Sciency Words A to Z: Oxygen Catastrophe

Welcome to a special A to Z Challenge edition of Sciency Words!  Sciency Words is an ongoing series here on Planet Pailly about the definitions and etymologies of science or science-related terms.  In today’s post, O is for:


Oxygen.  What could be more healthy or more wholesome to life than oxygen?  But oxygen is, in fact, one of the most dangerous and deadly chemicals on Earth.  It is, as you might easily guess, a powerful oxidizer, and it reacts with just about everything—especially organic matter!

True, life on Earth as we currently know it would not be possible without oxygen, but left to its own devices oxygen would eagerly burn us all up.

It started with cyanobacteria (also known as blue-green algae).  Cyanobacteria were the first organisms on this planet to develop photosynthesis, a process that uses sunlight to convert water and carbon dioxide into biomolecules.  But photosynthesis also produces hazardous oxygen as a waste product.

Things were okay for Earth’s biosphere for a while, but eventually the oxygen situation turned deadly.  As David Grinspoon explains in his book Earth in Human Hands:

For hundreds of millions of years, the cyanobacteria kept spitting out oxygen, but all the excess was hungrily snapped up by the abundant iron in Earth’s crust and interior.  Yet eventually, around 2.4 billion years ago, the crust was thoroughly oxidized and there was no more available iron lying around.

Grinspoon goes on to explain how life would eventually learn to protect itself from the harmful effects of oxygen exposure and even learn to control oxygen, transforming what was (and still is, in some respects) a deadly poison into a valuable and powerful new fuel.

Before we evolved that ability, however, the buildup of corrosive oxygen in the atmosphere was massively fatal for most of the species that existed on Earth at the time.

This event, approximately 2.4 billion years ago, is generally known as the Great Oxidation Event, but it’s also sometimes called the Oxygen Catastrophe.  I prefer calling it the Oxygen Catastrophe, because the consequences were truly catastrophic for almost everyone who was not a cyanobacteria. This was, in fact, Earth’s first mass extinction event.

As we continue our search for alien life, I think it’s important to keep oxygen’s true nature in mind.  To us, oxygen means life, but it could just as easily be seen as a deadly poison.  There may be other worlds out there was “poisonous” atmospheres, and those worlds may turn out to be the ones we should pay the most attention to.

Next time on Sciency Words A to Z, what if life on Earth started somewhere else?

Sciency Words A to Z: Noachian

Welcome to a special A to Z Challenge edition of Sciency Words!  Sciency Words is an ongoing series here on Planet Pailly about the definitions and etymologies of science or science-related terms.  In today’s post, N is for:


In the 1870’s, Italian astronomer Giovanni Schiaparelli began producing the most detailed and accurate maps of Mars anyone had ever seen.  Schiaparelli also assigned many of the names we still use today for Martian surface features today. One of those regions on Schiaparelli’s map got the name Noachis Terra—the Land of Noah.

In my opinion, no other name could have turned out to be more apt.  Schiaparelli got many of his names from the Bible, and I’m sure you remember the biblical story of Noah and the Great Flood.

Schiaparelli’s map of Mars.

Much like the geological history of Earth, Mars’s geological history is divided up into different periods.  Noachis Terra spawned the name for Mars’s Noachian Period, a time that roughly corresponds with the Archean Eon here on Earth—the time when the very first microbes were appearing on our planet.

So what was happening on Noachian Mars?  Based on the evidence presented in this textbook on Astrobiology, it wasn’t quite like the Great Flood in the Bible, but it was close!  Most if not all of Mars’s northern hemisphere was probably covered in water.  Circumstantial evidence of shorelines can be seen today.

And in the southern hemisphere, in regions like Noachis Terra, we see unambiguous evidence of ancient flowing water.  Craters show obvious signs of erosion.  There are dried up lakes and rivers, and those rivers appear to have been fed by tributaries, which tells us it used to rain on Mars.

And there’s more.  Many Noachian-aged minerals and rock formations are most easily explained if we assume there was water.  In some cases, water is the only possible explaination.  Our Mars rovers have found mudstone, clay minerals, sedimentary rock… iron and magnesium carbonate… hematite, jarosite, and more!  Some of these minerals would have required a hot and slightly acidic environment, like you might find in a hot spring or near a hydrothermal vent.

