Dedication

TO PROF BRUCE BALICK.

I always tell incoming students that choosing a good PhD advisor is the most important decision they’ll make in graduate school. I do this because I was so lucky in finding you. Thank you for your lived example of what a life in science should look like: creativity, kindness, delight, and precision.

Contents

Cover

Title Page

Dedication

Introduction

Chapter 1: How Did We Get Here? How Our Ancient Questions About Aliens Took Their Modern Form

A Really Old Question: Alien Debates Through History

Fermi’s Paradox: Is There a Great Silence?

The Drake Equation: Asking the Right Questions

The UFOs Arrive: Kenneth Arnold Sees Saucers. Roswell Gets Busy. The Government Reports.

Invasion of the Pop-Culture Extraterrestrials: They’re Here!

Chapter 2: So How Do We Do This? Critical Ideas That Shaped, and Still Shape, Our Search for Aliens

Project Ozma: The First Search

Habitable Zones: Goldilocks in Orbit

Dyson Spheres: Aliens Go Big with Megastructures

The Kardashev Scale: How to Measure an Alien Civilization

Chapter 3: WTF UFOs and UAPs? How They Do, or Do Not, Fit into the Search for Aliens

The Giggle Factor: How politics and UFOs Almost Killed the Search for Alien Life

Hoaxes and Hoaxers: A Good Con Never Dies

The McDonald Critique: So, About Those Unexplained Cases . . .

UFOs Become UAPs: The Modern Era Begins

How to Get Real About UFOs: What a True Scientific Study Would Look Like

Chapter 4: What If They Are Aliens? If UFOs Are ET, How’d They Get Here, and What the Hell Are They Doing?

Interstellar Travel: If UFOs Were Aliens, How Did They Get Here?

Alien Technology: Inside Luke Skywalker’s Garage

Interdimensional Aliens: Hey, Man, Get Off My Plane

But What Are They Doing Here? The High-Beam Argument and Other Questions

Chapter 5: Cosmic Curb Appeal? Where to Look for Aliens

The Origin of Life: The Miller-Urey Experiment and Abiogenesis

The Ocean Moons: Who Knew?

Exoplanets: The Revolution Will Be Telescoped

Planets Gone Wild: The Super-Earth Enigma

Snowball Worlds and Ocean Worlds: Winter is Coming, and So is the Flood

Ten Billion Trillion Chances to Roll the Dice: The Pessimism Line and What It Tells Us

Chapter 6: The Cosmic Stakeout: How We’re Going to Spy on ET

Biosignatures: How to Find Life From a Distance

Technospheres and Noospheres: When Smart Life Goes Boss

Technosignatures: The Day the Earth Stood Still-Ish

Attack of the Alien Megastructures: Boyajian’s Star

Pollution, City Lights, and Glint: What Alien Skies Can Tell Us About Aliens

Solar System Artifacts: Did You Leave These?

Was ‘Oumuamua an Alien Probe? You Have a Visitor

Terraforming: How to Engineer a Habitable Planet

Chapter 7: Do Aliens Do It Too? What Will We Find When We Find Aliens?

Beyond Carbon-Based Life? The Molecule of Love

Talking Tumbleweeds or Flying Forests: What Will Aliens Be Like?

Alien Minds: Can You Talk with an ET?

Alien Ethics: Should We Hide or Fire a Flare?

Will the Biological Era Be Short? Welcoming the Robot Overlords

Ancient Aliens: How to Think About Million-Year-Old Civilizations

Chapter 8: Why Aliens Matter: It’s More Than You Think

Acknowledgments

Notes

Recommended Reading

Index

About the Author

Also by Adam Frank

Copyright

About the Publisher

Introduction

EVERYBODY LOVES ALIENS. I KNOW THIS BECAUSE EVERYBODY TELLS ME they love aliens. Life in the universe is the first thing people ask me about when they hear I’m an astrophysicist. Do aliens exist? is one of those special questions, kind of like What happens after you die? Lots of opinions, no real answers, and, most important, actually knowing the answer would change the world.

The thing is: I love aliens too. In fact, I have been obsessed with them since I was a kid. I first got hooked when I found my dad’s pulp science-fiction magazines as a five-year-old. On the cover of every issue were images of spaceships, barren moons, and bug-eyed alien monsters. From that moment on, I was on a mission to learn everything I could about the stars and alien life. This obsession made me a pretty annoying kid (apparently, I liked to quote the speed of light to four decimal places), but it also drove me to watch all the documentaries, bad sci-fi movies, and Star Trek reruns in existence. Any depiction of an alien was good enough for me as I dreamed of possibilities out there waiting to be discovered.

