Stone age

I've only got a few real obsessions.  My dogs.  Doctor Who.  Anything to do with astronomy.  Lost in Space.  The X Files.  Star Trek - The Next Generation.  The movie Contact.

I bet you're sensing a theme, here.  Other than my dogs, all of these have to do with the universe, space travel, and alien life.  And given how oddly my dogs act some days, I find myself wondering if they might not be alien spies as well.  Especially Rosie, who so often seems to be judging us.

"Unless I start getting steak for dinner, the report I'll be sending to the Mothership will be highly unflattering."

But even with that possible exception, it's evident that I have a bit of a fixation on the possibility of extraterrestrial life.  I'm well aware of the fact that with regards to life, we've still got a sample size of one; despite decades of looking, we have yet to find any unequivocal biosignatures, signs that life exists, anywhere else but here.  (Much less any signs of extraterrestrial intelligent life.  Much as I would love for some astronomer to become a real-life Ellie Arroway, no such luck... yet.)

In spite of all this, I still am very much of the opinion that life elsewhere in the universe is likely to be abundant.  I base this on the known facts that there are trillions of stars out there, in billions of galaxies, and that exoplanetary systems are common (i.e. the formation of the Solar System wasn't just a lucky fluke).  Optimistic estimates of some of the other variables in the Drake Equation are harder to defend, but I stand by my statement: a purely statistical argument suggests that many star systems have planets that support some kind of life.

One of the things that in my mind argues for life existing elsewhere in the universe -- even in environments that we might consider inhospitable -- is how many extreme habitats here on Earth turn out to host living things.  There's life in the desiccated, perpetual cold of the dry valleys of Antarctica, in highly alkaline (or highly acidic) hot springs, in boreholes miles deep, in hydrothermal vents in the oceanic abyss.  The odd little animals called tardigrades can survive extremes in temperature and pressure, radiation, and dehydration; they've even survived exposure to the vacuum of space.

And we're still finding new ones in unexpected places.  Take, for example, the microorganism -- or, rather, the traces of it -- that was the subject of a study this week in the journal Geomicrobiology.  A team out of Johannes Gutenberg Universität Mainz was studying samples of marble and limestone quarried in the parched deserts of Namibia, Oman, and Saudi Arabia, and found microscopic tunnels apparently excavated by some as-yet-unidentified microbe.

"We were surprised because these tubes are clearly not the result of a geological process," said Cees Passchier, who co-authored the paper.  "We were looking at the structure of the rocks to find out how continents came together to form the supercontinent Gondwana five hundred to six hundred million years ago.  At that time, carbonate deposits formed in the ancient oceans and turned into marble due to pressure and heat...  We noticed strange structures in this marble that were not the result of geological events.  These are old structures, perhaps one or two million years old...  What is so exciting about our discovery is that we do not know which endolithic microorganism this is.  Is it a known form of life or a completely unknown organism?  It must be an organism that can survive without light because the tubes have formed deep inside the rock.  We don't currently know whether this is a life form that has become extinct or is still alive somewhere."

Samples of marble with the "microburrows" [Image credit: C. Passchier et al.]

It seems like everywhere we look on Earth, we find life, which strengthens the hope of those of us who'd like to find life out there amongst the stars as well.  That microorganisms can live by tunneling their way through solid rock certainly suggests we should expand the parameters of the phrase "capable of supporting life."

Although most of it may not be at the point of sending out messages that could be picked up by our radio telescopes, my surmise is that most even remotely hospitable locales in the universe will turn out to be inhabited.  And just judging by the diversity of our terrestrial organisms, I also strongly suspect that what is out there will indeed turn out to be, in Darwin's immortal words, "endless forms most beautiful and most wonderful."

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Revising Drake

Most of you probably know about the Drake Equation, a way to estimate the number of intelligent civilizations in the universe.  The Equation is one of those curiosities that is looked upon as valid science by some and as pointless speculation by others.  Here's what it looks like:

Math-phobes, fear not; it's not as hard as it looks.  The idea, which was dreamed up by cosmologist Frank Drake back in 1961, is that you can estimate the number of civilizations in the universe with whom communication might be possible (Nb) by multiplying the probabilities of seven other independent variables, to wit:

R* = the average rate of star formation in our galaxy
fp = the fraction of those stars that have planets
ne = the fraction of those stars with planets whose planets are in the habitable zone
fl = the fraction of planets in the habitable zone that develop life
fi = the fraction of those planets which eventually develop intelligent life
fc = the fraction of those planets with intelligent life whose inhabitants develop the capability of communicating over interstellar distances
L = the average lifetime of those civilizations

Some of those (such as R*) are considered to be understood well enough that we can make a fairly sure estimate of their magnitudes.  Others -- such as fp and ne -- were complete guesses in Drake's time.  How many stars have planets?  Seemed like it could have been nearly all of them, or it perhaps the Solar System was some incredibly fortunate fluke, and we're one of the only planetary systems in existence.

