Looking for a signature

Finding unequivocal evidence of extraterrestrial life is not as easy as science fiction makes it sound.

The problem is, we're biased toward detecting terrestrial life -- living things that have a similar chemistry to the familiar life forms here on Earth.  It's understandable; I mean, why would we not be?  While there is a great diversity of species here on our planet (something on the order of nine million extant, at an estimate), they all share the same basic biochemistry, including:

• ATP as an energy driver
• some form of sugar-fueled cellular respiration to produce that ATP
• phospholipid bilayers as cell membranes, and (for eukaryotes) for the internal membranes that compartmentalize the cell
• proteins to facilitate structure, movement, and catalysis (the latter are called enzymes)
• nucleic acids such as DNA and RNA for information storage and retrieval
• lipids for long-term energy storage

While there are obviously different twists on how exactly these things work, those features are common to just about all life on Earth.  (Interestingly, a 2010 paper claiming a microbe had been discovered in California's Mono Lake that incorporates arsenic into the backbone of its DNA instead of phosphorus was just retracted by the journal Nature -- although the retraction is controversial, and the authors are still defending their work as valid.)

The question remains unsolved, therefore, of the extent to which the genesis and evolution of life are constrained -- by which we mean that the pathways taken by biology might be expected to repeat on other Earth-like worlds.  (Or, to use Stephen Jay Gould's pithy phrase, we might find that evolution would produce similar forms again here on Earth if we were somehow able to "replay the tape of life.")  Would living things, down to the biochemical level, be at least somewhat like terrestrial life, or would they be entirely different?

To be fair to the science fiction writers, there have been instances where they've made a significant effort to consider what life "not as we know it" might look like.  Star Trek's Horta ("The Devil in the Dark"), Crystalline Entity ("Silicon Avatar"), and Tholians ("The Tholian Web") come to mind, as well as Doctor Who's Vashta Nerada ("Silence in the Library"), Boneless ("Flatline"), Not-Things ("Wild Blue Yonder"), and Midnight Entity ("Midnight").  

Even Lost in Space gave it a creditable try with the Bubble Creatures in "The Derelict."

The problem is more complex than that, though.  Since we can't actually go to other planets and search for living things -- even going to planets and moons in our own Solar System is crazy expensive and fraught with difficulties -- we're stuck with looking for biosignatures, traces (either structural or chemical) that show unequivocal evidence of being created by living things.

The sticking point is that word unequivocal.  Two good examples of this came to light in the last couple of weeks, one of them (from the standpoint of people like me who would love nothing better than to find out we're not alone in the universe) bad news, and the other one -- at least tentatively -- good news.

Let's start with the bad news first.

Back in 2005, NASA's Cassini mission spotted something exciting -- the presence of organic molecules in water-rich plumes erupted from Saturn's icy moon Enceladus.  The surmise was that these plumes were created by pressure in the moon's interior, where water might be kept liquid by tidal deformation from the huge gas giant's gravitational pull.  Now, organic doesn't mean produced by life; it's a chemistry term meaning "containing carbon and hydrogen."  (Formaldehyde, for example, is an organic compound, and has been found by its spectral signature in interstellar gas, and no one's claiming that it was made by aliens.  At least, no one I'd want to have a conversation with.)  On the other hand, lots of organic compounds are made by living things, so their presence in Enceladus's geysers certainly seemed to raise at least the possibility that underneath the ice, there might be a watery ocean that harbored life.

Well, a study led by scientists from Italy's Istituto Nazionale di Astrofisica e Planetologia Spaziale has found another possible explanation.  The organic molecules could be formed right there on the surface by radiation funneled in by Saturn's magnetic field, then blasted off the surface -- i.e., they didn't come from an interior ocean at all.  "While the identification of complex organic molecules in Enceladus's environment remains an important clue in assessing the moon's habitability, the results demonstrate that radiation-driven chemistry on the surface and in the plumes could also create these molecules," said Grace Richards, who led the study.  "Molecules considered prebiotic could plausibly form in situ through radiation processing, rather than necessarily originating from the subsurface ocean.  Although this doesn't rule out the possibility that Enceladus's ocean may be habitable, it does mean we need to be cautious in making that assumption just because of the composition of the plumes."