Back in the 1970s, at the height of my childhood obsession, the scientific search for life in the cosmos had barely begun. There were only a few very brave and determined pioneers carrying out the search for extraterrestrial intelligence (SETI), and most of them faced the scorn of their colleagues. SETI was considered a little out there, marginal at best in the scientific community. A big part of that dismissal was just bias. There just weren’t many astronomers who thought about the problem of life in its cosmic context back then. And it’s true, we really didn’t have much to go on in those days in terms of setting up a true scientific search for life among the stars, smart or otherwise.

Most of all, we didn’t know if there were any planets in the galaxy other than the eight that orbited our Sun. This was a killer point, since scientists expect planets to be necessary to get even simple life started. So not having a single example of an extrasolar planet (an exoplanet) meant we literally didn’t know where to look. We also didn’t know much about how planets and life evolve together in ways that might keep a world habitable for billions of years, long enough for higher animals and even technological civilizations to appear. In short, when it came to searching for alien life in the universe, we were pretty much in the dark.

Not anymore.

As you read these words, the human species is poised at the edge of its greatest and most important journey. Over the past three decades, the scientific search for life in the universe—a field called astrobiology—has exploded. We’ve discovered planets everywhere in the galaxy, and we’ve figured out how and where to look for signs of alien life in the atmospheres of these new worlds. We’ve also looked deep into Earth’s almost four-billion-year history as an inhabited world. From this view, we’ve gained new and powerful insights into how planets and life evolve together. Seeing the way life hijacked Earth’s evolution over the eons gives us clues about what to look for on distant planets (like oxygen, which generally can exist in an atmosphere only if life puts it there). We’ve also sent robot emissaries to every planet in our solar system. With their wheels or landing pads on the ground, we’ve begun searching these neighbor worlds for evidence of life existing now or perhaps deep in their past. Most important, we have launched and are building insanely powerful, next-generation telescopes. With these tools, we’ll finally go beyond just yelling our opinions about life in the universe at each other. Instead, we will get what matters most—a true scientific view of if, where, and when extraterrestrial life exists.

All these new discoveries, from exoplanets to Earth’s deep history, are transforming what we think of as SETI. A new research field is rising that scientists are calling technosignatures,* which embraces the classic efforts of SETI while taking the search for intelligent life into new forms and directions. Knowing that the galaxy is awash in planets means we now know exactly where and how to look for alien civilizations. Rather than hoping for someone to set a beacon announcing their presence (one premise of the first generation of SETI), we can now look directly at the planets where those civilizations might be just going about their civilization-ing. By searching for signatures of an alien society’s day-to-day activities (a technosignature), we’re building entirely new toolkits to find intelligent, civilization-building life. These toolkits will also allow us to find the kind of life that doesn’t build civilizations. Using our telescopes to find a signature of a planet covered in alien microbes or alien forests (a biosignature) would also be a game changer in terms of how humanity sees its place in the cosmos.

So now, finally, we are on the road to finding those aliens I was so obsessed with as a kid. Or we’re on the road to finding out we really are alone in the cosmos. Either answer would be stunning. It’s a pretty damn exciting moment.

But it’s also a confusing moment. Just as the scientific search for alien life is gaining steam, there’s also been an explosion of interest in aliens that are supposedly visiting Earth right now. Over the last few years, a handful of videos taken by US fighter pilots have cropped up online showing fuzzy blobs appearing to fly in ways that would be impossible for normal aircraft. The videos have brought unidentified aerial phenomena (UAPs) into the spotlight, raising the stakes on the alien debate. But the UAP furor also confuses the issue about the giant leap science is taking as it begins looking for aliens in the most likely place (i.e., alien planets).

UAPs are the US government’s new name for unidentified flying objects (UFOs), a subject that’s been around for years, holding modern culture in thrall. UFOs as alien visitors make for great science fiction (everything from The X-Files to Independence Day to Nope). The possibility of their actual existence has mostly been dismissed by scientists. The overwhelming majority of astronomers see UFOs as natural phenomena that get misidentified, objects related to national defense, or just purposeful hoaxes. In 2021, however, the US government revealed more than a hundred UAP sightings for which it had no obvious explanation. The media tornado over the UAP videos was unrelenting, even as most scientists emphasized that unexplained can mean there simply isn’t enough data, or good enough data, to even begin formulating an explanation. Still, in the wake of the new government interest, I am left wondering, Do these things really have anything to do with aliens?