We shouldn’t jump to conclusions.  After all, there’s still so much we don’t yet know about Mars, and new discoveries are being made all the time.  But I’m going to go ahead and call a spade a spade here: Noachian Mars sounds an awful lot like Arcean Earth, and it’s easy to imagine that whatever was happening on Arcean Earth (by which I mean LIFE!!!) must’ve also been happening on Noachian Mars.

However, the Noachian Period did not last long—a mere 400 million years.  Earth and Mars have had very different geological histories since then.  After the Noachian, Mars rapidly lost its internal heat, its atmosphere, and its oceans.  By the time of Earth’s Cambrian explosion, when complex, multi-cellular organisms really “exploded” onto the scene, Mars had fully transformed into the barren, inhospitable world we know today.

Modern day Mars has been trying really hard to get our attention and convince us that it might still support life.

And maybe that’s true.  During the Noachian, life had a great opportunity to get started on Mars, and it’s possible that some isolated remnant of a Noachian ecosystem has persisted to this day.  But in my opinion, it’s far more likely that we’ll find fossils left over from the Noachian Period (assuming we haven’t found some already).

Next time on Sciency Words A to Z, did you know there’s a deadly chemical in the air you breathe?  It’s called oxygen.

Sciency Words A to Z: METI

Welcome to a special A to Z Challenge edition of Sciency Words!  Sciency Words is an ongoing series here on Planet Pailly about the definitions and etymologies of science or science-related terms.  In today’s post, M is for:


In a sense, SETI researchers are just sitting by the phone waiting for somebody to call.  Maybe that’s the wrong way to go about it.  Maybe it’s time to pick up the phone, start dialing numbers, and see who picks up.

This idea is sometimes called active SETI, but it’s more common (and according to this paper, more appropriate) to use the term METI: the messaging of extraterrestrial intelligence.

Earth has been broadcasting TV and radio signals for over a century.  This has led to a common misconception that even now, aliens on some far off planet might be gathering around their equivalent of a television set, watching old episodes of Howdy Doody  or The Honeymooners.  Or perhaps, if the aliens live nearby, they’re currently listening to our more recent music.

But Humanity is only a Type 0 or Type I civilization, depending on which version of the Kardashev scale you’re using. Either way, our broadcasts are not actually that strong.  As David Grinspoon explains in his book Earth in Human Hands:

Our television signals are diffuse and not targeted at any star system.  It would take a huge antenna, much larger than anything we’ve built or planned, to pick up on them.  From a radio point of view our planet is not completely hidden, but it is hardly conspicuous.  This could easily change.  Targeted messages sent directly toward nearby stars would cause Earth suddenly to turn on like a spotlight, becoming an obvious beacon announcing, for better or worse, “We are here!”

Of course we’ve already done this.  Several times, in fact.  But not with enough consistency to truly make our presence known.

The first attempt was in 1974, when Frank Drake and Carl Sagan transmitted a message from the Arecibo radio telescope in Puerto Rico, aimed at the M13 globular cluster.  But according to Grinspoon, if aliens ever do pick up that signal, “[…] they might dismiss it as a momentary fluke.  We would.”  That’s because the Arecibo message was a quick, one-time thing.  By itself, it’s hardly proof beyond a reasonable doubt that life exists on Earth.

If we really want to get somebody’s attention, we have to send a sustained, repetitive signal, kind of like those repetitive radio pulses Jocelyn Bell detected in the 60’s.  We have the technology.  We can make METI a reality.  But should we?  Some say yes, others no.  After all, we have no idea who might hear our signal, or what form their response might take, and there is no guarantee that the aliens will be friendly.

METI is a discussion and a debate that maybe we all, as a species, should be part of.  Perhaps we should take a vote, because in the end, we all have a stake in what might happen.  And while we’re at it, there are some other issues we all, as a species, should vote on.  Or at least that’s what Grinspoon says we should do in his book.

Next time on Sciency Words A to Z, we’ll go back in time and check out the oceans of Mars.

Sciency Words A to Z: LGM-1

Welcome to a special A to Z Challenge edition of Sciency Words!  Sciency Words is an ongoing series here on Planet Pailly about the definitions and etymologies of science or science-related terms.  In today’s post, L is for:


In the mid-1960’s, Jocelyn Bell (later known as Jocelyn Bell Burnell) was a grad student at Cambridge.  Through Anthony Hewish, her Ph.D. advisor, she became involved with the construction and operation of a brand new radio telescope specially designed to hunt for quasars.  But that telescope ended up finding something more than just quasars.