The second paper had more hopeful news.  It comes out of Stony Brook University, and is an analysis of some mudstones found in Jezero Crater on Mars by the Perseverance rover.  The analysis -- done long-distance, obviously -- found traces of organic material, along with ferrous sulfate and ferric sulfide.  Most interestingly, the organic material seems to have undergone post-deposition redox reactions; redox reactions are the mechanism by which all terrestrial life forms harvest energy for their life processes (cellular respiration is, basically, one long string of redox reactions).  The minerals, the researchers say, "challenge some aspects of a purely abiotic explanation" -- cautious science-speak for, "Life?  Yeah, could be."

Of course, this is not proof, and Sagan's principle that I mentioned in a post a few days ago -- "Extraordinary claims require extraordinary evidence" -- certainly applies here.  But it is fascinating, and if further inquiries support the biotic explanation for the odd chemistry of the Jezero mudstones, it'll be somewhere beyond exciting.  I've always thought it likely that life is common in the universe, but having a real, honest-to-Gallifrey extraterrestrial example would be amazing.

In any case, keep your eyes on the science news.  Despite the ridiculous budget cuts NASA is facing, they're still doing some wonderful science.  And hopefully, at some point, we'll actually find proof of aliens.  Wouldn't that be cool?

I hope it's not the Vashta Nerada, though.  Those mofos are scary.

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Life converges

One of the most fascinating features of biological evolution -- particularly as it applies to the possibility of life on other planets -- has to do with the concept of constraint.

Which features of life on Earth are, in some sense, inevitable?  Are there characteristics of terrestrial organisms that we might expect to find on any inhabitable world?  Stephen Jay Gould looked at this question in his essay "Replaying the Tape," from his brilliant book on the Cambrian Explosion, Wonderful Life:

You press the rewind button and, making sure you thoroughly erase everything that actually happened, go back to any time and place in the past -– say, to the seas of the Burgess Shale.  Then let the tape run again and see if the repetition looks at all like the original.  If each replay strongly resembles life’s actual pathway, then we must conclude that what really happened pretty much had to occur.  But suppose that the experimental versions all yield sensible results strikingly different from the actual history of life?  What could we then say about the predictability of self-conscious intelligence?  or of mammals?

Some features that have been suggested as evolutionarily constrained, with arguments of varying levels of persuasiveness, are:

• a genetic code based on some kind of nucleic acid (DNA or RNA, or some chemical analogue)
• internal cell membranes made of phospholipids, to segregate competing chemical reactions from each other 
• multicellularity, with some level of tissue specialization
• in more complex organisms, some form of symmetry, with symmetrically-placed organs
• some kind of rapid-transit system for messages, analogous to our nervous system (but perhaps not structured the same way)
• cephalization -- concentration of the central processing centers and sensory organs near the head end

It's interesting when science fiction tackles this issue -- and sometimes comes up with possible pathways for evolution that don't result in humanoids with strangely-shaped ears and odd facial protuberances.  A few that come to mind are Star Trek's silicon-based Horta from the episode"Devil in the Dark," the blood-drinking fog creature from "Obsession," the giant single-celled neural parasites from "Operation Annihilate," and Doctor Who's Vashta Nerada, Not-Things, Gelth, and Midnight Entity.

So the search for extraterrestrial life requires we consider looking not only for "life as we know it, Jim," but life as we don't know it.  Or, more accurately, to consider to what extent our terrestrial biases might be blinding us to the possibility of what evolution could create.

It's worth considering, however, how often evolution here on Earth ends up landing on the same solutions to the problems of survival and reproduction over and over again, a phenomenon called convergent evolution.  Eyes, or analogous light receptor organs, have evolved multiple times -- some biologists have suggested as many as fifty different independent lineages that evolved some form of eye.  Wings occurred separately in four groups of animals -- birds, pterosaurs, insects, and bats.  (If you include structures for gliding, add flying squirrels, sugar gliders, colugos, flying fish, and flying lizards.)

Even biochemical pathways can reappear, something I find astonishing.  Take, for example, the research that came out this week in Nature Chemical Biology, which found that two only distantly-related plants -- ipecac (Carapichea ipecacuanha), in the gentian family, and sage-leaved alangium (Alangium salviifolium), in the dogwood family, have both come up with complex biochemical pathways to generate the same set of bitter, emetic compounds -- ipecacuanha alkaloids.

The last common ancestor of these two species was over a hundred million years ago, so there's a strong argument that they evolved this capacity independently.  And indeed, when the biochemists looked at the enzymatic pathways, they're different -- they found entirely different chemical synthesis methods for producing the same set of end products.  Weirdest of all, they both evolved an enzyme that cleaves a sugar molecule from the alkaloid precursor, and that's what activates it (i.e., makes it toxic).  In the living plant's tissues, the enzyme and the precursor are segregated from each other.  It's only when they're brought together -- such as when a herbivore chomps on the leaves -- that the sugar is split away from the precursor, the alkaloid is activated, and the herbivore starts puking its guts up.

