Sciency Words: The Fermi Paradox

March 24, 2017

Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us expand our scientific vocabularies together. Today’s term is:


Enrico Fermi was an Italian physicist, one of the many great scientists who immigrated to the United States right before the outbreak of World War II. He is most noted for creating the first nuclear reactor and the role he played in the development of the atomic bomb.

But that’s not what we’re going to talk about today. Today we’re talking about something Fermi said half-jokingly over lunch.

Where Is Everybody?

Based on some historical detective work, we can say this probably happened in the summer of 1950. Fermi was visiting the Los Alamos National Laboratory. He and a few colleagues were having a lunchtime conversation about flying saucers. Apparently there had been an amusing cartoon about little green men in a recent edition of the New Yorker.

The conversation got serious (sort of) when Fermi suddenly asked: “but where is everybody?” Everyone at the table laughed, but Fermi’s question and the not-entirely-serious discussion that followed would become the basis of what we now call the Fermi paradox.

As a matter of statistics and probability, it seems like advanced alien civilizations should be out there somewhere. There are over 100 billion stars in our galaxy. Many (if not most) of these stars have planets orbiting them. Some of these planets must surely support life, and in at least a few cases intelligent life—life capable of developing interstellar travel.

Even without faster-than-light technology, one or more of these space-faring civilizations could conceivably spread across the whole galaxy in just a few million years. The galaxy is far, far older than that. There’s been plenty of time for the aliens to do it. So where is everybody? Shouldn’t we have heard from somebody by now?

Or so Fermi argued over his club sandwich (or whatever he was eating) in a half-serious conversation about flying saucers. Of course there are plenty of objections to Fermi’s line of reasoning here, but I’m not going to weigh in on that. Not today. I’m saving my opinion for Monday’s post.

Life on Mars: The Hunt for Martian Dinosaurs

December 28, 2016

Can Mars support life? Is there anything living on Mars right now? It sometimes seems like Mars is desperately trying to convince us that the answer to both questions is yes.


If you’re hunting for alien life in the Solar System, there are four places you should pay attention to: Mars, Europa, Enceladus, and Titan. Now a thought recently occurred to me—a thought that I’m sure has occurred to other people before: in an astrobiological sense, these four worlds sort of represent the past, present, and future.

  • Mars: a place where alien life might have existed and thrived in the past.
  • Europa and Enceladus: places where life may exist and thrive in the present.
  • Titan: a place where life might start to evolve and thrive sometime in the future (assuming it hasn’t started already).

Regarding Mars, there was clearly a time when rivers, lakes, and oceans of liquid water covered the Martian surface. There’s growing evidence that at least some of the organic chemicals necessary for life were also present. Therefore it’s easy to imagine a time millions or perhaps billions of years ago when Mars had a biosphere as rich and robust as prehistoric Earth’s.

Obviously that robust biosphere is gone now. Even when we hear about the possibility that life still exists on present-day Mars, it’s generally assumed that this life would be only a remnant of what came before. The microbial survivors of whatever wiped out the Martian dinosaurs, so to speak.

Someday (hopefully soon), humans will travel to Mars. When we get there, we may find that all the Martians are long dead. That might seem a bit depressing, but actually I’m kind of excited by the idea that the fossilized remains of Martian dinosaurs might be there, waiting for us to come dig them up.

Sciency Words: The Zero-One-Infinity Rule

December 16, 2016

Sciency Words MATH

Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us expand our scientific vocabularies together. Today’s term is:


I came across this term in Time’s special edition on Scott Kelly’s year in space, which I reviewed on Wednesday. The term was used in an article about astrobiology, but it actually originates in the field of computer science.

Zero-One-Infinity in Computer Science

The zero-one-infinity rule is sort of a rule of thumb. It’s credited to Dutch computer scientist Willem Louis Van Der Poel. According to this rule, a computer program should either never allow a certain event (zero), or it should allow it only once (one), or it should allow it an unlimited number of times (infinity).

The logic here is that it makes sense to not allow something to happen. It also might make sense to allow something to happen only once, perhaps as an exception. But programmers shouldn’t create arbitrary limits (according to this rule) on what a program can do. If you’re willing to allow something to happen twice, why not three times? Or four? Or thirty-eight? Or as many times as the user wants (computer memory space permitting)?

I don’t have a whole lot of coding experience, but the zero-one-infinity rule makes sense to me. It seems like a good rule, although I could probably think up more than one exception to the rule if I really wanted to.

