Would panspermia, the idea that primitive life can spread from star to star, be theoretically observable? Henry Lin and Abraham Loeb (both associated with the Harvard-Smithsonian Center for Astrophysics) believe the answer is yes. In a paper accepted for publication in Astrophysical Journal Letters, the duo make the case that panspermia would create statistical correlations regarding the distribution of life. Detecting biosignatures in the atmospheres of exoplanets may eventually allow us to apply statistical tests in search of these clustering patterns. If panspermia occurs, the paper argues, we can in principle detect it.
“In our theory,” says Lin, “clusters of life form, grow, and overlap like bubbles in a pot of boiling water.” The paper argues that future surveys like TESS (Transiting Exoplanet Survey Satellite) could be an early step in building the statistical database needed. TESS could detect hundreds of terrestrial-class explanets, some of whose atmospheres will be subject to study by ground-based observatories and instruments like the James Webb Space Telescope. Next-generation instruments will do more, allowing us to look for detailed spectral signatures like the ‘red edge’ of chlorophyll or, conceivably, the pollution of a technological society.
Moreover, SETI searches at radio or optical wavelengths could produce detections that eventually allow us to test for clustering, the point being that life that arises by spreading through panspermia should exhibit more clustering than life that arises spontaneously. The statistical models that Lin and Loeb develop in the paper have observable consequences that could begin to turn up as we expand our investigations into astrobiology. Think of Lin’s ‘bubbles’ of life that grow and overlap, or consider the spread of life from host to host in terms of the spread of an epidemic. We may eventually have the data to confirm the idea. From the paper:
In a favorable scenario, our solar system could be on the edge of a bubble, in which case a survey of nearby stars would reveal that ∼ 1/2 of the sky is inhabited while the other half is uninhabited. In this favorable scenario, ∼ 25 targets confirmed to have biosignatures (supplemented with 25 null detections) would correspond to a 5σ deviation from the Poisson case [a probability distribution], and would constitute a smoking gun detection of panspermia.
Image: The center of the Milky Way as seen from the mountains of West Virginia. Is there life out there, and if so, does it arise spontaneously, or spread from star to star? Credit: Forest Wander.
The transition between an uninhabited to an inhabited galaxy occurs much faster through panspermia than through a gradual buildup of life arising spontaneously in random areas. There is even a Fermi implication here — if life started at roughly the same time everywhere, then we would expect fewer advanced civilizations at the present time than if life started at random times throughout the universe (the authors note that the Drake equation is based on the assumption that life arises independently everywhere, which contradicts efficient panspermia).
The paper continues:
A more generic placement would increase the number of required detections by a factor of a few, though an unusual bubble configuration could potentially reduce the number of required detections. It should be noted that the local environment of our solar system does not reflect the local environment ∼ 4 Gyr ago when life arose on earth, so the discovery of a bubble surrounding earth should be interpreted as the solar system “drifting” into a bubble which has already formed, or perhaps the earth seeding its environment with life.
The paper notes that any species capable of panspermia will have enormous fitness advantages as it can move from one stellar host to another. Lin and Loeb believe that if panspermia is not viable and Earth is the only inhabited planet, interstellar travel may lead to colonization of the galaxy. In this case panspermia models may still be relevant: A culture using starships may provide the opportunity for primitive forms of life including disease and viruses to spread efficiently, with the same processes of growth and diffusion occurring throughout space.
Can primitive life, then, spread on its own, or does it need intelligent life to create the conditions for its growth outward? Either way, we see life expanding in all directions, producing what this CfA news release calls “a series of life-bearing oases dotting the galactic landscape.” If such patterns exist, finding them will depend upon how quickly life spreads, for any ‘bubbles’ or ‘oases’ could be lost in the regular flow of stellar motion and redistribution about the galaxy. But whatever the biological mechanisms of panspermia might be, it is in principle detectable.
The paper is Lin and Loeb, “Statistical Signatures of Panspermia in Exoplanet Surveys,” accepted for publication at Astrophysical Journal Letters (preprint).
Comments on this entry are closed.
You write: “There is even a Fermi implication here — if life started at roughly the same time everywhere, then we would expect fewer advanced civilizations at the present time than if life started at random times throughout the universe…”
There is also the possibility (known as “neocatastrophism”), suggested by James Annis in “An astrophysical explanation for the ‘great silence'” (Journal of the British Interplanetary Society. 52, 19-20, 1999) that life started at roughly the same time everywhere in the galaxy because periodic massive gamma-ray bursts wipe out life everywhere in the galaxy, so that intelligent life must evolve in the “window” between these periodic sterilizing bursts (the frequency of which is decreasing over time due to stellar evolution).
