adjustments to human physiology passed as humanoid Aliens in 1961.

That same year, Betty and Barney Hill, a married New Hampshire couple with no history of delusion or other mental instabilities, recounted being abducted by extraterrestrials under hypnosis. Snatched while driving along a winding road in the White Mountains, their descriptions of the Aliens and the invasive probing of their bodies were widely publicized and helped establish the modern Alien archetype as well as the Aliens’ fascination with our body orifices.

By the 1970s, with books and television and films riding the zeitgeist, our collective agreement on the appearance of a bona fide, non-evil Alien converged rapidly: Gray colorless body. Can be green. Large eyes. Bald. Disproportionally large head—especially forehead. Long fingers. Thin limbs. Bipedal. And, of course, humanoid, but generally tinier and skinnier than we are. And just in case we all needed our imagination refreshed, in 1987 Whitley Strieber published his bestselling novel Communion: A True Story , a first-person account of the author’s own Alien abduction. The book spent six months on the fiction bestseller list with jacket artwork portraying the head of an extraterrestrial who, by the late 1980s, came from Alien central casting. That visage is here to stay. Even Roswell, New Mexico, the town near the site of a famous July 2, 1947, UFO crash, has monetized the Alien archetype into a local tourist industry. Apart from Alien mugs and T-shirts and innumerate other knickknacks, the streetlights sport classic Alien almond-eyed, head-shaped covers. And why not. July 2 is celebrated as “World UFO Day,” with skywatch parties around the globe. Not to be confused with “Alien Day,” April 26, a marketing ploy by 20th Century Studios to call attention to their lucrative Alien film franchise, with the date extracted from LV-426, the home moon of the original Alien.

Biologists may boast of the extraordinary diversity of life on Earth—a planet that harbors upward of 10 million plant and animal species, 13 most of which are insects, as well as 5 million species of fungus, 14 and possibly a trillion species of microbes. 15 But at the end of the day, and behind closed doors, biologists must confess to one another that the DNA double helix encodes all life on Earth and the commonality of genes points to a single genesis, nearly 4 billion years ago. Why is that a confession rather than a boast? A true quest for biodiversity would be the search for life, with or without DNA.

Could it be that DNA is an inevitable outcome of complex biochemistry anywhere in the Galaxy? If so, think of it as a biological analog to minerals and rocks. The geologist expects them to form the same way—under similar conditions of pressure and temperature—wherever those ingredients are found on Earth and across the Galaxy. If so, then DNA is cosmically fundamental, but we’d have no reason to think that Aliens, having evolved from scratch elsewhere, would contain any genes in common with Earth’s evolutionary tree of life. No banana connections at all.

One of the few Hollywood Aliens that passes the no-DNA-in-common-with-life-on-Earth test appeared in the 1958 film The Blob , starring Steven McQueen. The creature arrived via meteorite, and was a gelatinous, amorphous, flesh-hungry life-form aptly described by the film’s eponymous title. Nothing vertebrate about it. Perhaps more terrifying than your average Alien, since it could enter your home or office by oozing through air ducts and under doorways. And who could forget the microscopic Alien life in Michael Crichton’s first novel (and 1971 film) The Andromeda Strain. Arriving on one of our own sample-return satellites from space, the life-form was crystalline and lethal. Another candidate was the Horta in “The Devil in the Dark” episode from the original Star Trek TV series. 16 It was silicon-based life. And since silicon is the active ingredient in most rocks, this life-form was simply an animate rock. What those Aliens have in common apart from no DNA is that none of them were intelligent. Or more precisely, they wielded no technology.

The attraction of silicon as the basis for Alien life in science fiction storytelling has authentic biochemical foundations. On the Periodic Table, the location of elements is not random. One feature is the common configuration of outer electrons for all elements that appear in a single column. It’s the outer electrons that determine what other elements you get to bond with—by sharing, donating, or receiving them. That’s how you make molecules. The element that sits directly below carbon on the Table is silicon, which means all the molecules you can make with carbon you can also make with silicon. One for one. For example, we have CO 2 (carbon dioxide), but we also have SiO 2 (silicon dioxide). Since life on Earth is carbon-based, why not imagine an Alien biota with silicon atoms swapped in for carbon atoms?

As tempting as that sounds, it’s simply unnecessary. Carbon is the fourth most abundant element in the universe, and it’s everywhere. But more importantly, carbon is a factor of ten times more abundant than silicon. Furthermore, chemical bonds made with carbon tend to be more fungible than silicon bonds, granting Nature a much broader set of experimental recipes to spawn the diversity of life. Especially after episodes of major extinction, when ecological niches have been pried open and become ripe for new species to emerge and thrive. Although one could imagine, in principle, a lava-hot environment where silicon bonds become as fungible as carbon bonds. But suppose we discover life that isn’t encoded by the DNA molecule at all. That would render Earth’s entire tree of life as nothing more than a sapling in the cosmic forest.

These biases might include our collective dependence on liquid water, which leads to the famous “Goldilocks Zone” of star systems: if a planet orbits too close to its host star, heat from the star will evaporate its waterways and oceans; if the planet orbits too far, the water freezes, although important exceptions exist. Jupiter’s moon Europa sits far outside of the Sun’s Goldilocks Zone. Yet under a thick layer of insulating ice sits a moon-wide ocean of liquid water, kept warm by tidal stress induced by Jupiter and other moons. One of several moons in the Jovian and Saturnian systems with these properties.