Zero-One-Infinity in Astrobiology

Applying the zero-one-infinity rule to the search for alien life is, in my opinion, brilliant. How many locations in the universe can support life? There are really only three answers:

  • Life cannot exist anywhere in the universe (zero).
  • Life can exist only on Earth; Earth is a very special exception in a universe where life is otherwise not allowed (one).
  • Life can exist in an unlimited number of locations in the universe (infinity).

We already know the zero proposition is false.

There was a time (I remember it well) when many a scientist argued that Earth must be an exception: the one and only place in the universe where life could exist. Occasionally, I still hear people try to argue this.

All it would take is to find a second life-bearing world to prove the one proposition wrong (I’m looking at you, Europa). Because once we know about two living worlds, how could anyone argue that there can’t be three? Or four? Or thirty-eight? Or however many the universe feels like having?


Zero-One-Infinity Rule from The Jargon File.

Willem Louis Van Der Poel from Wiki Wiki Web.

Sciency Words: Thalassogen

November 18, 2016

Sciency Words PHYS copy

Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us expand our scientific vocabularies together. Today’s term is:


I stumbled upon this term while researching my recent Molecular Monday post on ammonia. The word thalassogen comes from the Greek words for “sea” and “creation,” and it was coined by one of the great luminaries of both science and science fiction: Isaac Asimov.

Basically, a thalassogen is a chemical substance that could, under realistic circumstances, form an ocean on a planet or moon. Obviously water qualifies. Just look at Earth. But what other substances could we call thalassogenic?

First, we need something that can be liquid and is capable of remaining in a liquid state across a reasonable wide range of temperatures and pressures.

We also need a chemical that is reasonably plentiful in the universe. According to Asimov, that rules out something like mercury. Mercury does a great job being a liquid, but it’s so rare that we can’t realistically expect to find a world covered in mercury oceans.

Asimov also wrote that “ideally” a thalassogen should be able to transition from liquid to both solid and gaseous states without too much difficulty. That way, we could have something analogous to Earth’s hydrocycle, with clouds and rain and snow and glaciers. Please note: that’s ideal, but not necessarily a requirement.


In my opinion, the most sensible way to use this term is to say that a substance is (or could be) a thalassogen in a specific environment. So methane is a thalassogen on Titan, but not Earth. You might also say water is a thalassogen on Earth but not on Venus. Or water is a thalassogen beneath the surface of Europa, but not on Europa’s surface.

So as we venture out into space, what sorts of chemicals might we find acting as thalassogens on alien worlds? Asimov suggested water, ammonia, and methane as the most likely candidates. Other possibilities include carbon dioxide, sulfur dioxide, and sulfuric acid. We should also consider mixtures of these and other chemicals.

And who knows? Given some of the strange, improbable-seeming exoplanets we’ve discovered so far, maybe Asimov was a little too quick to rule out mercury.

Molecular Monday: Life in an Ammonia Ocean

November 7, 2016

Molecular Mondays Header

Welcome to Molecular Monday! On the first Monday of the month, we take a closer look at the atoms and molecules that make up our physical universe. Today, we’re looking at:


Water, Water Everywhere…

You know how water has that Mickey Mouse shape? That shape is really important. That slight asymmetry allows electrical charges to accumulate on opposites sides of the water molecule.


The polarization of water molecules makes water a good solvent for other polar molecules, like amino acids. This is a big reason why water is essential to life (or at least, life on Earth). Without the ability to dissolve amino acids, we’d have an awfully hard time getting them to form peptides or proteins or DNA molecules.

But could life on some alien planet substitute another chemical for water?

Ammonia, Ammonia Everywhere…

This is an ammonia molecule (chemical formula NH3).


At first glance, you might think ammonia molecules are symmetrical, with three hydrogen atoms evenly spaced around the central nitrogen atom. Symmetrical molecules have all their electrical charges perfectly balanced, and therefore are non-polar and do not act as good solvents for amino acids.

But when you turn the ammonia sideways, things look rather more promising.


The three hydrogen atoms bend toward each other, just as the two hydrogens in water do. There’s a slight asymmetry, meaning electrical charges can form. Ammonia is a polar molecule after all!

And ammonia has a few other things in common with water:

  • They’re both fairly common in the universe (though water is more common).
  • They both can be liquid under fairly ordinary temperature/pressure ranges (though water’s liquid phase is wider than ammonia’s).
  • They can both act as a base, meaning they can accept a proton from an acid (though ammonia is slightly more basic than water).
  • They can both act as an acid, meaning they can both donate a proton to a base (though water is slightly more acidic than ammonia).