The encouraging thing, at least for people like me who would love nothing better than to find we lived in a Star Trek universe where there's intelligent life wherever you look, just about all of these parameters have been revised upward since Drake first put his equation together.  Exoplanets, including ones in the so-called "Goldilocks zone," have turned out to be pretty much everywhere; not having planets turns out to be a much rarer situation.  There are over a hundred billion stars in the Milky Way alone; the number of planets in our galaxy is almost certainly in the trillions.  

As far as developing life... well, that one is still open to question, given that thus far we have a sample size of one to draw inferences from.  But that parameter -- fl -- just got a significant boost from a study done collaboratively by Hokkaido University and NASA of samples brought back from the asteroid Bennu by NASA's OSIRIS-REx mission, which found significant traces of all five nitrogenous bases that make up the genetic material in every living thing known (adenine, cytosine, guanine, thymine, and uracil).

Not only that, but they found the organic compounds xanthine and hypoxanthine (precursors of many bioactive compounds, including caffeine and theobromine), and nicotinic acid (vitamin B3).

This is an absolutely astonishing result.

"In previous research, uracil and nicotinic acid were detected in the samples from asteroid Ryugu, but the other four nucleobases were absent," said Toshiki Koga, who co-authored the paper, which appeared last week in Nature Astronomy.  "The difference in abundance and complexity of N-heterocycles between Bennu and Ryugu could reflect the differences in the environment to which these asteroids have been exposed in space."

What it brings to mind for me, though, is that if these five critical compounds can form on an airless, icy rubble pile (which is what Bennu honestly is), they've got to be pretty much everywhere in the universe that isn't so hot they fall apart.  And in case I haven't made the case strenuously enough, they are the basis of the genetic information shared by all life on Earth.

I think N -- the all-important Drake Equation estimate of the number of technological civilizations in the universe -- just got revised upward again.

Of course, even with my excited leaping about, I have to admit there's still a great deal we don't know, especially about the parameters that are lower on the list.  How many planets that do develop life end up with intelligent, technological life?  A while back I did a post about the rather terrifying idea of the Great Filter, which looks at the roadblocks that might prevent technological civilizations from forming or persisting.  Because the fact remains that when we look out there, we don't see signals from other civilizations -- something called the "Fermi Paradox" after the great physicist Enrico Fermi, who after listening to all the arguments for extraterrestrial life, famously quipped, "Then where is everybody?"

And we still have no idea about the scary parameter L -- how long, on average, technological civilizations last.  Given recent horrific developments in U.S. politics, I rather think I'm revising my own estimate of this one in the downward direction.  Maybe a benevolent alien will come and fix the mess we're in.  I know who I'm hoping for:

But even so, the Bennu study is exciting, and gives me hope that we might still one day find extraterrestrial life.  Perhaps even from the recently-launched Europa Clipper mission, which in April 2030 will do flybys of Jupiter's moon Europa -- widely considered to be our best shot of a place hosting extraterrestrial life in our own Solar System -- in the hopes of picking up biosignatures.

So we continue to wait, and wonder, and learn.  And -- as astronomer Neil deGrasse Tyson always says, at the end of his talks -- "Keep looking up!"

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The planet detectors

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Long-time readers of Skeptophilia know I'm kind of obsessed with the idea of extraterrestrial life.  I guess it's natural enough; I'm a biologist who's also an amateur astronomer, and grew up on Lost in Space and Star Trek and The Invaders, and later The X Files and Star Wars.  (Although I'm aware this is kind of a chicken-and-egg situation, so what the ultimate origin of my obsession is, I'm not certain.)

Until fairly recently, there was no particularly good way to determine the likelihood of life on other worlds.  Decades ago astronomer Frank Drake came up with the famous Drake Equation, which uses the statistical principle that if you know the probabilities of various independent occurrences, to find the probability of all of them happening, you multiply them together.  Here's the Drake Equation:

[Image is in the Public Domain courtesy of NASA/JPL]

The problem, of course, is that the more uncertainty there is in the individual probabilities, the more uncertainty there is in the product.  And there was no way known to get even a close value -- or, worse, to know if the value you had reflected reality or was just a wild guess.