Developing Annis’s idea, Milan M. Ćirković and Branislav Vukotić suggest in “Astrobiological Phase Transition: Towards Resolution of Fermi’s Paradox” (Orig Life Evol Biosph. 2008 Dec; 38(6):535-47) that panspermia can be incorporated into such a model:
“In the forthcoming study, we shall generalize the present APT [astrobiological phase transition] model with more phenomenological details, notably […] non-zero probability of interstellar panspermia”
Very interesting approach. The approach is simple enough that this model could be explored more fully to determine the statistical patterns under different parameter values for infection rate and stellar drift. My understanding was that infection rates are likely to be very low for lithospermia. Just how efficient does panspermia need to be before the patterns are not detectable? As teh paper suggests, inefficient panspermia might that patterns are more indicative of directed panspermia or colonization.
In any case, I hope that real data collected over the next decades will indicate whether or not these spatial patterns exist.
“the authors note that the Drake equation is based on the assumption that life arises independently everywhere, which contradicts efficient panspermia”
That isn’t quite true as the Drake equation has a term for star formation rates. Since stars mostly [?] form in clusters, we might still expect local bubbles of higher than average life bearing stars.
Great piece. There is an article on arXiv in 2012 related to this and the concept of “lithopanspermia” , the tranferance of microbes via meteorites of different sizes when stars ( and their protoplanetary disks ) are still in their original open clusters and therefore much nearer each other . Survival of microbes in meteorites appears to be related to the size and thus resistance to radiation , so for instance ranging from 10 million years for a 3cm meteor to half a billion years for a 9 footer. This is important as the receiving planet has to be sufficiently developed to facilitate the development of any ready delivered life , say 300 million years for the Earth , thus offering a window of opportunity. Anyway, the authors ,Moro-Martin and Malhotra describe it better than me on arXiv. Their names plus ” lithopanspermia ” will find on Google.
Nice points, but the paper is devoid of information on the
mode of transport of these interstellar microbe travelers.
How does a microbial ‘cyst’ exit the gravitational well of
its primary. It is not easy, an impact event sure, but like anything else it has to be a special hit. and statistically it is only likely to occur if there
are many thousands of planets with microbial life in the galaxy. The
vastness of empty space demands this IMO.
If this transport of life were likely our galaxy would be much different today. I would not rule out that life in the solar system may have been
seeded like this, But it sounds like a very remote chance.
On the other hand, if we find life to be very rare we may find that it is more probable that life will be transported via random chance, rather than evolve
naturally out of a chemical soup.
Within the solar system however, I would posit that there is a higher probability that a life form will some how make an interplanetary crossing, rather than new life evolving on it own.
> The transition between an uninhabited to an
> inhabited galaxy occurs much faster through
> panspermia than through a gradual buildup of
> life arising spontaneously in random areas.
This claim seems completely unsupported. There’s a characteristic time constant for the spontaneous arising of life, which is not known. (Or rather, several of them, none of which are known.) There’s a characteristic time constant for panspermia (or several), also which is not known. How is it possible to compare two unknown and say which is lesser or greater? It’s not possible to compare a known to an unknown, so why two unknowns?
I am not sure statistical ‘bubbles’ would exist as it implies the local stellar geography is static over extended periods of time, which is not the case. For example the solar system has orbited the galaxy 16 to 20 times since life appeared here, so any natural lithopansperma effects from earth for example would be smeared along our orbit, and those planets/stars receiving earth life would also have moved throughout the galaxy. The same applies to other life bearing stars. If a bubble was found it would imply a rapid pansperma in the very recent past (million or so years) for the stellar geography to still have some coherence around the source star/planet.
@David – if you get a chance to read the paper, you can see they have a term, D, for the drift rate of stars from each other. They model this as a displacement (swap stars between cells randomly) on their lattice model. This is one parameter that could be explored to see how it impacts the patterns.
[i]”There is even a Fermi implication here — if life started at roughly the same time everywhere, then we would expect fewer advanced civilizations at the present time than if life started at random times throughout the universe (the authors note that the Drake equation is based on the assumption that life arises independently everywhere, which contradicts efficient panspermia).”[/i]
This is an interesting line of thought on many levels, also for fiction.