Clever strategy.  So clever, in fact, that it was stumbled upon by two entirely separate lineages of plants.  The rules organisms play by are the same, so perhaps not surprising there are similar outcomes sometimes.

The whole thing highlights the fact that there is a limited range of solutions for the fundamental difficulties of existence.  It has to make you wonder if, when we do find life elsewhere in the universe, it might look a lot more familiar that we're expecting.  I don't think it's likely we'll bump into Romulans or Ice Warriors or Krillitane, but maybe there are features of life on Earth that will re-evolve in just about any conceivable habitable planet.

But hopefully there won't be any Vashta Nerada.  Those things are terrifying.

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The signature

As much as I love the movie Contact, trying to find extraterrestrial life isn't just a matter of tuning in to the right radio frequency.

There's no guarantee that even intelligent life would use radio waves to communicate, and if they did, that they'd do it in such a way that we could decipher the message.  I must admit, though, that the whole "sequence of prime numbers" thing as a beacon was a pretty cool idea; it's hard to imagine a natural phenomenon that would result in blips in a pattern of prime numbers.

Even besides the issue with how exactly a technological species would choose to communicate, there's the problem that this method would miss the vast majority of life that's potentially out there.  Consider the fact that there's been life on Earth for 3.8 billion years, give or take a day or two, and until about a hundred years ago, we weren't producing any radio waves ourselves.  To a civilization two hundred light years away -- so, seeing us as we were two hundred years ago -- Earth would be, to borrow C. S. Lewis's pithy phrase, a completely silent planet, even though there was a thriving biosphere that included at least one intelligent, soon-to-be-technological species.

So except for those presumably few planets that host intelligent beings who communicate kind of like we do, detecting extraterrestrial life is a tricky question.  The most promising approach has been to look for biosignatures -- chemical traces that (as far as we know) can only be produced by living things.  One example on Earth is the fact that our atmosphere contains both oxygen and methane.  Both are highly reactive (especially with each other); to keep stable levels of these gases in the atmosphere requires that something is continuously producing them, because they're constantly being removed by oxidation/reduction reactions.  In this case, photosynthesis and bacterial methanogenesis, respectively, pump them into the atmosphere as fast as they're being destroyed, so the levels remain relatively stable over time.

Two other chemicals that, on the Earth at least, are entirely biological in origin are dimethyl sulfide and dimethyl disulfide.  You've undoubtedly encountered these before; they're partly responsible for the unpleasant smell when you cook cabbage.  They're produced by a variety of living things, including bacteria, plants, and fungi -- dimethyl sulfide is what truffle-hunting pigs are homing in on when they're after truffles. 

Well, data from the James Webb Space Telescope showed that an exoplanet called K2-18b has measurable quantities of both dimethyl sulfide and dimethyl disulfide -- to the point that even the astronomers, who ordinarily have zero patience with the "It's aliens!" crowd, are saying "this is the strongest hint yet of biological life on another planet."

So far, the spectroscopic data that found the chemicals is at a significance level of "3-sigma" -- meaning there's a 0.3% chance that the signal was a statistical fluke (or, put another way, a 99.7% chance that it's the real deal).  It's exciting, but we've seen 3-sigma data do a faceplant before, so I'm trying to restrain myself.  Generally 5-sigma -- a 0.00006% chance of it being a fluke -- is the standard for busting out the champagne.  But even so, this is pretty amazing.

K2-18b is 124 light years away, and is thought to be a "Hycean world" -- an ocean-covered world with a thick, hydrogen-rich atmosphere.  So whatever life is there is very likely to be marine.  But even if we're not talking about your typical Star Trek-style planet with lots of rocks and an orange sky and aliens that look like humans but with rubber facial appendages, the levels of DMS and DMDS suggest a thriving biosphere.

"Earlier theoretical work had predicted that high levels of sulfur-based gases like DMS and DMDS are possible on Hycean worlds," said Nikku Madhusudhan of Cambridge University, who co-authored the study, which appeared this week in Astrophysical Journal Letters.  "And now we've observed it, in line with what was predicted. Given everything we know about this planet, a Hycean world with an ocean that is teeming with life is the scenario that best fits the data we have."