Bell Burnell recounts the story in this speech, as published by Cosmic Search Magazine.  Part of her job was analyzing data from the telescope, which came in the form of chart paper—literally miles worth of paper—produced by a set of “3-track pen recorders.”  And on those chart papers, Bell saw some odd markings, which she described as bits of “scruff.”

Naturally, that scruff required further investigation. Faster, more accurate recordings were made, and the scruff resolved itself into a series of regular radio pulses.  These pulses were so consistent, so evenly spaced, that you’d think they must be artificial. It was almost like someone out there in space was trying to get our attention!

Bell named the source of those radio pulses LGM-1, which stood for little green men #1.  But as I said in a previous post, when it comes to discovering alien life, scientists must hold themselves to the same standard as a court of law: proof beyond a reasonable doubt.  While Bell may have been happy to joke about little green men, she did not actually believe that’s what she’d discovered.  As she explained in her speech:

Just before Christmas I went to see Tony Hewish about something and walked into a high-level conference about how to present these results.  We did not really believe that we had picked up signals from another civilization, but obviously the idea had crossed our minds and we had no proof that it was an entirely natural radio emission. It is an interesting problem—if one thinks one may have detected life elsewhere in the universe how does one announce the results responsibly?  Who does one tell first?

After her Chistmas break, Bell returned to work and found a big pile of fresh data to analyze, and there was more “scruff.” In total, Bell found four distinct radio sources, located in completely different parts of the sky.

And that finally put the “little green men” hypothesis to rest.  It seemed highly unlikely that four different alien civilizations, located in completely different regions of space, would all try to get our attention at the exact same time, in the exact same way, using the exact same radio frequencies.

LGM-1 is now believed to be a neutron star, spinning rapidly, projecting twin beams of radio waves out into space like some sort of cosmic lighthouse.  It’s an entirely natural phenomenon, the result of a supernova explosion.  Today, we call this sort of object a pulsar.

Next time on Sciency Words A to Z, maybe it’s time to stop waiting for aliens to contact us.  Maybe it’s time we tried to contact them.

Sciency Words A to Z: Kardashev Scale

Welcome to a special A to Z Challenge edition of Sciency Words!  Sciency Words is an ongoing series here on Planet Pailly about the definitions and etymologies of science or science-related terms.  In today’s post, K is for:


In 1963, Soviet scientist Nikolai Kardashev published this paper concerning the search for extraterrestrial intelligence. Kardashev seems to have been primarily interested in how much information aliens might be able to transmit to us across the vastness of space.  This, in turn, relates to how much energy an alien civilization is able to produce, because the more energy you have, the stronger your radio signals can be.

Kardashev summarized his thoughts on this by devising a scale—now known as the Kardashev scale.  In Kardashev’s original system, there were only three types of civilizations:

  • Type I: a civilization that has harnessed energy on a planet-wide scale.  Kardashev considered Earth to be a Type I civilization.
  • Type II: a civilization that has harnessed the energy of an entire star, perhaps by building a Dyson sphere or some other megastructure around their own sun.
  • Type III: a civilization that has harnessed the energy of an entire galaxy.  Kardashev doesn’t offer any examples of this, but I might point to something like the Galactic Republic/Galactic Empire in Star Wars—they’re approaching Type III status.

Later scientists have expanded on the Kardashev scale.  Humanity has been demoted to a Type 0 civilization, because we don’t really use all the energy available to us on our planet.  Not yet, at least.

We can also talk about Type IV civilizations, which can harness the energy of the whole universe, and Type V civilizations, which can harness all the energy of the multiverse, or perhaps all the energy of alternative timelines, or something like that. Examples?  I don’t know, maybe the Timelords from Doctor Who or the Q-Continuum from Star Trek. Or maybe these people.

So which of these civilizations should we expect to find out there? What sort of transmissions do we expect to see?

The problem with Type IV and V civilizations is that their activities would be, to us mere mortals, virtually indistinguishable from nature.  As for Type 0 and Type I, their radio signals (if they’re sending any) may be too weak for us to detect over all the background radiation of the cosmos.

But the Type II and Type III civilizations… Kardashev was pretty optimistic about our chances of finding them.  In his 1963 paper, Kardashev argues that it’s absurd to think Earth is the only planet with intelligent life, and furthermore most alien civilizations should be far older and far more advanced than we presently are.  You may recall Enrico Fermi made a similar argument.