If you think about it, it quite odd when sentient races stat their interstellar efforts at the same time, something that always struck me as rather odd. This is a very good framework for such scenarios, for example in a computer game, where you need somewhat equal footing in the beginning to preserve overall balance.
Fiction aside, transit time is the major roadblock regarding interstellar transfer of viable microbes. Exchange between planets in a solar system is averaged to a few million years. Regarding interstellar transfer the timescale is much, much more complicated. We are talking billions of years at best, if we are optimistic.
Still transfer of material does happen, since stars are recyclers and form in generations, but the distances are abysmally vast and so are the involved timescales. Considering such transfers really leads to questioning decay rates of material components, for starters. It is hard to even consider anything could make it preserving its functionality.
The process, if it exists, might rely on intrinsic mechanics of stellar mixing, such as the deep black smoker communities, which have long eluded us in pinpointing how exactly the communities spread to new sites. Thermal mixing distributes juvenile larva across the area. It might be a comparable mechanism, albeit on a much grander scale.
For example, the Oort cloud extends , if i am not totally mistaken, approximately a light-year around our solar system. Sometimes other stars get close enough to shake this cloud up, showering the inner planets with comets. Assuming that other systems share a similar cloud, depending on gravity of course, material exchange could already happen at a distance of 2 Ly. Alpha Centauri is 4 Ly away, the distances are vast but considering stellar mixing not insurmountable in my opinion.
Worse, lately our observations are no longer in harmony at great cosmic distances. Things are too heavy and seem to drift apart for reasons eluding us currently. We may be off by wide margin.
Another thing to consider is that microbes not only show remarkable survival capabilities in space exposure experiments, they are also very small. The smaller, the better, because you need less energy for transfers. That is almost too much of a coincidence to bear. Radiation exposure is usually lethal but then again a bit cosmic dust should provide aptly shielding for the journey. Biofilms do behave differently under zero-g conditions and microbes do alter behavior, this is all subject to active research currently.
Frankly i think its not only possible but increasingly likely.
Plus, if i want to shoot a nano-device to another star system, that can be done relatively easy. We do it in our particle accelerators and relativity also plays into that favorably (decay rates slow down at relativistic speeds). Plus, the smaller the device is, the less likely it is hitting something else.
[i]”How is it possible to compare two unknown and say which is lesser or greater? It’s not possible to compare a known to an unknown, so why two unknowns?”[/i]
Transfer is less complex than spontaneous assembly. Lets assume you encounter… an object. Would you argue for spontaneous formation or did somebody carry it there? Increasing complexity of the object does impact the likeliness of formation increasingly negatively. Life is rather complex and… what do we mean by “spontaneous” assembly in any case? Artificial?
Creationists do have a point here, however they fail when they mix up a spatial problem with systemic one, which is of course a faith(=origin)-based approach (for a creator, hence artificial, without acknowledging that this again raises questions about the origin of a creator, which is considered sacrosanct). But they are not necessarily automatically mistaken on all counts, because they do try to explain scientifically, they just run a top-down approach, which is of course, the entire problem (“here is the answer, how do we get there?”).
At the moment it seems to me this particular scenario seems to gather more credibility through observations, however i could be totally mistaken. I don’t think so, but i might.
Microbial spores have a high surface charge-to-mass ratio. It may be that Earth’s magnetic lines of force are sufficient to convey spores into space at the poles. Once lofted above the ionosphere these spores might be swept away in the solar wind.
If so, Earth would become a source of panspermaic life seeds. I have no idea how long various spores could survive in space. Maybe long enough to travel to another body such as the ocean moons that seem to abound in our system.
Part of this idea is at least test-able today. A few cube-sats in polar orbit could be designed to function as “Nansen bottles” by opening spore-detection sensors to the vacuum at timed intervals. Then we could use the space shuttle to retreive…whats that?? The Smithsonian you say?? Oh…nevermind.
I believe the author of the paper means that life would fill a galaxy faster if life could spread via panspermia and spontaneously emerge. Panspermia relies on life to spontaneously emerge somewhere. It is the additional vector provided by panspermia that would create the mentioned bubbles.
Microbial life can exist 4km beneath the surface on Earth. If there was a disaster that tossed massive amount of such material into space, then I imagine that microbial life could make an interstellar journey on the order of billions of years.
