By Robert J Hansen

“There are a million different worlds

A million different suns

And we have just one world

But we live in different ones.”

— Dire Straits, Brothers in Arms

When humanity finally succeeds in getting direct images of potentially inhabited extrasolar planets, there’s going to be enormous rejoicing: not only for the opportunity to see what we expect, but the opportunity to see what we don’t.  If our understanding says we should expect to see a methane field and the spectrographics say there’s very little of it, there are really only two possibilities: our understanding of planetary formation is wrong, or — much more tantalising — aliens need it, mined it, used it, and depleted it.

So how could astronomers determine the difference? That’s a tremendously important question which needs to be answered.  Fortunately, we get to cheat: we can look at how humans have transformed our own planet, and imagine how we’d look to alien astronomers!


What do nuclear power plants have in common with a farmer tilling a field?  The answer turns out to be surprisingly deep.  To begin answering it we have to go back to one of the oldest weapons: the sling.  These aren’t the slingshots you may have played with as a child, but real weapons capable of killing animals and people.

Start with a small stone about of about 150 grams (five ounces).  If you were to throw this at a running deer you’d be unlikely to do much more than annoy it, even if you could hit it.  The human hand and arm are capable of exerting a lot of force, but we can’t move them all that fast.  Even a professional baseball player in peak physical condition would be hard-pressed to throw it faster than about 160 kilometers per hour (100 miles per hour).

Now take a broad strip of cloth, like an old necktie with the rear stitches torn open and the fabric unfolded.  Hold both ends in your hand, letting the rest hang free with the stone held in the lowest part of the loop.  Spin the necktie overhead and you can hear that stone whir the air.  If you’re very careful you can let go of one end of the necktie and that stone will streak out of the sling faster than the eye can really track, and much faster than you could ever hope to throw it.  (If you decide to do this experiment, make sure to do it far away from people or anything breakable.  You’re throwing rocks, after all!)

Congratulations: any anthropologist who could see that sling would immediately know it was created by intelligent life — life capable of purposeful design, life that was smart enough to grasp a problem and formulate a solution.

  • Problem: the human arm can’t impart sufficient kinetic energy.
  • Solution: build a tool to harness kinetic energy and impart it to a stone.
  • Result: you can now hunt that deer — which is very important if you’re a Palaeolithic civilisation.

Keep this basic idea in mind: intelligent life harnesses and redirects energy to address problems.  Pretty much every way humanity has transformed this planet — and every way we expect to see aliens transform theirs — is tied back to that pattern.

And to imagine that it all starts with a stone and a strip of cloth!


Imagine that billions and billions of years ago the astronomers of the third planet from Altair sent a starchip probe — a one-gram computer propelled by a laser sail — to orbit the earth.  Altairian technology is pretty awesome, but even it would have trouble spotting a plains ape using a sling to bring down a gazelle (assuming apes or gazelles existed two billion years ago, that is).  It would be looking for the same basic pattern, though, just on a much larger scale. To sum up the pattern in a single sentence, the starchip would be looking for the large-scale harnessing and redirection of energy to address problems.

Unfortunately, that can be a lot more complicated than it sounds!

Consider nuclear power.  One would think splitting the atom was such a difficult process that it would of course be proof of an advanced civilization, except for the fact Earth’s first nuclear reactor ran about 1.7 billion years ago in what is today the African nation of Gabon.  That’s not just prehistoric, that’s prior to the first appearance of fungi.

Naturally-occurring uranium is a mixture of different isotopes, of which only one, called U-235, can be easily used for energy.  U-235 is not only radioactive, but more radioactive (with a faster half-life) than the more stable isotope of U-238, so over extremely long periods the fraction of U-235 in our natural uranium reserves will decrease.  1.7 billion years ago natural uranium had enough U-235 to be useful in nuclear reactors without any processing at all: all that was needed was some water to serve as a neutron moderator.

This happened in Gabon when porous rocks containing uranium ores became saturated with groundwater.  The U-235 split and an enormous amount of heat was released, converting the water to steam and boiling it away.  Without water the reaction stopped until more water flowed in to resaturate the rock.  This process repeated for literally hundreds of thousands of years on a predictable schedule (30 minutes of fission boiling water away, 150 minutes of water seeping back into the rock) until the U-235 fuel was exhausted and the landscape was left as a natural Chernobyl.