What's cool for us alien enthusiasts, though, is that our research and techniques have improved to the point where we do have decent estimates for the values of some of these.  Even better, every time one of them is revised, it's revised upward.  Today I'd like to look at two of them -- f(p) and n(e) -- respectively, the fraction of stars that have planetary systems, and the fraction of those systems that have at least one planet in the habitable zone.

Given that we started out with a sample size of one (1) solar system, no one knew whether the coalescence of stellar debris into planets was likely, or simply a lucky fluke.  Same for planets in the habitable zone; here we have only a single planet that is habitable for organisms like ourselves.  Again, is that some kind of happy accident, or would most planetary systems have at least one potentially habitable planet?

Once we started to find exoplanets, though, they seemed to be everywhere we looked.  The earliest ones were massive (probably Jupiter-like) planets, often in fast, close orbit, so they'd be pretty hostile places from our perspective.  (Although, as I dealt with in a recent post, what we're finding out about the resilience of life may mean we'll have to revise our definition of what constitutes the "habitable zone.")

So the estimates for f(p) and n(e) crept upward, but still, it was hard to get reliable numbers.  But just last week, two studies have suggested that f(p) -- the percentage of stars with multiple-planet systems -- may be very close to 100%.

In the first, we hear about a recently-developed technique to improve our ability to detect exoplanets even at great distances.  Before this, most exoplanets were discovered using one of two methods -- looking for stellar wobble as a planet and its star circle their mutual center of gravity (which only works for nearby stars with massive planets capable of generating a detectable wobble), and luminosity dips as a planet occludes (passes in front of) its host star (which only works if the orbital plane is lined up in such a way that the planet passes in front of the star as seen from Earth).  As you might imagine, those restrictions mean that we might well be missing most of the exoplanets out there.

Now, a new orbiting telescope developed by NASA -- called WFIRST (Wide Field Infrared Survey Telescope) -- has the capability to detect microlensing.  Microlensing occurs because of the warping of the fabric of space-time by massive objects.  As a planet rotates around its host star, that warp moves, creating a ripple -- and the light from any stars behind the planet gets deflected.  An analogy is when you're looking down to the bottom of a clear pond and a ripple on the surface passes you; the image of the pebbles on the bottom appears to waver.  That wavering of light from distant stars is what WFIRST is designed to detect.

The nice thing is that WFIRST isn't dependent on visible wobbles or planets with precisely-aligned orbital planes; it can see pretty much any planet out there with sufficient mass.  And it can detect them from much farther away than previous telescopes -- the Kepler Space Telescope could detect planets up to around a thousand light years away, while WFIRST extends that reach by a factor of ten.  It's also capable of scanning a great many more stars; the estimate is that the first sweep will look at two hundred million stars, which is a thousand times the number Kepler studied.

So chances are, we're going to see an exponential jump in the number of exoplanets we know of, and a corresponding uptick in the estimate for f(p).

The second study is much closer to home -- about as close as you can get without being in our own Solar System.  Proxima Centauri is the nearest star to us other than the Sun, at 4.244 light years away.  In 2016 we were all blown away by the announcement that not only did Proxima Centauri have a planet, it was (1) Earth-sized, and (2) in the habitable zone.  (Anyone want to board the Jupiter 2?)

Now, astronomers have discovered a second planet around Proxima, at a distance about 1.5 times the orbit of the Earth, and a mass of about twelve times Earth's.  This means it's probably something like Neptune, and very cold -- Proxima is a dim star, so the habitable zone is a lot closer to it than the Sun's is -- the estimate is that its average temperature is -200 C.

Even though it probably doesn't host life, it's exciting from the standpoint that Proxima's planetary system is looking more and more like ours.  As astronomer Phil Plait put it, over at his fantastic blog Bad Astronomy, "I hope this new planet candidate turns out to be real.  Having one planet orbiting the star is already pretty amazing, but having two?  In my mind that makes it a solar system.  And if two, why not more?  How about moons orbiting the planets, or asteroids and comets around the star, too?"

The impression I'm getting is that f(p) (the fraction of stars with planetary systems) and n(e) (the fraction of stars with at least one planet in the habitable zone) are both extremely high.  This bodes well for our search for life -- and as the techniques improve, my sense is that we'll find planets like ours pretty much everywhere we look.  So life is looking more and more likely to be plentiful out there.  Now, intelligent life that is sufficiently technological to communicate across interstellar space... that's another matter entirely.