So there should be plenty of Type II civilizations out there, and perhaps a few Type IIIs as well, all chattering away in loud, easy-to-detect radio transmissions.  Or so Kardashev claims.  “In any case, the deciding word on this question is left to experimental verification,” he wrote.  But after fifty years of trying to detect something… anything… what has the experimental evidence shown us?

That’s a fair question.  And yet I have to agree with Kardashev: it is absurd to think Earth is the only planet with intelligent life.  So once again, in the immortal words of Enrico Fermi, where is everybody?

Next time on Sciency Words A to Z… wait, did we detect a signal?  Nope.  False alarm.

Sciency Words A to Z: JUICE

Welcome to a special A to Z Challenge edition of Sciency Words!  Sciency Words is an ongoing series here on Planet Pailly about the definitions and etymologies of science or science-related terms.  In today’s post, J is for:


Speaking as a space enthusiast and a citizen of the United States, I have to confess I’m a bit disappointed with the status of the American space program.  While there have been some success stories—New Horizons, Curiosity, Scott Kelly’s year in space—I can’t help but feel like NASA has spent the last decade or so floundering.

However, it’s encouraging to see that so many other space agencies around the world are starting to pick up the slack.  My favorite example of this is the JUICE mission, a project of the European Space Agency (E.S.A.).

Astrobiologists have taken a keen interest in the icy moons of Jupiter.  There’s compelling evidence that one of those moons (Europa) has an ocean of liquid water beneath its surface.  There’s also a growing suspicion that two more of those moons (Ganymede and Callisto) may have subsurface oceans as well.

The original plan was for NASA and the E.S.A. to pool their resources for one big, giant mission to the Jupiter system.  But then the 2008 financial crisis hit.  The U.S. Congress was loath to spend money on anything—especially space stuff.  “Due to the unavailability of the proposed international partnerships […]”—that’s how this E.S.A. report describes the matter.

So the E.S.A. decided to go it alone. Personally, I think this was a very brave move.  E.S.A. has never done a mission to the outer Solar System before, not without NASA’s help.  But there has to be a first time for everything, right?  And so JUICE—the JUpiter ICy moons Explorer—began.  It’s not my favorite acronym, but it works.

According to E.S.A.’s website, JUICE will conduct multiple flybys of Europa and Callisto before settling into orbit around Ganymede.  You may be wondering why JUICE won’t be orbiting Europa.  This is in large part because of the radiation environment around Jupiter.  Europa may be more exciting to astrobiologists, but Ganymede is a safer place to park your spacecraft.

Meanwhile, NASA has recovered much of the funding it lost after the 2008 financial crisis, and they’re once again planning to send their own mission to the Jupiter system.  So maybe NASA and E.S.A. will get to explore those icy moons together after all!  Or maybe not.  According to this article from the Planetary Society, NASA’s budget is under threat once again.

I guess we’ll have to wait and see, but no matter what happens to NASA’s budget, E.S.A. seems fully committed to JUICE.  So speaking as a space enthusiast, at least I have that to look forward to.

Next time on Sciency Words A to Z, how do you measure the size of an alien civilization?

Sciency Words A to Z: Intelligence

Welcome to a special A to Z Challenge edition of Sciency Words!  Sciency Words is an ongoing series here on Planet Pailly about the definitions and etymologies of science or science-related terms.  In today’s post, I is for:


In 1959, this paper by Giuseppi Cocconi and Philip Morrison appeared in the journal Nature.  The ideas Cocconi and Morrison laid out in that paper were bold, and maybe a little presumptuous, but they became the foundation for a very important subfield of astrobiology: the search for extraterrestrial intelligence, or SETI for short.

The A to Z Challenge being what it is, it’s too early for us to start digging in to the subject of SETI research.  But we can talk about part of it.  Specifically the I part—“intelligence.”  It’s fairly obvious what “search” means, and “extraterrestrial” simply refers to something that’s not from Earth. But what is the definition of “intelligence”?

What does it mean to be intelligent? How would we recognize an extraterrestrial intelligence if and when we find one?  Are we sure we humans are a good example of what an intelligent life form is like?  (No, wait, maybe don’t answer that last one!)

In this article from, the famous SETI scientist Jill Tarter is quoted as saying:

SETI is not the search for extraterrestrial intelligence.  We can’t define intelligence, and we sure as hell don’t know how to detect it remotely.  [SETI]… is searching for evidence of someone else’s technology.  We use technology as a proxy for intelligence.

This reminds me of a joke from the Hitchhiker’s Guide to the Galaxy, that humans think we are the most intelligent creatures on Earth because we built cities and nuclear weapons and things like that, while dolphins believe they are more intelligent than us because they chose not to do those things.