Think about that for a second.  One of the most cutting-edge techniques in power generation, nuclear power, splits U-235 to boil water and create steam… exactly like naturally-occurring nuclear power.  If you were to imagine an alien probe orbiting Earth and collecting information, how could they differentiate the natural reactors of Gabon from, say, the Duane Arnold Energy Center in Iowa?  They’re both doing fundamentally the same thing, after all, and from as far away as Altair they’d both appear to be identical neutrino beacons: one on a 30-minute/150-minute on/off cycle, and the other tightly coupled to whether it was receiving solar radiation.  (Nuclear power plants generate more power — and more neutrinos — during the daytime in order to meet daytime power demand.)

Going back to our thought experiment of the Altairian astronomers, it’s now easy to figure out why they sent a starchip probe to Earth.  Imagine their excitement at discovering that Earth was generating pulses of neutrinos on a regular repeating basis.  How would we react if we were to discover a neutrino beacon on another world?  A great many people would leap to think the beacon was clear and obvious proof of an advanced society seeking to signal us; only a few would pause to say, “but is it really a beacon at all, if it’s not conveying any information?”


The nuclear reactors of Gabon each generated hundreds of kilowatts of power for hundreds of thousands of years, but had what to show for it?  All they did was boil water and pollute the soil to no end.  If our alien starchip probe were to measure the rate of steam formation and the rate at which uranium was being consumed, the two would be in perfect lockstep.  No useful work was being extracted from the system, which would be a clear indicator intelligent life wasn’t behind it.

Remember what we learned from the sling: intelligent life harnesses and redirects energy to address problems.  Since no useful work was being extracted from the system, there’s no evidence of energy being harnessed or redirected.  Our Altairian starchip probe would conclude (sadly — if the AI aboard a one-gram processor was complex enough to feel emotion) the reactors of Gabon were a purely natural phenomenon.

But if that starchip were to wait 1.7 billion years, reactivate, and look down on the banks of Iowa’s Cedar River, a much different picture would emerge.  If the starchip were to measure the rate at which the Duane Arnold Energy Centre consumed uranium and compare it to the rate at which it formed steam and released heat, an enormous disparity would become obvious.  What the starchip probe should measure, versus what it actually measured, would disagree to the tune of 600 megawatts.  Energy is clearly being harnessed here; but is it being used to solve a problem?

About fifteen kilometers away from the Duane Arnold Energy Center sits the city of Cedar Rapids, Iowa.  The city’s energy signature would be tremendously complicated.  The starchip would have to begin somewhere: it might as well start with its light output.  Lo and behold, whereas most of the Earth just reflects light, this city is actually making light — light that is very unlike the Sun’s light. Generating that much light requires a lot of energy; fifteen kilometers away there’s an energy discrepancy; perhaps, the starchip AI would think, there might be a connection; perhaps energy is being harnessed and redirected for a purpose!


What else could our starchip astronomer look for?  How else is humanity harnessing and redirecting energy in ways that are visible from space?

The starchip has been watching us for over two billion years. It saw the nuclear reactors at Gabon, it saw the rise of the dinosaurs, it saw the cataclysm of meteor impact.  By this point it has a sophisticated understanding of Earth’s ecosystems.  It knows plant life exists and its basic cycle: it takes in solar energy, some chemical energy from soil, and repurposes carbon dioxide into a polysaccharide called ‘cellulose’.

But in the last ten thousand years something unprecedented has happened. Some blight has wiped out large tracts of naturally growing plant life, and in the aftermath monocultures have appeared with layouts optimised to maximise the vegetation per unit area. Something has transformed the natural order. Something isn’t just content with natural plant growth: something is working on ensuring as much as possible of one particular thing grows, and nothing of anything else.

Far below in the gentle hills of eastern Iowa or the flat terraces of the Korean peninsula, a farmer finishes planting hundreds of acres.  “A monoculture with optimised vegetation per unit area” is another way of saying the farmer gets paid for grain, not weeds, and wants to bring to market as many bushels as possible. 