But in my opinion, any time we can revise some part of the Drake Equation upward, it's a good thing.

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This week's Skeptophilia book recommendation of the week is brand new -- only published three weeks ago.  Neil Shubin, who became famous for his wonderful book on human evolution Your Inner Fish, has a fantastic new book out -- Some Assembly Required: Decoding Four Billion Years of Life, from Ancient Fossils to DNA.

Shubin's lucid prose makes for fascinating reading, as he takes you down the four-billion-year path from the first simple cells to the biodiversity of the modern Earth, wrapping in not only what we've discovered from the fossil record but the most recent innovations in DNA analysis that demonstrate our common ancestry with every other life form on the planet.  It's a wonderful survey of our current state of knowledge of evolutionary science, and will engage both scientist and layperson alike.  Get Shubin's latest -- and fasten your seatbelts for a wild ride through time.

The Great Filter and the three f's

In yesterday's post, we looked at how the Drake Equation predicts the number of intelligent civilizations out there in the galaxy, and that more than one of the variables has been revised upward in the last few years because of recent research in astronomy.  This suggests that life is probably super-common in the universe -- and intelligent life undoubtedly is out there, as well.

But we ended with a puzzle.  Physicist Enrico Fermi famously responded to Frank Drake with four words: "Then where is everybody?"  This was true back when it was said (1961) and is even more true now; in the intervening 57 years, we've done huge amounts of surveying of the sky, looking for any sign of an extraterrestrial intelligence, and found... nothing.

Now, to be fair, "huge amounts of surveying" still covers a minuscule fraction of the stars out there.  All that would have to happen is the radio signal saying, "Hi, y'all, here we are!" hitting Earth while our radio telescopes were aimed at a different star, or tuned to a different frequency, and we could well miss it.

Messier 51, the Whirlpool Galaxy [Image courtesy of NASA/JPL]

But there's a more sinister possibility, and that possibility goes by the nickname of "The Great Filter."

I looked at this concept in a post a while back, especially apropos of the variable "L" in the Drake Equation -- once a planet hosts intelligent life, how long does it last?  If we were to time-travel two thousand years into the future, would there still be a human civilization, or are we doomed to destroy ourselves, either by our own fondness for weaponry capable of killing large numbers of people at once, or because our rampant population growth exceeded the planet's carrying capacity, and we experienced what the ecologists somewhat euphemistically call "overshoot-and-rebound?"

But today I want to look at the Great Filter in a larger perspective.  Given that most astronomers think that the Drake Equation leads to the conclusion that life, and even intelligent life, is common out there, Fermi's quip is well taken.  And the answers to that question can be sorted into three basic categories, which have been nicknamed the "three f's":

1. We're first.
2. We're fortunate.
3. We're fucked.

Could we be the first planet in our region of the galaxy to harbor intelligent life?  It's certainly possible, especially given the time gap between our developing life (four-odd-billion years ago) and our developing the technology not only to send, but to detect, signals from other planets (about fifty years ago).  Consider, for example, that if there was a civilization on Alpha Centauri at the technological stage we had two hundred years ago, they would have a thriving society made up of individuals that are highly intelligent, but to us here on Earth, they would be completely silent (and also wouldn't know it if we were talking to them).

However, considering the number of stars with planets, even in our region of the Milky Way, I think that's unlikely.  Even if we were all on a similar time table -- a contention that is not supported by what we know of stellar evolution -- it's nearly certain that there'd be someone out there at, or ahead of, our level of technology.  Add to that the fact that there are a lot of planet-hosting stars out there that are much older than the Sun, and I think option #1 is really not that likely.

Might we just be fortunate?  There are a number of hurdles we had to overcome to get where we are, none of which were at all sure bets.  The development of complex multicellular life, the evolution of symbiosis between our cells and what would eventually become our mitochondria (allowing us not only to avoid the toxic reactiveness of atmospheric oxygen, but to hitch that to our energy production systems, an innovation that improved our energy efficiency by a factor of 18).  None of those are at all guaranteed, and although it's conceivable to have intelligent life that lacks those characteristics, it's kind of hard to imagine how it would advance this much.

Then there's the evolution of sexual reproduction, which is critical not only because it's fun, but because it allows recombination of our genetic material each generation.  This allows us to avoid the dual problems of genetically-identical individuals being susceptible to the same pathogens, and also Muller's Ratchet (a problem faced by asexual species that is best understood as a genetic game of Telephone -- at each replication, mutations build up and eventually turn the DNA into nonsense).