So it is time we change SETI to SETT—the search for extraterrestrial technology?  It sounds like Tarter would support that change.  She calls the SETI acronym “problematic” and suggests that we “talk about a search for technosignatures” instead.  But as regular readers of Sciency Words should know by now, once a word gets embedded in the scientific lexicon, it’s really, really, really hard to change it, no matter how problematic it might seem. Don’t believe me?  Click here or here or here or here or here.

And I suspect that Jill Tarter knows this.  In this report on SETI nomenclature, which is co-authored by Tarter, it says, “Definitions of intelligence are slippery […]” however, “[the word’s] use in the acronym SETI is sufficiently entrenched that we recommend against a more precise rebranding of the field.”

So what does it mean to be intelligent?  For the purposes of SETI, no one knows.  The term is vague to the point of being unusable for official scientific discourse.  But scientists have been talking about and writing about this for decades—remember, that Cocconi-Morrison paper came out in 1959—so at this point we’re sort of stuck with the I in SETI.

Next time on Sciency Words A to Z, we’ll get the latest juicy gossip from the moons of Jupiter.

Sciency Words A to Z: Hydrothermal Vents

Welcome to a special A to Z Challenge edition of Sciency Words!  Sciency Words is an ongoing series here on Planet Pailly about the definitions and etymologies of science or science-related terms.  In today’s post, H is for:


In his book All These Worlds Are Yours, Canadian astronomer Jon Willis recounts the story of how hydrothermal (hot water) vents were first discovered here on Earth.  It was 1977.  A scientific research vessel was towing a deep-sea probe along the ocean floor in the Pacific when the probe detected a temperature anomaly.

This was exactly what the crew of that research vessel was hoping to find: a sort of underwater volcano, right where two tectonic plates were moving apart.  But the real surprise came when that research team brought their deep-sea probe back to the surface and developed all its photographs.  They saw the hydrothermal vent they were expecting to see, but they also saw things living—yes, living!—all around it.

Marine microbiologist Holger Jannasch, who was part of a follow-up expedition in 1979, had this to say:

We were struck by the thought, and its fundamental implications, that here solar energy, which is so prevalent in running life on our planet, appears to be largely replaced by terrestrial energy—chemolithoautotrophic bacteria taking over the role of green plants.  This was a powerful new concept and, in my mind, one of the major biological discoveries of the 20th Century.

It’s become fashionable to suppose that, rather than the “warm little pond” that Charles Darwin once wrote about, perhaps life began its conquest of Earth in an environment like this: a place deep under water where heat and chemicals come spewing up out of the planet’s crust.

An Introduction to Astrobiology actually cites science fiction writer Arthur C. Clarke as the first to realize what all this might mean for life in our Solar System.  Specifically, Clarke thought of the icy moons of Jupiter.  In his 2001: A Space Odyssey novels, Clarke tells us of a hydrothermal vent on Europa—a “warm oasis” populated by plant-like, slug-like, and crab-like creatures.

The idea of life on Europa (or Saturn’s moon Enceladus) clustered around hydrothermal vents may have started out as science fiction, but it is now a possibility that astrobiologists take very seriously. But we’ll talk about that later this week.     

Next time on Sciency Words A to Z, what’s wrong with the I in SETI

Sciency Words A to Z: Goldilocks Zone

Welcome to a special A to Z Challenge edition of Sciency Words!  Sciency Words is an ongoing series here on Planet Pailly about the definitions and etymologies of science or science-related terms.  In today’s post, G is for:


Once upon a time, there was a little girl from outer space who came to visit the Solar System.  Her name was Goldilocks.  First, she landed on Mars, but she didn’t like it there.  It was too cold.  Then she tried to land on Venus, but she didn’t like it there either. It was too hot—way too hot.  And then finally, Goldilocks landed her spaceship on Earth.  When she came out of the airlock and walked down the landing ramp, she said to the astonished Earthlings, “Ah yes, this planet is just right!”

At least that’s how my version of the Goldilocks story goes.

Anyway, the concept of a Goldilocks zone (also known as a habitable zone, continuously habitable zone, or circumstellar habitable zone) is pretty simple.  Fairy tale simple, you might say.  The Goldilocks zone is the region of space around a star where liquid water can exist on a planet’s surface.  And as you know, if a planet has liquid water on its surface, then it could have life!