The liquid water prerequisite is surely our biggest bias. One can even make a stronger statement: almost every place on Earth where there’s liquid water, we have found life. Even the Dead Sea, between Israel and Jordan, contains happy, salt-resistant microbes swimming about. Whoever first named that body of water did not have a microscope. Regardless, maybe life does not require liquid water at all, but instead requires just a liquid.

In the 2026 film Project Hail Mary , starring Ryan Gosling, the Eridians are a friendly race of craboid aliens from the Eridani star system. They thrive in a high-pressure and high-temperature atmosphere of ammonia. Andy Weir, the software engineer turned sci-fi novelist, and author of the 2021 book on which the film is based, conceived of Eridians as a challenge to both the humanoid and water biases of our Alien imaginings.

So, if Aliens crash-land in any of Earth’s hot, sandy deserts, and if they crawl forward with thirsty tongues hanging from their mouths chanting, “Ammonia, Ammonia,” don’t question their means of nourishment. Just honor the request.

Maybe the best rendering of an Alien is no rendering at all. In the 1968 Stanley Kubrick epic, 2001: A Space Odyssey , and in the 1997 film Contact , starring Jodi Foster, based on a novel by American astronomer Carl Sagan, we never see the Aliens. Their form and substance have been left to our imagination—which is perhaps where they should stay until we gather actual evidence on the morphology of extraterrestrials. Until then, we’re compelled to ask, what’s the most different kind of Alien one can imagine?

In the 1500s, the Catholic Church was not in the mood to entertain such possibilities. The Italian monk-philosopher Giordano Bruno was particularly enchanted by Nicolaus Copernicus’s 1543 seminal tract: De Revolutionibus Orbium Coelestium ( On the Revolution of the Heavenly Bodies ), in which he establishes the Sun, and not Earth, as the center of all motion. This thesis directly contradicts a literal read of the biblical Genesis, where you have to wait until the sixteenth verse—after God creates the heavens, the earth, the waters, and the plants—to get the Sun and Moon in the sky.

And God made two great lights; the greater light to rule the day, and the lesser light to rule the night: he made the stars also. 17

Copernicus’s heliocentric idea spooked him enough to wait until he was mostly dead before releasing the book, dying within days of publication. You can’t persecute someone who is already dead. But Bruno took Copernicus’s heliocentric model to its logical conclusion. He reasoned that if Earth is just a planet like all the others in orbit around the Sun, then maybe the stars of the night sky are also suns. And if so, then maybe they too have planets that harbor life.

This idea not only decentralized Earth, it decentralized life on Earth as the object of God’s creation. The Holy Roman Inquisition found the concept so heretical that they put Bruno on trial, found him guilty, and burned him at the stake—upside down, naked, and with a plug hammered into his mouth so that even in Hell he could not preach such heresies. One of his last utterances to his accusers was, “Your God is too small!” Just what a religious, God-fearing Alien might say of us as well.

By 1889, a contrite monument to Bruno would be erected in the Campo de’ Fiori piazza in Rome, the site of his execution. Fast-forward to November 2009: Inspired by rampant reports of UFOs and Alien abductions, the Pontifical Academy of Sciences held a conference of scientists and religious leaders in Rome. The subject? Life on other planets. Apparently, it’s no longer heresy to think such things, so at least for now, we can count the Catholic Church as being all-in on Aliens.

If there were an Alien sighting guide, like the kind birdwatchers might reference but for birds, the Alien you least want to meet is the one made of antimatter. One of the great mysteries of modern astrophysics is the asymmetry between the matter and antimatter in the universe. In every experiment ever conducted, when you create matter out of energy—understood via Albert Einstein’s famous equation: E = mc 2 —you get matter-antimatter particle pairs. Each particle has exactly the same mass as the other, but opposite charge and other opposite properties in the quantum realm. This favorite fuel of sci-fi rocket propulsion is real. First predicted in 1928 by the English physicist Paul A. M. Dirac, it was discovered four years later by the American physicist Carl Anderson. During the early universe, when everything was a seething cauldron of energy, matter and antimatter would spontaneously pop in and out of existence, transforming back and forth into photons—light energy. But as the universe expanded and cooled, there would be one last time that a photon became a matter-antimatter particle pair. Except that one in a hundred million of the photons mysteriously created a lone particle without an annihilation partner, which would evermore remain adrift in space with an empty dance card. The rest would find partners and annihilate, becoming photons one last time.

That one-in-a-hundred-million particle was made of matter, with no antimatter particle to match it. Nobody knows how or why that happened. But it did, leaving us with a universe of a hundred million photons for every particle of matter. And no antimatter. Without that tiny asymmetry, the universe would contain only photons and no matter, and we wouldn’t exist. So we’re glad for the asymmetry, but the question remains, what caused it? Where did the antimatter particles go, if in fact they were created at all? And most importantly, did Aliens gather them up and create an anti-universe of their own?

On the off chance that your first encounter is with an Alien made entirely of antimatter, always carry something in your pocket that you can toss. A coin will do. If, upon catching it, the Alien spontaneously explodes with the power of two hundred million sticks of dynamite, then it was made of antimatter. Otherwise, you’re good to go.