The issue, of course, is not just the statistical significance; 99.7% seems pretty good to me, even if it doesn't satisfy the scientists.  The problem is that sneaky little phrase that was in my description of biosignatures earlier; "as far as we know."  We don't know of a way to produce DMS and DMDS in significant quantities except by biological processes, but that doesn't mean one doesn't exist.  It could be that in the weird chemical soup on an planet in another star system, there's an abiotic way to produce a stable amount of these two compounds, and we just haven't figured it out yet.

Be that as it may, it's still pretty damn exciting.  It's certainly the closest we've gotten to "there's life out there."  And being only 124 light years away -- in our stellar neighborhood, really -- it's right there for us to study more intensively.  Which the astronomers will definitely be doing.

So that's our cool news for today.  I don't know about you, but now I'm daydreaming about what kind of life there might be on a world entirely covered by water.  I'm sure that whatever they are, they'll be "forms most beautiful and most wonderful" beyond Charles Darwin's wildest dreams.

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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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What Listen heard

Regular readers of Skeptophilia -- and, heaven knows, my friends and family -- are well aware that one of my obsessions is the possibility of extraterrestrial life, and perhaps even extraterrestrial intelligence.

I grew up watching Lost in Space and The Invaders and the original Star Trek, and later The X Files and Star Trek: The Next Generation and Doctor Who.  But while those classic shows piqued my budding interest in exobiology, my training in actual biology taught me that whatever the aliens look like, they will almost certainly not be humans with odd facial protuberances and strange accents.  How evolution plays out on other planets is impossible to say, but it's likely to be vastly different from the pathways taken by life on Earth.  I still remember reading Stephen Jay Gould's essay "Replaying the Tape" from his excellent book on the Cambrian-age Burgess Shale fauna, Wonderful Life, and being blown away by the following passage:

You press the rewind button and, making sure you thoroughly erase everything that actually happened, go back to any time and place in the past -– say, to the seas of the Burgess Shale.  Then let the tape run again and see if the repetition looks at all like the original.  If each replay strongly resembles life’s actual pathway, then we must conclude that what really happened pretty much had to occur.  But suppose that the experimental versions all yield sensible results strikingly different from the actual history of life?  What could we then say about the predictability of self-conscious intelligence? or of mammals?

His point was that a great deal of evolution is contingent -- dependent on events and occurrences that would be unlikely to repeat in exactly the same way.  And while there's no way to re-run the tape on the Earth, this has profound implications regarding what we're likely to find elsewhere in the universe.

If we do find intelligent aliens, chances are they won't be Klingons or Romulans or Andorians.  To be fair, the aforementioned shows did make some attempts to represent what truly different life might be like; the Horta from the original Star Trek and the Vashta Nerada and the Midnight Entity from Doctor Who come to mind.  Most likely, though, whatever we find out there will be -- to pilfer a phrase from J. B. S. Haldane -- "queerer than we can imagine."

All of this is just a preface to my telling you about an article I read today, that should have had me excited, but ended up leaving me looking like this:

The link I'm referring to was sent to me by a loyal reader of Skeptophilia, and I've now seen the story in a number of different news sources.  This particular iteration has the title, "Huge Alien Announcement 'Could Happen Within Weeks' as Professor Says 'We've Found It'."  "It," in this case, is apparently definitive proof of extraterrestrial intelligence.  The guy claiming this is one Simon Holland; two different scientific teams, he says, are "in a race to publish the first confirmed evidence."

And not just evidence, but actual transmissions of some kind, bringing to mind the movie Contact and the breathtaking moment astronomer Ellie Arroway finds a radio signal from another planet.  Like the one in Contact, the signal Holland tells us about is some kind of narrow-band radio message, and was apparently discovered by Yuri Milner's Breakthrough Listen program.

"It’s a single point source, not just noise," he said.  "The signal, instead of being the giant buzz of everything in the universe that we hear through all radio telescopes, was a narrow electromagnetic spectrum."

Which sounds awesome, right?

But.

First, Simon Holland isn't a professor, he's a YouTuber and filmmaker.  He says he "taught at a major UK university" -- no name given -- and his nickname is "Prof."  And here are a few of his recent YouTube videos:

• "Cattle Mutilation -- a Horrible 'Big Picture'"
• "Nuclear Explosions Over the Atlantic"
• "The Science Film YOU ARE NOT ALLOWED TO SEE"
• "Antigravity Machine Finished"
• "Faster Than Light: CIA and the UFO"

Not exactly a testimony to scientific rigor, right there.  So how would this guy know about some find at Breakthrough Listen, especially one that is being kept hush-hush so the scientific teams themselves don't get scooped?