The most noteworthy difference seems to be that ammonia burns easily in the presence of oxygen. That could pose serious challenges to the evolution of complex, multi-cellular organisms that need the extra kick of energy oxygen provides.

Still, water and ammonia are similar enough to attract the attention of astrobiologists, and a lot has been written about the possibility of life emerging on some distant planet in an ammonia sea.


Hypothetical Types of Biochemistry from Wikipedia.

Alternatives to Water from Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization.

Thalassogens from Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization.

* * *

A special thank you to Kirov99 for suggesting this topic. My research tends to focus on the planets and moons of the Solar System, rather than hypothetical environments we might find elsewhere in the universe, so without the recommendation I would have probably missed this.

Sciency Words: Shadow Biosphere

October 14, 2016

Sciency Words MATH

Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us all expand our scientific vocabularies together. Today’s term is:


Crazy Talk

We are not alone on planet Earth. There are aliens among us. Their existence has gone unnoticed and unsuspected for millions of years.

Truth be told, I shouldn’t call them “aliens.” They evolved here on Earth, side by side with what we, in our arrogance, call “organic life.”

They’re everywhere. There’s a whole biosphere of these weird creatures sharing our planet with us. It’s called the shadow biosphere.

Not Crazy Talk

I first heard about the shadow biosphere on an episode of SciShow, and I’ve been seeing the term more and more lately. It seems like some sort of astrobiology buzzword at the moment.

The idea is that an alternative form of life could have evolved here on Earth, and we just haven’t discovered it yet. Maybe it lives in areas totally inaccessible to us, like deep beneath the Earth’s crust. Or maybe it’s so different from us that we don’t yet recognize it as a life form.

Personally, I take this as more of an astrobiology thought experiment than a serious hypothesis about life on our planet. It’s a way of reminding us how limited our understanding of life is and show how difficult it might be to identify alien life should we happen to find it.

You see, to determine if something is alive, we must try to identify ways in which it is similar to other living things. Does it move? Grow? Reproduce? On a more fundamental level, is it cellular in structure? Does it have a carbon-based biochemistry? A DNA-like genetic code?


Little did the humans suspect that their “pet rocks” were in fact silicon-based life forms.

But all these questions presuppose that newly discovered life forms will be similar to life forms we already know about. What if we’re dealing with a life form totally dissimilar to life as we know it? What if they’re non-cellular, non-carbon-based organisms that don’t have anything resembling DNA?

Why, such organisms might be so strange to us that they could exist all around us, even right here on Earth, and we wouldn’t know it. Or so this type of thought experiment may lead you to conclude.

Back to Crazy Talk

It’s not just a thought experiment. The shadow biosphere is real. It’s real, I tell you! Wait, where are you taking me? No, I don’t want to take my medicine. Are you working for them? Did the pet rocks send you?

All These Worlds Are Yours: A Book Review

October 11, 2016

In his book All These Worlds Are Yours: The Scientific Search for Alien Life, author Jon Willis gives you $4 billion. How many authors do that? Okay, it’s imaginary money, and you’re only allowed to spend it on astrobiological research. But still… $4 billion, just for reading a book!

If you’re new to the subject of astrobiology, All These Worlds is an excellent introduction. It covers all the astrobiological hotspots of the Solar System and beyond, and unlike most books on this subject, it doesn’t gloss over the issue of money.

There are so many exciting possibilities, so many opportunities to try to find alien life. But realistically, you can only afford one or maybe two missions on your $4 billion budget. So you’ll have to pick and choose. You’ll have to make some educated guesses about where to look.

Do you want to gamble everything on Mars, or would you rather spend your money on Titan or Europa? Or do you want to build a space telescope and go hunting for exoplanets? Or donate all your money to SETI? Willis lays out the pros and cons of all your best options.

My only complaint about this book is that Enceladus (a moon of Saturn) didn’t get its own chapter. Instead, there’s a chapter on Europa and Enceladus, which was really a chapter about Europa with a few pages on Enceladus at the end.


I agree, Enceladus. On the other hand, Enceladus is sort of like Europa’s mini-me. So while I disagree with the decision to lump the two together, I do understand it.

In summary, I’d highly recommend this book to anyone interested in space exploration, and especially to those who are new or relatively knew to the subject of astrobiology. Minimal prior scientific knowledge is required, although some basic familiarity with the planets of the Solar System would help.

P.S.: How would you spend your $4 billion? I’d spend mine on a mission to Europa, paying special attention to the weird reddish-brown material found in Europa’s lineae and maculae.