Of course, not all life on Earth likes the same things we do. Some bacteria like extreme cold, some like it hot, others eat sulfur, and some like the extreme pressures found deep underwater or underground. These extremophiles are abundant on Earth, and may exist on Mars as well. But even if they are there, deep under the Martian surface, it’s unlikely they’d get scooped up and carried away by an asteroid impact.
Looking to other worlds in the solar system for potential bacterial breeding grounds is even more futile. Europa, a moon of Jupiter that is covered in ice, may harbor a vast water ocean under its surface. It’s an excellent candidate to look for life beyond Earth, but it’s a low-probability location for anything that’ll think our environment is cozy. The ice on Europa is probably ten miles or more thick; any impact that could loft a subsurface ocean-dwelling microbe into space would also be powerful enough to vaporize said microbe.
Another potential home for life is Titan, one of Saturn’s moons. Titan is aptly named: it’s over 3,000 miles in diameter (about the size of Mercury) and sports a thick atmosphere of nitrogen, argon, and methane. It rains there, but the drops are liquid methane! It’s cold on Titan, about −300 degrees Fahrenheit. Any water on the surface is frozen into a solid harder than terrestrial rocks. And while biochemists have speculated that life could arise in such a weird environment, it would be utterly alien to us. Any bug capable of living there would find itself in the equivalent of a blast furnace on Earth.
It seems that as incubators go, we’ve struck out of potential bugs in the solar system. Any alien microbes that would have evolved for Earthlike conditions almost certainly wouldn’t survive the trip.
Of course, this assumes that any form of life Out There is just sitting back and waiting for a ride. Maybe, though, the more sophisticated types would prefer to drive.
WHERE ARE THEY?
The question was asked so succinctly by the physicist Enrico Fermi in the early 1950s, over lunch with some other scientists. They were discussing the recent spate of flying saucer sightings and considering interstellar travel, human or otherwise. When the topic turned to aliens, Fermi asked, “Where are they?”68
The question, simple though it is, has a rich backstory. The basic idea is that by now either we should have detected intelligent life in our galaxy or it should have come visiting. Since neither has occurred,69 asking where the aliens are is a reasonable thing to do.
Let’s assume that for aliens to come knocking, their circumstances must be something like ours: Sunlike star, Earthlike planet, development and evolution of life over billions of years, discovery of technology, then the capability to travel between the stars. How likely is all this to happen?
For that we can turn to the Drake Equation. Named for the astronomer Frank Drake, it categorizes all the necessities of advanced life and assigns probabilities to them. If you fill in all the terms correctly what pops out is the number of advanced civilizations in the galaxy (where “advanced” is defined as being able to send signals into space—which is how we’d know they’re out there).
For example, the Milky Way Galaxy has roughly 200 billion stars in it. About 10 percent of these stars are like the Sun: similar mass, size, and so on. That gives us 20 billion stars to work with. We’re just now learning how planets form around other stars—the first planet around a Sunlike star was discovered in 1995—but we’re finding that stars like the Sun are rather likely to have planets. Even if we assign a ridiculously low probability of there being planets around another star (say, 1 percent), there are still hundreds of millions of stars out there with planets. If we assign a ridiculously low probability of these planets being Earthlike (again, say, 1 percent), then there are still millions of Earthlike planets. You can continue to play this game, estimating how many planets can support life, how many have life, how many have life capable of technology . . . each step in the chain is a little less firm than the last, but even the most pessimistic view of this series indicates we shouldn’t be alone in the galaxy. The estimates of the number of aliens out there vary widely, literally from zero to millions.
ARE WE ALONE?
That’s not terribly satisfying, of course. The lower estimate is sobering. Maybe, just maybe, we really are alone. In all the galaxy, in all the vast trillions of cubic light-years of emptiness, ours is the very first planet to harbor creatures that can ponder their own existence.70 This is a humbling and in some ways frightening possibility. And it’s possibly true.
Another possibility is that life might be common, but “advanced” life is rare. Books have been written on this topic, and it makes for an interesting argument. Maybe once life gets to a certain stage, it tends to go navel-gazing and never develop or care about technology (alien psychology is a difficult topic to get too deeply into). And I hope that by the time you get to this point in this book, I’ve made it clear that civilization-ending events occur uncomfortably often over geologic time scales. Maybe every civilization eventually gets wiped out by some natural event before it can develop space travel advanced enough to prevent it.
I don’t think that’s a good answer, actually. We are within years of being able to prevent devastating asteroid impacts on Earth. We know we can properly shield ourselves from solar events. Our astronomy is good enough to pick out nearby stars that might explode, so if we saw one ticking away we could devote ourselves to getting away from it. All of these advances are quite recent, happening in a blink of the eye compared to how long life has existed on Earth. It’s almost impossible for me to imagine a civilization intelligent enough to explore the heavens yet not advanced enough to preserve its own existence.
TALK IS CHEAP
I’m suspicious of the other end of the estimates of the Drake Equation as well, that there are millions of aliens out there as advanced as we are or more. If that were true, I think we’d have unequivocal evidence of them by now.