Between the remarkable progress in astrobiology and technosignatures on the one hand, and the blizzard of coverage about UAPs on the other, aliens are big news. More than ever, we want to know: Is anyone out there? I wrote this book to help people understand that question as scientists see it, the definitive answers scientists are working to find, and, most amazing of all, how close we are to getting some of those answers.

For a chunk of my career as an astrophysicist, I studied less freaky-deaky stuff. At the University of Rochester, I ran a computational astrophysics research group in which my students and I used the world’s most powerful computers to explore how stars form from giant clouds of interstellar gas and how they die by tearing themselves apart in titanic stellar winds. These were very cool projects, and I loved the vistas they opened for me. But I never lost my little-kid interest in cosmic life. So, about a decade ago, I started a research program in astrobiology, doing work on exoplanets and their atmospheres. Then I started thinking about climate change from the perspective of astrobiology, positing that maybe every civilization triggers its own version of global warming.

My life really changed, however, in 2019 when a group of colleagues and I were awarded NASA’s first grant to study exoplanet technosignatures. That is, NASA began funding us to think about the best ways to look for alien civilizations. We applied for the grant because, over dinners (and beers) at international meetings, we all got way too excited (it was the beers) about those exoplanet discoveries and how they could rewire the search for intelligent life. But NASA had never funded a project like the one we were thinking of. In fact, after years of getting burned by Congress for funding SETI research as a waste of taxpayer dollars, the space agency had barely funded any work on intelligent life in the cosmos.

So when we put in our proposal, we kept our hopes low. But then to our surprise, amazement, and joy (and more beers), it was accepted. The frontier was opening. We’d been given a chance to help shape the most exciting quest humanity had ever taken on. It was a milestone for the field and a recognition of how much had changed in the scientific thinking about life in the universe. Since then, we and other researchers have been pushing into new terrain. We’re all preparing for a truly systematic, scientific search for alien life and alien civilizations. That search is just getting started now.

It’s from this vantage point that I see, and understand in my bones, why everyone wants to know about aliens. But if you’re interested in the science—from SETI to astrobiology to technosignatures—where do you begin? There is a mess of history, concepts, and terminology floating around that you need to know to understand what’s about to happen. What, for example, is the Drake equation, and why does it matter so much? What’s the Fermi paradox, and how much SETI searching has actually been done to resolve the paradox? How many exoplanets are there, and which of them matter? What is a technosignature (or a biosignature), and how is anyone going to find one? And what about the UFOs/UAPs? Should we take them seriously? If we do, what are the questions we should ask, and how should we ask them?

The aim of this book is to give you a good ten-thousand-foot overview of what’s happening now, what’s going to happen soon, and why it matters so much. My biggest goal in writing it was to give you a fast, fun path into all the amazing questions and issues swirling around that mother of all questions:

Are we alone?

So, suit up. It’s time to get started on our journey. We have a lot of ground to cover. By the time we’re done, though, you’ll have everything you need to know about everything there is to know (for now, at least) about aliens. From that point on, you’ll be ready to join this great voyage of discovery, and you’ll be ready when someone says we’ve found them. Because in the end, we don’t want to just believe; we have to know.

Chapter 1

How Did We Get Here?

How Our Ancient Questions About Aliens Took Their Modern Form

LOOK AT YOUR HAND. I KNOW, IT’S A STUPID REQUEST, BUT JUST LOOK AT IT for a moment. Inside every cell in your hand and the rest of your body is the genetic memory of every ancestor, going back to the origin of Homo sapiens almost three hundred thousand years ago. That’s more than fifteen thousand great, great, great, etc. grandparents. You carry multitudes within. You can also bet that all of those grandmas and grandpas stretching back through time spent some of their lives staring up into the clear night sky, with the sentinel stars staring back. And what does that mean? It means you’re not the only one who’s into aliens. Your parents were too. So were your grandparents, your great-grandparents, your great-great-grandparents, and so on.

OK, to be clear, maybe your parents or your distant ancestor in the fourteenth century wasn’t obsessing about alien life. You can, however, be pretty damn sure someone else in each of those generations was thinking hard about it. That’s because the argument about life in the universe is as old as arguing itself. Are we alone? turns out to be a really, really old question.

Debates about the existence of other inhabited planets go way, way back, and it’s important to understand the form those arguments took, because they could get pretty heated. More important, those older debates lie as a kind of unspoken background for the great shift that happened in the middle of the twentieth century and the explosion of possibilities happening today. After the Second World War, the technologies of rockets, radio, radar, and atom bombs transformed how we thought about space and the possibilities of alien civilizations. It also brought about the first wave of widely publicized and recorded UFO sightings, which drove the idea of aliens deep into popular consciousness. In this first chapter we’re unpacking that history so we can see exactly how we got to this crazy, amazing moment when a question as old as humanity is poised to get its answer.