Such life would also need little from any world it did eventually crash into – just for the planet to be not too close to a star, and not be a gas giant. It would eventually replicate, and spread, evolving to fill new niches. Even a wanderer could have such life.
It’ll be interesting to see what we find when we can drill down that far on Mars.
This is an interesting article. While I’m not a huge fan of panspermia per se, I’m glad we’re now getting some direction in the form of a testable hypothesis. I wonder if our first alien-bug sample that is most definately not earth-contaminated (eg found in a comet on a hyperbolic trajectory) will be DNA based or truely alien…
I am quite a panspermia skeptic, I mean the probability of a rock of sufficient size to act as a protector getting into space would be low been launched from the Earth. Bacteria would need to live in the cracks which are themselves weak points and prone to splitting open under great forces which would expose them to the harshness of space. However if single life organisms evolved in or on a low gravity proto planet such as say Ceres when there was a lot of heat of formation and radioactive decay I would think it could be much more probable.
Now in the future if we look at the various objects in our solar system and do not find life then wouldn’t panspermia be dealt a death blow after all the probability of life’s expansion into the solar system would be billions of times more likely than between the stars.
You are right, it would indicate another explanation as more likely.
When it comes to microbes on extraterrestrial bodies, it is the search for the needle in a haystack, especially considering that it may lie waiting in dormancy for the right conditions to awaken.
That being said, it could be far more common than we ever dared to imagine. Maybe we should simply create such conditions and see what happens.
Of course there is a contamination issue and in a way, if you think about it, that is in itself already very suspicious.
We participate already in interplanetary Panspermia because our space vehicles certainly already contaminated a host of system bodies. That is already happening RIGHT NOW.
Total sterilization, if you really look into it is virtually impossible. If you want to amuse yourself you can look at the lists of organisms found in NASA “clean” rooms. The lists are, despite all our efforts, extensive and not even complete. Our procedures are killing most of it, but to kill all of it defies our current capabilities. There is no know procedure achieving a 100% sterile result. It doesn’t work.
Spreading the “stuff” is, of course, still a long way from it actually being placed under conditions that allow it to thrive and grow.
Even if this is not exactly the process we are looking for, one depending on natural (we could argue if accidental distribution by sentient species is “natural”, but lets leave it there for the moment) distribution on an interstellar scale, it is a start.
I think it is IMPOSSIBLE tho have an universe which includes conditions for forming life and not having it spreading around. Thats how life works on our planet. I don’t really think life cares for an arbitrary boundary we draw to separate our “world” from the rest of matter.
My money is on spontaneous evolution of life in the right settings which may not be what we would term habitable. Mercury has ices and organics at the poles, any cracks there and there is possible conditions there that could harbour or even allow life’s creation, you could include the moons poles as well as impacts cause deep cracks going down 10’s of kilometers to the warmer interior.
@Swage September 4, 2015 at 5:53
‘Total sterilization, if you really look into it is virtually impossible. If you want to amuse yourself you can look at the lists of organisms found in NASA “clean” rooms. The lists are, despite all our efforts, extensive and not even complete. Our procedures are killing most of it, but to kill all of it defies our current capabilities. There is no know procedure achieving a 100% sterile result. It doesn’t work.’
I work in the pharmaceutical industry with a working knowledge of autoclaves and it is possible to kill all organisms and that is with steam and ethylene oxide.
Steam causes an interact with cell membranes rendering them non functional over a time and temperature dependence, generally the higher the temperature the shorter the duration needed.
Ethylene oxide also interacts with the cell membrane but at a lower temperature, it is extremely flammable and explosive and is not to be messed with. Both processes have pros and cons on the interaction with the electronics which limits the process effectiveness.
If you are expecting to see bubble-like structures, think again. By their nature, such bubbles will expand, and eventually all merge into one large, homogeneous, galaxy spanning “bubble”. Nothing to see with the author’s method, at that point. If life on Earth was really seeded by panspermia 2-3 Gy ago, we could have been at a bubble boundary, at best, back then. By now, the bubble boundary would have moved on (by natural growth) or disappeared (by galactic mixing), so it would not be here for us to observe, anymore.
And then there is this:
Gamma-ray burst? sterilize? Really? Gamma rays cannot even penetrate our rather tenuous Earthly atmosphere, today, much less reach the deeper places in oceans and the ground where life thrives. The whole idea of a cosmic catastrophy “sterilizing” planets is ludicrous and absurd. The distances are far too great. Even the most intense energy bursts imaginable are reduced to a mild disturbance that can be measured, but not felt, at a few light years distance.