The other thing, though, is that we've been down this road before.  Last year, we had all the hoopla over military whistleblower David Grusch, alleging that the United States had hard evidence not only of alien technology but of "biological material not of earthly origin" -- there were even extensive hearings in Congress over the matter.  And the whole thing came to nothing.  The upshot was, "Okay, yeah, if there are actual UFOs from another world zipping around on Earth, it would be a matter of national security," but when asked to present the actual evidence itself, all we got was a shoulder shrug.  

So forgive me for being dubious about Simon Holland's claims.  I'll say what I've said before; if there is proof of alien intelligence, stop acting coy and show us the goods.  Until then, I'm perhaps to be forgiven for being dubious.

I'll end, however, by saying that this is one case where I devoutly hope I'm wrong.  If in "a few weeks" we have publication of a paper in a peer-reviewed science journal about a radio transmission from an intelligent civilization on another planet, I will be beyond thrilled to eat my words.

But I'm not holding my breath.

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Tales of a Death Star

One of the most promising areas of study for astrobiologists -- scientists who are interested in the possibility of life elsewhere in the universe -- is the potential for life on the moons of Jupiter, Saturn, Uranus, and Neptune.  We're beginning to develop the technology to detect biosignatures -- chemical traces of living things in the atmospheres of moons or exoplanets -- but it's a hell of a lot easier to find those in our own Solar System than it is around the barely-visible specks of light that are all we can see of most exoplanetary systems.

Despite their distance from the Sun, due to tidal heating there are several of these moons that are thought to have liquid water beneath a frozen crust.  Four commonly-discussed possibilities are Europa (Jupiter), Enceladus and Titan (Saturn) and Triton (Neptune); the case is nearly certain for Europa and Enceladus, where fly-bys have detected liquid water geysers erupting from surface cracks in the ice sheet.

What could be down there, I wonder?  Single-celled life is the most likely, but with no further information... well, anything's possible.  We only have a sample size of one regarding how life forms and evolves, so trying to predict what it would look like somewhere else is going to be speculation at best.

The conventional wisdom has been that the smaller moons are unlikely places to look for life; being smaller, they lose heat faster, so any heat gains they get from the Sun and from tidal compression are far offset by heat loss from their small thermal mass. 

That assessment will have to be revised, apparently.  A new study -- out this week in Nature -- found that Saturn's moon Mimas, best known for having a huge crater that makes it look like the Death Star from Star Wars, has an ocean of liquid water underneath a crust of ice and frozen methane.  It's only four hundred kilometers in diameter, over eight times smaller than our own Moon.

A photograph of Mimas from the 2010 pass by the probe Cassini [Image is in the Public Domain courtesy of NASA/JPL]

The frozen crust of Mimas is thought to be so thick (something on the order of twenty to thirty kilometers) that it precludes the cracks that cause the geysers on Enceladus and Europa.  So the liquid water inside is trapped -- but the effects of tidal heating from the enormous planet it orbits are apparently enough to keep it well above freezing, and therefore very likely to enable the convection currents which overturn nutrients in our own oceans and are essential for the maintenance of ecosystems.  

Based on what we know about the formation of moons and their stability in orbit around their host planet, Mimas is estimated to be quite young, something on the order of between five and fifteen million years old.  This seems like a very short time even to evolve simple single-celled organisms, but as I said before -- it's not like we have a bunch of test cases from which to draw inferences.

"Mimas was probably the most unlikely place to look for a global ocean — and liquid water more generally," said study co-author Valéry Lainey, of the Paris Observatory.  "So that looks like a potential habitable world.  But nobody knows how much time is needed for life to arise."

I'm always fascinated when we find this sort of thing, because it seems like every time we get new information affecting the terms of the Drake Equation, the estimates are revised upward.  At first, we didn't know if planet formation was at all likely, or if the Solar System was a fluke; now it seems like exoplanets are kind of everywhere we look, and most stars have planetary systems.  Most stars that have been studied have at least one planet in the habitable zone, and the size of the habitable zone is way bigger than we used to think.  Forming the biochemistry of life turns out to be simple; like exoplanets, complex organic molecules turn out to be all over the place.  And so on.

So could Mimas host life?  Entirely possible.  "Not life as we know it, Jim" -- but life nonetheless.  I still think that Europa and Enceladus are more likely (remember the end of the movie 2010?  "All of these worlds are yours except Europa, attempt no landing there") but life could well be common, not just out in the galaxy but right here in our own Solar System.