Remember, besides being vast, the galaxy is old. The Milky Way is at least 12 billion years old, and the Sun only 4.6 billion. If we imagine a star like the Sun forming just 100 million years earlier—a drop in the bucket compared to the age of the galaxy—then it’s not hard to imagine an alien civilization rising many millions of years before humans did. We know that life arose easily enough on Earth; it got started as soon as the bombardment period ended and the surface of the Earth calmed down enough for long-term growth of life to occur. This implies strongly that life takes hold given the smallest opportunity, which in turn means it should be abundant in our galaxy. And, despite a list of disasters epic and sweeping, life on Earth has managed to get this far. We are intelligent, we are technologically advanced, and we are a space-faring species. Where will we be in a hundred million years?
Given that stretch of time and space, an alien species really should have knocked on our door by now.
They should have at least placed a call. Communicating across the vastness of space is easier than actually going there. We’ve been sending signals into space since the 1930s. These are relatively faint, and an alien would have a hard time hearing them from more than a few light-years away, but we’ve leaked out stronger signals as time has gone on. If we wanted to target a specific star, it’s not hard to focus an easily detectable radio signal to any star in the galaxy.
The reverse is true as well: any alien race with a strong urge to chat with us could do so without too much effort. The Search for Extraterrestrial Intelligence (SETI) is banking on just that. This group of engineers and astronomers is combing the sky, scanning for radio-wave signals. They are almost literally listening for aliens. The technology is getting so good that the astronomer Seth Shostak estimates that within the next two dozen years, we’ll be able to examine the million or two interesting star systems within a thousand light-years of Earth. This will go a long way toward our discovering whether we are alone or not.
The one drawback with SETI is that the conversations will tend to be a bit boring. If we detect a signal from a star that is really close on the galact
ic scale, say, a thousand light-years away, the dialogue will really be a monologue. We’d receive the signal, reply, and have to wait two thousand years for them to get back to us (the time it takes for our signal to reach them plus theirs to come to us again). While SETI is a great and worthwhile endeavor—and if they find a signal it will be one of the most important events in the history of science—we’re still used to thinking of aliens actually coming here. Face-to-face, as it were, assuming they have faces.
But a thousand light-years is a long way (6,000,000,000,000,000 miles). That’s quite a hike, yet it’s practically in our laps compared to the size of the Milky Way.
Is that why we haven’t been visited? Maybe the distances are simply too great!
Actually, not so much. A trip to the stars wouldn’t take that long at all, if you maintain a sense of scale.
TO BOLDLY GO
Let’s assume that we humans suddenly decide to fund the space program. And fund it really well: we want to send probes to other stars. That’s no easy feat! The nearest star system, Alpha Centauri (which has a Sunlike star and is worth a look-see), is 26 trillion miles away. The fastest space probe ever built would take thousands of years to get there, so we couldn’t really expect a payoff in the form of pretty pictures anytime soon.
However, that’s the fastest probe ever built so far. There are ideas out there on the drawing board that would make much faster unmanned probes, even ones that can move at a goodly fraction of the speed of light. Some of these include fusion power, ion drives (which start off slowly but accelerate continuously over years, building up ferocious speeds), and even a ship that explodes nuclear bombs behind it to provide a huge impulse in speed.71 These methods can drop the trip time from millennia to mere decades.
This might be worth doing. It’s expensive, sure. But there are no technological barriers to this idea, just social ones (funding, politics, etc.). Let me be clear: if we had the will, we could build spaceships like these right now. In less than a century we could be sending dozens of interstellar emissaries to other stars, investigating our own neighborhood in the galaxy.
Of course, the trip times and the actual construction of the fleet make it difficult to explore much real estate. The galaxy has billions upon billions of stars, and building that many starships is impossible. Sending one probe per star isn’t cost-effective. Even if we let the probe simply sweep through a star system on a fly-by, moving on to the next star, exploring the galaxy would take forever. Space is big.
But there’s a solution: self-replicating probes.
Picture this: an unmanned spaceship from Earth arrives at the star Tau Ceti after a journey of fifty years. It finds a series of small planets and begins its scientific observations. This includes a census of sorts—taking measure of all the bodies in the system, including planets, comets, moons, and asteroids. After some months of surveying, the probe will move on to the next star on its docket, but before it leaves, it sends a package down to a particularly promising nickel-iron asteroid. This package is in fact a self-starting factory. Once it lands, it mines the asteroid, smelts the metal, refines out the necessary substances, and then autonomously builds more probes. Let’s say it builds just one probe that, after a few years of construction and testing, blasts off for another star system. Now we have two probes. A few decades later they arrive at their destinations, find appropriate accommodations, and then go forth and multiply again. Now we have four probes, and the process repeats.
The number of robot ambassadors builds very rapidly; it’s exponential. If this takes exactly one hundred years per probe, by the end of a millennium we have 210 = 1,024 probes. In two millennia there are a million probes. In three thousand years there will be more than a billion. Now, in reality, it’s not that simple, of course, but even a pessimistic approach shows that we can explore every single star in this galaxy in something like 50 million years, maybe a bit less.