But no one knows how likely the evolution of sexual reproduction is -- nor, honestly, if it's really as critical as I've suggested.

The last possibility, though -- "we're fucked" -- is the most alarming.  This postulates that the Great Filter lies ahead of us.  The reasons are varied, and all rather depressing.  It could be the "L" in the Drake Equation is a small number -- on the order of decades -- because we'll destroy ourselves somehow.  It could be that there are inevitable cosmic catastrophes that eventually wipe out the life on a planet, things like Wolf-Rayet stars and gamma-ray bursters, either of which would be seriously bad news if one went boom near the Solar System.

Then there's Elon Musk's worry, that intelligent civilizations eventually develop artificial intelligence, which backfires spectacularly.  In 2017 he urged a halt, or at least a slowdown, in AI research, because there's no reason to think sentient AI would consider us all that valuable.  "With artificial intelligence," Musk said, "we are summoning the demon.  You know all those stories where there’s the guy with the pentagram and the holy water and he’s like, yeah, he’s sure he can control the demon?  Doesn’t work out."

But by far the most sinister idea is that we're doomed because eventually, a civilization reaches the point where they're able to send out radio signals.  We've been doing this ever since radio and television were invented, so there's an expanding bubble of our transmissions zooming out into the galaxy at the speed of light.  And the idea here is that we'll eventually attract the attention of a considerably more powerful civilization, which will respond by stomping on us.  Stephen Hawking actually thought this was fairly likely -- back in 2015, he said, "We don't know much about aliens, but we know about humans.  If you look at history, contact between humans and less intelligent organisms have often been disastrous from their point of view, and encounters between civilizations with advanced versus primitive technologies have gone badly for the less advanced.  A civilization reading one of our messages could be billions of years ahead of us.  If so, they will be vastly more powerful, and may not see us as any more valuable than we see bacteria."

Which, considering that the first traces the aliens will see of us are Leave it to Beaver and The Andy Griffith Show, is an understandable reaction.

So there you have it.  If we did contact another civilization, it would be good news in one sense -- the Great Filter hasn't wiped everyone out but us -- but could be a seriously bad one in another respect.  I guess stuff like this is always a mixed bag.

Me, I still would love to live long enough to see it happen.  If an alien spaceship landed in my back yard, man, I would be thrilled.  It'd suck if it turned out to be an invasion by Daleks or Cybermen or whatnot, but man, at least for the first three minutes, it would be a hell of a rush.

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This week's Skeptophilia book recommendation is from one of my favorite thinkers -- Irish science historian James Burke.  Burke has made several documentaries, including Connections, The Day the Universe Changed, and After the Warming -- the last-mentioned an absolutely prescient investigation into climate change that came out in 1991 and predicted damn near everything that would happen, climate-wise, in the twenty-seven years since then.

I'm going to go back to Burke's first really popular book, the one that was the genesis of the TV series of the same name -- Connections.  In this book, he looks at how one invention, one happenstance occurrence, one accidental discovery, leads to another, and finally results in something earthshattering.  (One of my favorites is how the technology of hand-weaving led to the invention of the computer.)  It's simply great fun to watch how Burke's mind works -- each of his little filigrees is only a few pages long, but you'll learn some fascinating ins and outs of history as he takes you on these journeys.  It's an absolutely delightful read.

[If you purchase the book from Amazon using the image/link below, part of the proceeds goes to supporting Skeptophilia!]

Parsing the Drake Equation

The Drake Equation is one of those curiosities that is looked upon as valid science by some and as pointless speculation by others.  Here's what it looks like:

Math-phobes, fear not; it's not as hard as it looks.  The idea, which was dreamed up by cosmologist Frank Drake back in 1961, is that you can estimate the number of civilizations in the universe with whom communication might be possible (Nb) by multiplying the probabilities of seven other independent variables, to wit:

R* = the average rate of star formation in our galaxy
fp = the fraction of those stars that have planets
ne = the fraction of those stars with planets whose planets are in the habitable zone
fl = the fraction of planets in the habitable zone that develop life
fi = the fraction of those planets which eventually develop intelligent life
fc = the fraction of those planets with intelligent life whose inhabitants develop the capability of communicating over interstellar distances
L = the average lifetime of those civilizations

Some of those (such as R*) are considered to be understood well enough that we can make a fairly sure estimate of its magnitude.  Others -- such as fp and ne -- were complete guesses in Drake's time.  How many stars had planets?  Could be nearly 100%, or it could be the Solar System was some incredibly fortunate fluke, and we're one of the only planetary systems in existence.  But now, with improvements in the techniques for surveying stars, we're finding planets everywhere we look -- most stars seem to have planets, and some research published just last month by a team of astronomers at the University of Witwatersrand (South Africa) has shown that planets could form stable orbits in multiple-star systems, something previously thought extremely unlikely.