And maybe I'll live to see confirmation of it.  What a monumental overturning of our self-importance that would be.  It'd be a total game changer.  Proving once and for all that life is abundant in the cosmos... and that we are not alone.

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The fingerprint of life

Springboarding off yesterday's post, which suggested that -- from a biochemical standpoint, at least -- extraterrestrial life might be way more common than we'd thought, today we look at how we might find out where it lives.

This is a thornier problem than it might seem at first.  Despite hopeful movies like Contact, picking up an alien radio signal makes looking for a needle in a haystack seem like child's play.  Consider the difficulties; you have to have your radio telescope pointed at exactly the right place in the sky, at exactly the right time, and tuned to exactly the right frequency, to pick it up as it sweeps by the Earth at the speed of light.  Even if you posit an extremely simple message, which repeats indefinitely (like Ellie Arroway's string of prime number blips), there's the problem that any kind of electromagnetic signaling follows the inverse-square law, meaning if you double the distance between the sender and the receiver, the intensity of the received signal goes down by a factor of four.  Triple it, and it goes down by a factor of nine, and so forth.

And the fact is, the distances we're talking about here are...

... astronomical.  (*rimshot*)

So the possibility of detecting some sort of radio signal (whether or not deliberately sent to attract our attention) is not zero, but pretty damn small.  And the other downside is that if that's all we're looking for, we're going to miss a huge slice of the living creatures that could be out there -- we'd only see the ones that have a technological civilization that uses radio waves to communicate.  From that approach, Earth itself would have appeared to be barren and lifeless until the use of radio became widespread, back in the 1930s.

Is there another way?

An alternate approach -- one that avoids at least some of these pitfalls -- is to look for biosignatures, chemical traces that might indicate the presence of life on a planet even if it hasn't reached the point of being technological.  The studies done on Mars that attempted to find Martian microbes took this approach; take a sample of soil, add some likely nutrients, and look for a sign of metabolism.  But this, too, has its inherent difficulties.  How do you tell the difference between Martian microbes chowing down on the food you gave them, and some exotic but abiotic chemical reaction?

A team of astronomers and biologists from the University of Birmingham and MIT have come up with a possible answer.  According to a paper in Nature Astronomy last week, there is a pair of dead giveaways; an atmosphere depleted in carbon dioxide but enriched in ozone.

Carbon dioxide is a highly stable compound, and on lifeless, dry planets like Venus and Mars, it makes up a significant percentage of the atmosphere.  (96.5% on Venus, 95.3% on Mars.)  The fact that despite the amount of carbon on the Earth, the quantity in the air is only 0.04%, is due mostly to the fact that the water in the oceans acts as a huge carbon sink, first dissolving the carbon dioxide, then reacting it with dissolved metal ions like calcium and magnesium to form minerals like the calcite and magnesite in limestone.  Without the oceans, all of that carbon would stay in the atmosphere -- and we'd be a lot more like the inferno that is Venus than the temperate world where we reside.

As far as ozone, the real tipoff for the presence of life would be gaseous oxygen, which is a highly reactive substance that, in the absence of something producing it pretty much continuously, would all be bound up chemically.  Ozone -- a chemical relative of oxygen, O3 instead of O2 -- is expected to be present in small amounts in any atmosphere with free oxygen, but is the astronomers' choice because its spectral signature is much easier to detect than oxygen's.

Likewise, carbon dioxide's spectral fingerprint is obvious because of its strong absorption in the infrared (a property that is directly related to the greenhouse effect and carbon dioxide's warming effect on atmospheres).

So it should be possible to analyze the light reflected from the surface of exoplanets that seem to be in the right temperature range, and look for two things -- low carbon dioxide (indicating liquid water on the surface) and high ozone (indicating something, possibly life, keeping molecular oxygen in the atmosphere).  See both of those things, the team said, and you're very likely looking at a planet that is inhabited.

Like I said yesterday, of course, "inhabited" doesn't mean "inhabited by bipedal humanoids with spaceships and laser guns."  But even so, the technique is intriguing in its simplicity.  The team suggested starting with relatively nearby planetary systems like TRAPPIST-1, which has seven known exoplanets and is only a little over forty light years away from Earth.

TRAPPIST-1 and its lineup of seven planets [Image is in the Public Domain courtesy of NASA/JPL]

So this is all tremendously exciting -- that astronomers are now taking the possibility of extraterrestrial life seriously enough to start proposing methods for searching for it other than just scanning the skies and hoping for the best.  After all, to go back to the movie Contact -- "if we're all alone in the universe, it seems like an awful waste of space."