A Really Old Question

Alien debates through history

We can track the debate about aliens, in writing at least, back to the ancient Greeks. Aristotle, one of the most famous of famous Greek philosophers, was what we’ll call an alien pessimist. You may know of him, but in case not: he lived around 350 BCE* and developed sophisticated ideas about everything from the nature of art to the nature of biology. When it came to life on other planets, Aristotle was sure that Earth was entirely unique. That’s because for him the Earth was literally the center of the universe: the Sun orbited Earth and so did the other five planets (the ones you could see without a telescope—Mercury, Venus, Mars, Jupiter, and Saturn). Aristotle saw the Earth as being so special that he divided the cosmos into a sublunar realm, meaning below the Moon’s orbit, and a celestial realm. Life and all its changes could only play out in the sublunar domain. The celestial realm was eternal and unchanging. It’s from this perspective that Aristotle made his famous proclamation: There cannot be more worlds than one. He meant that there can’t be any place like Earth (with its unique life-forms) anywhere else in the whole universe.

Aristotle was, of course, an exemplar of big ideas at the time, and for nineteen centuries afterward. Eventually even the Catholic Church would adopt some of his views into its doctrine. But that doesn’t mean all the other Greek philosophers of the Hellenistic period, about 2,350 to 2,050 years ago, agreed with him. There was, for example, a group of thinkers collectively known as atomists, who thought the split between sublunar and celestial realms was kind of stupid. For atomists like Epicurus, who lived around 300 BCE, everything everywhere in the universe was built from tiny bits of indestructible matter called atoms (atoms, atomists, duh). As these atoms sped around the cosmos, they collided and, in the process, combined in all kinds of ways. Here in our part of the cosmos, they collided to form the Earth and all of Earth’s living stuff.

However, since atoms were everywhere and everything was made of them, Epicurus reasoned that the universe must contain lots of other planets, and many of them had to be inhabited. Nothing else made sense. If atoms were universal, how could Earth be special? That viewpoint made Epicurus an alien optimist, and he struck back at Aristotle’s perspective, writing, There are infinite worlds both like and unlike this world of ours. . . . Furthermore, we must believe that in all worlds there are living creatures and plants and other things we see in this world.

Alien optimists and alien pessimists—while the details of these two positions are going to change over the centuries, it’s pretty remarkable to see their basic outline appear in writing more than two millennia ago. That’s about a hundred greats in the your great-great-great-grandparent way of describing the past. So, yes, people have been arguing about aliens for a long time.

AFTER THE FALL OF THE ROMAN EMPIRE AROUND 500 CE, PROGRESS IN astronomy shifted to the Islamic empires of Persia and elsewhere. Astronomers of the great Muslim societies continued to build on Greek astronomy, creating new and more accurate star charts, as well as adding new ideas about an Earth-centered universe of the kind Aristotle believed in. They, too, had their arguments between optimists and pessimists, adding Islamic theological perspectives to the split. Some scholars claimed the Quran supported the possibilities of other worlds and other humans. Then, as Europe emerged from the Dark Ages and began new scientific inquiries in the 1500s, the debate got even more heated.

In the late decades of that century, a radical Dominican monk, Giordano Bruno, appeared on the scene. Bruno was a Copernican, meaning he believed Polish astronomer Nicolaus Copernicus’s view of the solar system with the Sun, not the Earth, at the center. Since all the planets orbited the Sun, that meant Earth had been demoted to just another planet, nothing special. This heliocentric (Sun-centered) model stood against Aristotle’s geocentric (Earth-centered) version, which also happened to be the official Church view. Copernicus knew the Church didn’t take kindly to having its nose tweaked with astronomical heresies, so he held off publishing his theory until he was safely dead.

Giordano Bruno didn’t think that way. He was both intellectually courageous and kind of an asshole, alienating almost everyone who supported him. Despite getting chased from one European capital to another, Bruno was willing to push hard on the limits of what the Catholic Church would tolerate in terms of heretical ideas. For Bruno, if you accepted that all the planets, including Earth, went around the Sun, you might as well accept that the stars were just other suns. From there he reasoned that all the stars had their own families of planets in orbit and some of these worlds must host life just as Earth does. The Church was very much of the Oh no, they don’t opinion. Eventually the churchmen caught up with Bruno, and he was dragged before the dreaded Inquisition. For a variety of official reasons, Bruno was declared a heretic, hauled out into a square in Rome, tied upside down to