“Gamma-ray burst? sterilize? Really? Gamma rays cannot even penetrate our rather tenuous Earthly atmosphere, today, much less reach the deeper places in oceans and the ground where life thrives. The whole idea of a cosmic catastrophy “sterilizing” planets is ludicrous and absurd. The distances are far too great. Even the most intense energy bursts imaginable are reduced to a mild disturbance that can be measured, but not felt, at a few light years distance.”
I admit the word “sterilize” was misleading. We are not talking about the possiblity of the total destruction of life on Earth. However, there have been studies suggesting that a gamma-ray burst close enough in our own galaxy could damage the stratosphere, and also cause some direct damage through radiation exposure. For example, A.L. Melott et al. write in “Did a gamma-ray burst initiate the late Ordovician mass extinction?”:
“We present as a credible working hypothesis that a GRB could have contributed to the late Ordovician extinction first by subjecting the Earth’s biota to elevated levels of UV radiation, partially due to the initial burst but primarily by depleting the ozone layer, and then by precipitating relatively rapid global cooling which was followed by rapid global warming.”
Wikipedia cites the same article, and writes:
“The greatest danger is believed to come from Wolf–Rayet stars, regarded by astronomers as likely GRB candidates. When such stars transition to supernovae, they may emit intense beams of gamma rays, and if Earth were to lie in the beam zone, devastating effects may occur. Gamma rays would not penetrate Earth’s atmosphere to impact the surface directly, but they would chemically damage the stratosphere.
“For example, if WR 104, at a distance of 8,000 light-years, were to hit Earth with a burst of 10 seconds duration, its gamma rays could deplete about 25 percent of the world’s ozone layer. This would result in mass extinction, food chain depletion, and starvation. The side of Earth facing the GRB would receive potentially lethal radiation exposure, which can cause radiation sickness in the short term, and, in the long term, results in serious impacts to life due to ozone layer depletion.”
If we are talking about a hypothetical resolution of Fermi’s Paradox, then “sterilization” (taken to mean total elimination of life) would not be required. A gamma-ray burst could damage the biosphere sufficiently that the evolution of intelligent life would be halted, and would then have to resume from a more primitive state after the damage caused by the gamma-ray burst had passed.
Maybe a more useful paradigm would be to encompass “catastrophism” as an inevitable consequence of life in the Cosmos. In this view we would recognise that the clock is ticking and we dither at our own peril.
The sooner we start preparations to fulfil our destiny the sooner we increase the odds of our species (and others) having a destiny. Heh.
“My money is on spontaneous evolution of life”
Absolutely. In my case both, actually. It may seem counterintuitive at first but these thing don’t seem exclusive of each other, if you think about it.
“it is possible to kill all organisms”
Of course. Its not practical.
“Both processes have pros and cons on the interaction with the electronics which limits the process effectiveness.”
What you basically saying is that the electronics, which have usually high durability requirments for space missions, wouldn’t survive a thoroughly sterilization process. That is very interesting, don’t you agree?
“If you are expecting to see bubble-like structures, think again. By their nature, such bubbles will expand, and eventually all merge into one large, homogeneous, galaxy spanning “bubble”.”
You are right, of course. But let me remind you that by spatial observational distance also temporal distance incrases. We could observe such bubbles from the beginning of the big bang in theory.
If we assume it is not there, then our work is done. If we assume it is there the it is all down to signal/noise. Our work begins
True, but then again, we were talking about nearby stars, where this is not applicable. Further away, we cannot really see exoplanets, much less study their spectra.
I am not going to re-argue the Melott “catastrophy”, except to note that much worse catastrophies have befallen the Earth regularly than a 25% reduction in stratospheric ozone. Weren’t we supposed to have suffered almost as much from the man-made ozone hole of the eighties and nineties? I am feeling distinctly unextinct, at this time….
@swage September 7, 2015 at 12:56
“Both processes have pros and cons on the interaction with the electronics which limits the process effectiveness.”
‘What you basically saying is that the electronics, which have usually high durability requirments for space missions, wouldn’t survive a thoroughly sterilization process. That is very interesting, don’t you agree?’
The electronics on spacecraft are normally hardened against radiation that could cause error signals within the sensitive electronic components. Both of these sterilising processes can at elevated temperatures corrode electronic components badly.