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The biochemical zoo

The human/alien hybrid is a common trope in science fiction.  From the angst-ridden half-Vulcan Mr. Spock, to the ultra-competent and powerful half-Klingon B'Elanna Torres, to the half-Betazoid empath Deanna Troi, the idea of having two intelligent humanoid species produce children together is responsible for dozens of plot twists in Star Trek alone.

Much as I love the idea (and the show), the likelihood of a human being able to engage in any hot bow-chicka-bow-wow with an alien, and have that union produce an offspring, is damn near zero.  Even if the two in question had all the various protrusions and indentations more or less lined up, the main issue is the compatibility of the genetic material.  I mean, consider it; it's usually impossible for two ordinary terrestrial species to hybridize -- even related ones (say, a Red-tailed Hawk and a Peregrine Falcon) are far enough apart genetically that any chance mating would produce an unviable embryo.

Now consider how likely it is to have genetic compatibility between a terrestrial species and one from the fourth planet orbiting Alpha Centauri.

Any hope you might have had for a steamy tryst with an alien was smashed even further by a study that came out of a study from the Tokyo Institute of Technology, Emory University, and the German Aerospace Center.  Entitled, "One Among Millions: The Chemical Space of Nucleic Acid-Like Molecules," by Henderson James Cleaves II, Christopher Butch, Pieter Buys Burger, Jay Goodwin, and Markus Meringer, the study shows that the DNA and RNA that underlies the genetics of all life on Earth is only one of millions of possible information-encoding molecules that could be out there in the universe.

It was amazing how diverse these molecules were, even given some pretty rigid parameters.  Restricting the selection to linear polymers (so the building blocks have to have attachment points that allow for the formation of chains), and three constituent atoms -- carbon, hydrogen, and oxygen, like our own carbohydrates -- the researchers found 706,568 possible combinations (counting configurations and their mirror images, pairs of molecules that are called stereoisomers).  Adding nitrogen (so, hooking in chemicals like proteins and the DNA and RNA nitrogenous bases, the letters of the DNA and RNA alphabets) complicated matters some -- but they still got 454,442 possible configurations.

The results were a surprise even to the researchers.  "There are two kinds of nucleic acids in biology, and maybe twenty or thirty effective nucleic acid-binding nucleic acid analogs," said Henderson James Cleaves, who led the study, in an interview in SciTechDaily.  "We wanted to know if there is one more to be found...  The answer is, there seem to be many, many more than was expected."

Co-author Pieter Burger of Emory University is excited about the possible medical applications of this study.  "It is absolutely fascinating to think that by using modern computational techniques we might stumble upon new drugs when searching for alternative molecules to DNA and RNA that can store hereditary information," Burger said.  "It is cross-disciplinary studies such as this that make science challenging and fun yet impactful."

While I certainly can appreciate the implications of this research from an Earth-based standpoint, I was immediately struck by its application to the search for extraterrestrial life.  As I mentioned earlier, it was already nearly impossible that humans and aliens would have cross-compatible DNA, but now it appears that alien life might well not be constrained to a DNA-based genetic code at all.  I always thought that DNA, or something very close to it, would be found in any life form we run across, whether on this planet or another; but the Cleaves et al. study suggests that there are a million or more other ways that organisms might spell out their genetic code.

So this drastically increases the likelihood of life on other planets. The tighter the parameters for life, the less likely it is -- so the discovery of a vast diversity of biochemistry opens up the field in a manner that is breathtaking.

... but the chance that the aliens will look like this is, sadly, pretty low.

This raises the problem of whether we'll recognize alien life when we see it.  The typical things you look for if you're trying to figure out if something's alive -- such as a metabolism involving the familiar organic compounds all our cells contain -- might cause us to overlook something that is alive but is being carried along by a completely different chemistry.

And what an organism with that completely different chemistry might look like -- how it would move, eat, sense its environment, reproduce, and think -- well, there'd be an embarrassment of riches.  The possibilities are far beyond even the Star Trek universe, with their fanciful aliens that look basically human but with odd facial structures and funny accents.

The whole thing boggles the mind.  And it further reinforces a conclusion I've held for a very long time; I suspect that we'll find life out there pretty much everywhere we look, and even on some planets we'd have thought completely inhospitable.  The "Goldilocks Zone" -- the region surrounding a star where orbiting planets would have conditions that are "just right" for life to form -- is looking like it might be a vaster territory than we'd ever dreamed.