For a long time, our search for alien life has focused almost exclusively on Goldilocks planets.  But there are problems with limiting our search in that way.

In my post on carbon chauvinism, I told you there are other chauvinisms that astrobiologists have to deal with. One of them is water chauvinism, the presumption that water is necessary for life.  Another is surface chauvinism, the presumption that life can only exist on a planet’s surface.  Our obsession with Goldilocks zones is largely based on those two chauvinisms.

But looking to the moons of Jupiter and Saturn, we’ve already learned that there is more liquid water outside the Goldilocks zone than in it!  Several of those moons have vast oceans of liquid water beneath their surface, with only a relatively thin crust of ice overtop.  These subsurface oceans might be ideal environments for alien life.  So much for our surface chauvinism.

And then there’s Titan, a moon of Saturn, which has lakes of liquid methane and ethane on its surface.  Could those liquid hydrocarbons serve as a substitute for water in an alien biochemistry?  We don’t know.  It’s possible.  We certainly shouldn’t rule that possibility out.  And thus, so much for our water chauvinism.

To quote from Exoplanets by Michael Summers and James Trefil, “[…] the current focus on finding a Goldilocks planet amounts to a search for the least likely location of water and, presumably, life.”  I think there’s a bit of hyperbole in that statement, but I agree with the general point.  There are probably far more worlds in our galaxy like Europa, Enceladus, or Titan than there are like Earth.

Next time on Sciency Words A to Z, we’ll crack the surface of one of those icy moons and see what might be hidden in those dark, extraterrestrial depths.

Sciency Words A to Z: The Fermi Paradox

Welcome to a special A to Z Challenge edition of Sciency Words!  Sciency Words is an ongoing series here on Planet Pailly about the definitions and etymologies of science or science-related terms.  In today’s post, F is for:


The birth of the Fermi Paradox is, perhaps, one of the most poorly documented scientific events in recent history.  Nuclear physicist Enrico Fermi did not present his famous paradox at some scientific symposium or write it up for some academic journal. No, the whole thing started (apparently) with a comment Fermi made half-jokingly over lunch.

I normally draw all the illustrations on this blog, but I’m making an exception today.  In 1950, New York City was suffering an epidemic of disappearing garbage cans.  No one could figure out where the city’s garbage cans were going or who was taking them, so the New Yorker published this cartoon offering one possible explanation:

According to the historical narrative reconstructed in this report, that summer (or sometime thereabout) Fermi was visiting the Los Alamos National Laboratory in New Mexico.  He and a bunch of old friends from the Manhattan Project had seen that cartoon and were joking about extraterrestrial life over lunch.

As the conversation progressed, Fermi suddenly, almost out of the blue, said these fateful words: “But where is everybody?”  He then proceeded to lay out the fundamental problem that is now known as the Fermi Paradox.

In short, our galaxy is old—over ten billion years old by most estimations.  Earth is less than half that age, and our civilization—why, we’ve been around for barely a blink of an eye on the cosmic scale.  If civilizations like ours can pop up so suddenly, so abruptly, then over the last ten billion years advanced civilizations should have filled up the whole galaxy.  The aliens should be everywhere, and yet we can’t seem to find any evidence of their existence.

So where is everybody?

Many answers to that question have been proposed over the years.  Fermi and company are said to have run through most of them that day while they finished up their lunch.

  • Maybe Earth is part of a galactic nature preserve, or maybe intergalactic law forbids anyone from making contact with “primitive” cultures like our own.
  • Maybe Earth is out in the boondocks of the galaxy, far, far away from where all the aliens like to hang out.
  • Maybe interstellar travel is harder than we think, and so all the alien civilizations tend to keep to themselves and never leave their home planets or home solar systems.
  • Maybe intelligent life has an innate tendency to destroy itself.

That last one is a sobering thought, especially when you remember that these were the people who worked on the Manhattan Project!

Personally, I kind of like the notion that we’re part of a nature preserve.  I have no scientific justification for thinking that; I just find it comforting to suppose that maybe the aliens do know about us and think we’re worth preserving.  But what do you think the solution to the Fermi Paradox might be?  Let me know in the comments!

Next time on Sciency Words: A to Z, why is Earth “just right” for life?

Correction/Clarification: After reading some of the responses to this post, I think I may have been a little too flippant about the galactic nature preserve thing. I think that’s a cool idea, and I think it’s a fun thing to think about. But there is absolutely no scientific evidence to support that hypothesis at this point, and I do not actually take the idea seriously. I should have been clearer about that.