That they can do so is fortunate not only for alien intelligence enthusiasts like myself -- as much as half of all stars are thought to be part of multiple-star systems -- but for this guy:

So the estimates keep being revised upward.  The one we still have no real idea about is L -- how long civilizations tend to last.  Carl Sagan, when he described the Drake Equation in his amazing series Cosmos, was pessimistic -- many civilizations, he suggested, lasted long enough to develop weapons of mass destruction, then proceed to blow themselves to smithereens.

But the fact is, we just don't know about L.  But one that was complete speculation -- fl, the fraction of planets in the habitable zone that develop life -- just got a bit of a boost from a study done at the University of Bristol (England).  The researchers, Holly C. Betts, Mark N. Puttick, James W. Clark, Tom A. Williams, Philip C. J. Donoghue, and Davide Pisani, published their results in Nature: Ecology and Evolution last week in a paper titled "Integrated Genomic and Fossil Evidence Illuminates Life's Early Evolution and Eukaryote Origin."  And one of the points the team makes is that once the Earth's surface had cooled sufficiently that water was able to exist in liquid form, life appeared in a relative flash -- while it was still being clobbered every other day by meteorites.

The authors write:

Establishing a unified timescale for the early evolution of Earth and life is challenging and mired in controversy because of the paucity of fossil evidence, the difficulty of interpreting it and dispute over the deepest branching relationships in the tree of life.  Surprisingly, it remains perhaps the only episode in the history of life where literal interpretations of the fossil record hold sway, revised with every new discovery and reinterpretation.  We derive a timescale of life, combining a reappraisal of the fossil material with new molecular clock analyses.  We find the last universal common ancestor of cellular life to have predated the end of late heavy bombardment (>3.9 billion years ago (Ga)).

Besides being of obvious interest to evolutionary geneticists, this should get astronomers' blood pumping; it implies that life originated on Earth when the conditions were still nothing short of hostile, with the corollary that once a planet has conditions that allow liquid water, life probably follows soon thereafter.

The implication being that it's likely that every planet with water that sits in its star's habitable zone has some form of life.

So understandably enough, I think this is way cool.  It doesn't give us any information about the remaining variables we have little information about, especially fi, fc, and L.  There's no particular reason to believe that intelligence is a necessary outcome of evolution; it's tempting to think that the process always drives organisms to be bigger, better, stronger, and smarter, but that's not supported by the evidence.  After all, it bears remembering that by far the dominant life-forms on Earth right now, both in terms of biodiversity and overall numbers, are... insects.

It might be that intelligence sufficient to communicate over interstellar distances is a very uncommon occurrence, which leads to the most likely scenario (in my opinion) being plentiful planets with huge diversity of life, but few that have anything like us.

Still, the galaxy is a big place, with billions of stars, so even if it's unlikely, intelligent life probably exists somewhere.  Which segues into tomorrow's post, which is about the Fermi Paradox.  When told about the Drake Equation, physicist Enrico Fermi famously shrugged his shoulders and said, "Then where is everybody?"

Tomorrow we'll look at a few possible answers -- some of which are considerably more cheerful than others.

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This week's Skeptophilia book recommendation is from one of my favorite thinkers -- Irish science historian James Burke.  Burke has made several documentaries, including Connections, The Day the Universe Changed, and After the Warming -- the last-mentioned an absolutely prescient investigation into climate change that came out in 1991 and predicted damn near everything that would happen, climate-wise, in the twenty-seven years since then.

I'm going to go back to Burke's first really popular book, the one that was the genesis of the TV series of the same name -- Connections.  In this book, he looks at how one invention, one happenstance occurrence, one accidental discovery, leads to another, and finally results in something earthshattering.  (One of my favorites is how the technology of hand-weaving led to the invention of the computer.)  It's simply great fun to watch how Burke's mind works -- each of his little filigrees is only a few pages long, but you'll learn some fascinating ins and outs of history as he takes you on these journeys.  It's an absolutely delightful read.

[If you purchase the book from Amazon using the image/link below, part of the proceeds goes to supporting Skeptophilia!]