Whether we’re talking about cornfields or rice paddies, humans have terraformed the planet on an unbelievable scale — a scale that’s easily visible from space.  When we begin to image other planets we’ll also be looking for evidence of organized, systematic agriculture.


The downside of humanity’s craving for energy is the ecological damage we inflict. Wholesale deforestation and burning (“slash and burn”) of vast regions is done to create poor-quality farmland that’s exhausted after just a couple of years.  This photograph from Thailand shows the heartbreaking cost of our material gains.

Up until the late 1800s there were many places where oil was so close to the surface it naturally seeped to the top.  (You may have heard of one such famous place: the La Brea Tar Pits.)  Surface oil used to be considered a defect in real estate: it made the land less useful for any productive purpose.  It wasn’t until the development of modern chemistry that we realized how useful oil was, and in just a few short decades these easily-recovered oil reserves were all tapped out.  We chased oil throughout all of North America and, along the way, invented modern geology.

First we chased surface reserves, then shallow reserves, then deep reserves.  Today we’re building floating cities that exist only to run drills down into the ocean floor and using hydraulic fracturing to shatter huge regions of rock strata to liberate the petrochemicals locked up within them.  Regardless of how one feels about the economic prosperity that has come from petroleum mining, it’s impossible to argue there hasn’t been widespread ecological damage.  Could this be seen as evidence of efforts to harness and redirect energy?

Well — yes — with some caveats.  Consider the reactors of Gabon, which shut down after hundreds of thousands of years when they ran out of U-235.  We discovered them in 1972 when prospecting for uranium ore and found Gabonese reserves were curiously low in U-235.  Depletion, by itself, is not a sign of intelligent life.  Nor is ecological damage: running an unshielded, unregulated nuclear reactor for a hundred thousand years will really mess a place up.

The difference lies in the accessibility of the depleted resource.  The reactors of Gabon depleted surface reserves of uranium, but depleting an oil reservoir two kilometers beneath the ocean floor requires someone to put on a thinking cap.

Our starchip astronomer will discover Earth’s coal and oil reserves are depleted far in excess of what any known natural process can explain, and the atmospheric carbon levels have skyrocketed in similarly inexplicable ways.  Further, the rate of coal and oil depletion is tied to the rise in atmospheric carbon.  The ecological impact combined with the liberation of energy is a very strong indicator of life on Earth — although whether we count as “intelligent” is up in the air, given the damage we’re doing to our planet.


2The Gabonese nuclear reactors were all-natural and all-disastrous: imagine, if you can, the ecological consequences of hundreds of thousands of years of steam explosions spewing nuclear waste everywhere.  It took hundreds of millions of years for the local ecosystem to recover, but it did: Earth can forgive almost any insult, given enough time.

Intelligent life, though, is impatient.  We don’t want to wait a billion years for Nature to deal with our nuclear waste, so we’re engaging in tons of research and development to find out how to best limit the damage and rebuild our shared home.  Some say we should reprocess waste into new fuel, some advocate storage in Yucca Mountain, some push for glassification: although we don’t know today how we’ll handle nuclear waste, it’s almost unthinkable to imagine we won’t have good mechanisms developed within the next century.

Ecological depletion from oil extraction is countered by a vigorous recycling industry, by finding ways to make plastics from plant materials, by increased automobile efficiency standards, by biofuels, and more. We’re beginning to research how to mitigate our carbon pollution, and humanity’s history of creativity is enough to give us hope that in a few decades we’ll have a good idea of how to do that.

Just as ecological depletion can be seen from space, so can renewal.  Our Altairian starchip can not only see if vegetation is being cleared faster than nature allows (harvesting), but if it’s being restored more quickly, too (replanting).


If the “neutrino beacons” of our terrestrial nuclear reactors are in fact just noise, then what constitutes a real beacon?  What’s something that could uniquely and powerfully say, “only intelligent life could do this”?

The problem with thinking the Gabonese reactors were neutrino beacons is the same as thinking the Old Faithful geyser is a visual beacon.  Both of them are periodic events.  It’s the interruptions in a periodic event which can be used to determine if intelligent life is behind it.