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Crying wolf

While I understand being deeply fascinated with a specific subject, there's a point at which an interest becomes an idée fixe.  The result, especially for a scientist, is such a single-minded focus that it can cloud judgment with regards to the strength of the evidence.  We've seen that here at Skeptophilia before -- two examples that immediately come to mind are the Sasquatch-hunting geneticist Melba Ketchum and the British proponent of extraterrestrial panspermia, Chandra Wickramasinghe.  And the problem is -- for them, at least -- their obsessions have had the effect of completely destroying their credibility in the scientific community.

I can already hear the objections -- that (1) said scientific community is a hidebound, reactionary bunch of sticks-in-the-mud who resist like mad any new ideas, and (2) there are times the mavericks have been vindicated (sometimes after a long and arduous battle to get someone, anyone, to take them seriously).  The former can sometimes be true, but almost all scientists are well aware that groundbreaking ideas -- as long as they are supported by adequate evidence -- are how careers are made.  Look at the list of Nobel Prizes in the sciences in the past fifty years if you want examples.  Virtually all of them were awarded for research that expanded our scientific models dramatically (in some cases, overturned them entirely).  

As far as the second -- that sometimes the fringe-dwelling researchers who say "our entire prior understanding of the science is wrong" turn out to be correct -- okay, yeah, it happens, but if you consider the history of scientific paradigm shifts, what will jump out at you is how seldom that actually occurs.  The Copernican/Galilean/Keplerian heliocentric theory, Newton's Theory of Gravity, Maxwell's and Faraday's studies of electromagnetism, the Germ Theory of Disease, Einstein's Theories of Relativity, quantum/atomic theory, thermodynamics, Darwin's evolutionary model, Hubble and the Big Bang, the gene as the carrier of inheritance, and the plate tectonic model of Vine and Matthews. 

And that's about it, in the last five hundred years.

The point is, we're in a position now where the amount of evidence amassed to support the edifice we call science is so colossal that the "it could all be proven wrong tomorrow" objection I used to hear from my students (especially the lazy ones) is about as close to absurd as you want to get.  Sure, there will be some modifications made to science in the future.  A few -- probably very few -- will be major revisions.  But there's no reason to think that science as it stands is in any way unstable.

And people who come at it with earthshattering claims based on extremely slim evidence are almost certainly wrong.

Which brings us to Avi Loeb.

Loeb is an astrophysicist at Harvard University who has garnered significant notice (and notoriety) in the past few years from his fixation on the extrasolar source of some astronomical objects.  (By extrasolar I mean "originating from outside the Solar System.")  In 2017 he made headlines by claiming that the oddball astronomical object 'Oumuamua was not only extrasolar -- something fairly certain given its trajectory -- but that it was the remnant of a spacecraft from an intelligent extraterrestrial civilization.  Since then, his obsession with extraterrestrials visiting the Solar System has become so intense that it has drawn unfortunate comparisons with this guy:

The latest salvo from Loeb et al. is a sample of metallic beads scavenged from the floor of the Pacific Ocean near Papua New Guinea, that Loeb says are the remnants of a meteor that exploded in 2014.  So far, nothing to raise an eyebrow; meteoritic debris is cool but hardly uncommon.

But (as always) he goes one rather enormous step further, and claims that the meteor it came from was extrasolar, and the concentrations of metals in the beads indicate the object that exploded may have been an alien spacecraft.

Look, I'm as eager as the next Doctor Who aficionado to have a meet-cute with intelligent aliens.  (As long as they don't turn out to be Daleks, Sontarans, or Stenza.  I do have my boundaries.)  Hell, the way things are going down here on Earth, I might even ask to be taken on as a crew member when they leave.  But if you're asking me to believe you have bits of an alien spaceship, I'm gonna need more than a few oddball microscopic metal beads.

Extraordinary claims require extraordinary evidence, as Carl Sagan used to put it.  And this ain't it.  

At the moment, Avi Loeb is increasingly reminding me of a famous character from fiction -- The Boy Who Cried Wolf.  I have no problem with Loeb and his friends continuing to search; maybe (to quote a luminary of the field) The Truth Is Out There, and Loeb's dogged determination will eventually pay off.  But the problem is, there's a significant chance that (like The Boy in the fable) if he ever actually does find the hard evidence he's looking for, by that time he'll have exhausted people's patience to the point that everyone will have stopped paying attention.

So sorry to rain on the UFOs-and-aliens parade, but me, I don't think we've got anything here but some pieces of a curious metallic meteorite.  Worthy of study, no doubt, but as far as what it tells us about extraterrestrial intelligence, the answer seems to be: nothing whatsoever.

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