Well, that sounds like a long wait! And we’re still a long way off from being able to do it. The technology is formidable.
But hang on—remember that civilization we considered, just 100 million years in advance of us? Given that much time, they could easily have examined every single star in the Milky Way, looking for life. If they saw our warm, blue world, one would think they’d make some note of it. Still, it’s possible they came here 50 million years ago and missed us humans (mining the Moon for a monolith à la 2001: A Space Odyssey maybe isn’t as silly as it sounds), or maybe they just haven’t gotten here yet.
But given the time scales, that seems unlikely. It just doesn’t take that long to map out a whole galaxy and visit the appropriate planets. That’s why I don’t think the “millions of civilizations” number from the Drake Equation is correct. We’d have seen them by now, or at least heard from them.72
IN SPACE NO ONE CAN HEAR YOU SCREAM
But sometimes I wonder. Given all this information: the likelihood of life, the relative ease of galactic exploration, the time spans involved . . . and the fact that we have not detected any other life in our galaxy at all, there is another possibility that is worth mulling over.
Consider: what spurs technological advancement more than anything else?
War.
The first Cro-Magnon who beat an opponent over the head with a tree branch was also the one who was most likely to live a bit longer, and be able to reproduce. The army with rifles will (in general) beat the one with spears. The country with missiles will (in general) beat the one with cannons. The ones with electronic remote-controlled drones, spy satellites, and instantaneous communication will outmaneuver the ones without.
Nothing advances technology like good old-fashioned aggression. Even one of the most noble events in human history—man walking on the surface of the Moon—was initiated because of the cold war, the space race with an enemy vast and powerful. Americans imagined Soviet missile bases in orbit and on the Moon, and the motivation to beat them was in place.
When I was a kid, it was fashionable in more intelligent science-fiction stories to assume that any aliens we meet were bound to be friendly—no warlike race would be able to get their act together long enough to reach the stars.
Humans are on the verge of falsifying that statement.
Putting the pieces together, we find that warlike races are perhaps more likely to achieve space travel. The ones with a history of victories will have the best technology, and will be most motivated to be at the very least wary of outsiders, if not openly hostile. This case can certainly be made for our own provincial example.
This hypothetical advanced civilization will be xenophobic, fearful of aliens. We’ve already seen that it’s technologically possible to create interstellar starships, and it’s also possible to engineer them to create duplicates of themselves, to speed up the time it takes to comb over the whole galaxy.
What happens when you take a paranoid species and give them the ability to build such spaceships?
Uh-oh.
The scenario plays out in the vignette at the start of this chapter. It makes a creepy kind of sense to me: any aliens that are that aggressive would want to wipe out potential enemies before they got sophisticated enough to pose a threat. The easy way to do it is to create space probes like the ones described, and use them to ruthlessly wipe out all life they find.
Death from the skies, indeed.
I’ve wrestled with this idea, wondering if it’s possible. One potential saving grace is the same as before: exploring the whole galaxy in this way doesn’t take long compared with the age of the galaxy itself. Therefore, according to the same logic above, it’s likely that if such a xenophobic civilization were to evolve, it would have been here by now.
Yet we’re still here. We know life has been around for billions of years. There have been the odd interruptions, but we’ve never been sterilized back down to the microscopic level. Like so many of the natural disasters we’ve seen, this puts a pretty good damper on the odds of being wiped out by nasty aliens. Si
mply put, if they were out there, we wouldn’t be here.73
I honestly don’t know if we’re alone in the Universe; no one does. However, given the immensity of space, and the grand depth of time, it sure seems unlikely. And if we do get out there, it also seems unlikely we’ll meet any nasty races like Klingons, Romulans, Vogons, Reavers, Daleks, or Kzinti. Natural disasters will still probably be our biggest worry.
But the galaxy is big, with room enough for lots of things. I may not know if we’re alone, but I’d love the chance to find out.
CHAPTER 7
The Death of the Sun
THE PLANET IS FAIR-SIZED, CLEARLY BIG ENOUGH TO sustain a healthy atmosphere, though none is currently present. Given its distance from its parent star, it could easily have held liquid water on its surface too, once, in the far distant past. The outlines of continents are visible on its surface, though difficult to make out because of the lack of contrast. Were those deep, broad basins once ocean floors?
It’s hard to tell now. The planet may have once been green, or even blue, but now it’s all browns and grays and blacks. If any liquid water—or even water vapor—once existed there, it’s long gone, evaporated a billion years before. Without an atmosphere there can be no liquid water.
The planet’s star begins to peek over the planet’s horizon. Swollen, distorted, nebulous, and very, very red, the star rises ponderously. It almost appears flat, it’s so large. But after a few minutes the gently curved nature of the limb becomes more obvious, clarifying just how big the star is. An hour later it still hasn’t fully risen, less than half of it showing above the horizon. It looms menacingly there, glaring like a bloody half-closed eye.
Death From the Skies!: These Are the Ways the World Will End... Page 19