Consider Marconi.  The wireless telegraph consisted of a spark-gap transmitter — a huge power sink that broadcast over gargantuan swaths of the radio spectrum — and a switch Marconi could use to interrupt power to his transmitter.  By carefully controlling the pattern of interruptions, Marconi could send signals to a far-away radio receiver.  A short interruption followed by a longer one would be the letter ‘A’, for instance; and in this manner, the whole of Morse Code can be sent via the radio.

Of course, Marconi wasn’t just sending his wireless telegraph to London.  Three seconds later, his message arrived in a variety of locations back on Earth, having traveled round-trip to the Moon.  Radio propagates out to the entire universe in all directions at the speed of light, so any alien culture that’s sophisticated enough to have radio receivers will someday pick up a wailing howl of radio noise that gets punctuated with silence.  At first they’ll think it’s natural, up until they realize there are some patterns that never appear, and others that appear again and again.  They might think the radio source might be natural, but these interruptions are odd: they’re definitely not random, so what could they be?

Once they’ve satisfied themselves the interruptions aren’t random and don’t conform to a repeating natural pattern, the conclusion will be inevitable and inexorable: it’s someone harnessing radio energy and using it for the purpose of communication!

Of all the ways humans have transformed our world, the most vivid is one we can’t see at all.  The wifi signals from your laptop, the satellite radio morning show you listened to on your way to work, the cell phone in your pocket, all of these are filling the radio spectrum with information carried in the interruptions of carrier waves, and communicating our presence to the rest of the universe at the speed of light!


Now that we have a brief imaginary tour of Earth as seen from an Altairian starchip, what can we learn about both Earth and how to look for intelligent life on other planets?

First, the history of intelligent life on Earth is the history of energy and how we’ve used it.  Every bit of food ultimately used chlorophyll to convert sunlight to energy.  Trees converted sunlight into cellulose to give us paper, and our lights stay on because nuclear reactors liberate energy from atoms to boil steam to drive turbines.  Humans have terraformed the planet in ways that optimize energy collection and distribution to our ends, sometimes at the expense of all other life.

Second, not all energy is indicative of life.  If it’s possible to have a naturally occurring nuclear reactor, who’s to say we can’t have naturally occurring lasers or other things we normally associate with intelligent life?  We can’t look at a specific thing, no matter how high-tech it might first appear to be: we need to look at them in the context of how they relate to what’s around them.  The reactors of Gabon served no useful purpose, but the Duane Arnold Energy Center exists in a symbiotic relationship with the city of Cedar Rapids.

Third, ecological disruption — even of the gravest kind, like nuclear waste — isn’t a sign of intelligent life by itself.  But when it occurs in places or to degrees that can’t be explained by known processes, we might be observing the consequences of intelligent life.

Fourth, ecological renewal is the mirror of disruption.  Like it, when it occurs in unusual places or to unusual rates we’ve got good odds of finding intelligent life.

Fifth, using radiative processes (radio, lasers, neutrino generation, whatever) to encode information is very much in line with what we expect of technologically sophisticated life.  Should we ever see a neutrino source on a far off planet, we would be amazed; should we see one that was encoding the Fibonacci sequence, we’d realize we’d just made contact with intelligent life!


The search for extraterrestrial intelligence has been a staggeringly successful failure: every time we’ve seen what we think is a smoking gun indicating intelligent life, we’ve ultimately discovered new things about the cosmos that have transformed our understanding of the possible.  When our extrasolar planetary imaging develops to the point we can see surfaces we’ll undoubtedly be fooled again, too. We shouldn’t be afraid of this.

Within the next few decades we’ll start to develop detailed knowledge of the geological and biological processes of alien worlds.  Getting a comprehensive body of understanding may take a century or more.  There will be many false starts, many times we think we’ve seen intelligent life and instead have discovered yet another natural process we couldn’t have imagined.

And then, on some otherwise normal Thursday, a research team will announce their study of the recent imaging data of the third planet from Altair.  “Once upon a time they were a sophisticated alien race,” the spokesman will say.  “Their planetary crust is completely bare of nuclear fuel, in ways that can’t be explained by natural processes.  We propose sending a starchip probe to the third planet from Altair…”