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Life from a Passing Star

Remember ‘Nemesis’? The idea was that mass extinctions on Earth recur on a timescale of between 20 and 40 million years, and that this recurrence could be accounted for by the existence of a faint star in a highly elliptical orbit of the Sun. Put this object on a 26 million year orbit and it would, so the theory ran, destabilize Oort cloud comets, causing some to fall into the inner system at a rate matching the record of extinctions. Thus a cometary bombardment was to be expected on a regular basis, as were the mass extinctions that were its consequence.

No one has found Nemesis, though other theories about recurring mass extinctions are in play, including recent work from Lisa Randall and Matthew Reece that explores dark matter as the trigger, with the Sun periodically passing through a disk of the stuff. Of course, finding dark matter itself continues to be a problem. Moreover, the wide range in the proposed recurrences gives rise to the possibility that these events are not periodic at all but simply random.

Robert Zubrin now offers a paper arguing that random encounters between our Solar System and passing stars can account for the Oort Cloud disruptions leading to extinctions without the need for Nemesis. Appearing in the International Journal of Astrobiology, the paper weighs other discussions of periodicity and goes on to propose a model for calculating the frequency of these encounters. The model rides on the treatment of the galaxy as a gas, with stars as particles at a density of approximately 0.003 stars per cubic light year.

These stars, argues Zubrin, are clearly not in synchronized motion but have random velocities with respect to each other on the order of 10 kilometers per second. Much rides on the effective encounter distance — when do stars pass closely enough to disrupt the outer cometary shell? One encounter every 26 million years occurs in Zubrin’s calculations if the distance of effective encounter is taken to be 22,000 AU, which would send a passing star through the Sun’s Oort Cloud, while at the same time exposing the Sun to the cometary cloud around the passing star.

Image: The layout of the solar system, including the Oort Cloud, on a logarithmic scale. Credit: NASA.

We also have to take into account that the Sun is among the larger stars, the most common type of encounter being with far less massive M dwarfs. Zubrin assumes such stars have Oort Cloud analogs of their own, though we have as yet no observational evidence for this. He makes the case that comet bombardment of Earth will more likely occur from disrupted objects in the passing star’s Oort cloud than through objects native to the Sun’s Oort Cloud.

From the paper:

If the Sun were to travel through the alien star’s Oort Cloud at a range of 20,000 AU, it would probably be in the cloud for about that distance. Assuming a disruption range of 10 AU, it would sweep out a path with a volume of π(102) 20,000=6.3 million cubic AU. Assuming 4 Oort cloud objects per 1000 cubic AU, this implies that approximately 25,000 alien cloud objects could potentially be captured per pass, providing a significant chance of impact events to follow.

Indeed, disrupted Oort objects can in Zubrin’s calculations fall into their target stellar systems quickly, creating a cloud of short period comets and potential planet impacts while the other star is still relatively nearby. In the case where an M-dwarf passes through the Sun’s Oort Cloud while the Sun remains outside the smaller cloud of the dwarf, the dwarf star’s planetary system would be bombarded by objects from the Sun’s Oort Cloud while our own Solar System remained unaffected. By the calculations of this paper, while we might expect bombardments on the order of 30 million years or so in our system, our Oort Cloud would be delivering bombardments to a passing dwarf star every 7.5 million years.

Crucially, these events, transferring objects from one system to another, could happen fairly swiftly. If an object from an M-dwarf’s comet cloud were destabilized as it passed through our Solar System at a range of 10 AU, for example, it would have the possibility of reaching perihelion swiftly, in a matter of years. Note that Zubrin derives the figure of 10 AU for the distance a visiting star needs to come to an Oort Cloud object to turn it into a comet; i.e., the Sun can capture a visiting star’s Oort cloud objects if it passes within 10 AU of the object.

Here, then, is the hook for astrobiology:

If we estimate that each Oort Cloud object disrupted has an average mass of 1 billion tons, then an encounter [with a star] at 20,000 AU would appear to have the potential to import about 25 trillion tons of mass from another solar system into our own. Of course, only a tiny fraction if it would hit the Earth. But even so, the potential to transfer biological material is evident.

Most of these bombardments of our own system would occur from M-dwarf comet clouds, given the high percentage of M-dwarfs in the galaxy. The table below shows the distribution.

Table 1. Comparative responsibility of star types for cometary bombardment of our Solar System. Credit: Robert Zubrin.

We have the prospect, then, of material from one stellar system impacting a planet in the other, or at least, being captured in that system’s Oort cloud and stored until another encounter with a passing star causes it to be disrupted and fall inward. Notice that Zubrin is talking about microbes in the transferred material that would have to survive a journey far less than the multiple light years assumed necessary for interstellar panspermia, though they would have to survive Oort-like conditions, having traveled from their inner system to the comet cloud.

It may also be noted that with a typical time between incoming encounters of 25 million years, it is probable that our Solar System has had about 140 incoming-delivery encounters with other stars since life first appeared on Earth some 3.6 billion years ago… If each encounter with a dwarf star typically releases 1000 solar system Oort cloud objects, then our Solar System has been responsible for releasing some 140,000 objects into others over this period. But, as a large G star, the sun probably delivered at least three times as many bombardments on other stellar systems as it received.

Our star keeps orbiting galactic center somewhere in the range of every 225 to 250 million years. A lot of material could be exchanged in this way:

…while we have only travelled through the Oort clouds of other, mostly dwarf, stars 140 times, dwarf stars have probably travelled through our own Oort Cloud about 420 times. If only 10% of encounters actually result in the transfer of microbial life from the Earth to another solar system, then we have been responsible for seeding 42 other solar systems with life. If each of these were then to act as a similar microbial transmitter, the result would be billions of inhabited worlds seeded by Earth.

Here it’s interesting to speculate, as Zubrin goes on to do, about whether there is an optimal impact rate for the evolution of advanced life. More frequent impacts might actually be a useful evolutionary driver — the author notes that the biosphere recovered from the K-T impact within 5 million years, offering up mammals and birds that proved long-term survivors. But too frequent an impact rate would not allow sufficient recovery time. Thus it is conceivable that areas of the galaxy with perhaps double our population of stars might be those more likely to feature advanced species and civilizations.

So we have no ‘Nemesis’ to fall back on — a cometary impact of roughly 1 every 25 million years has no specific driver within our own system, but results from the movement of stars, a random motion as the Sun moves through the Milky Way. Harvesting objects from passing stars, most of them red dwarfs, we collect them on timescales of years or decades rather than millions of years, the result of their relatively close disruption. We wind up with a mechanism for exchanging materials with other stellar systems that could have implications for life.

A key question: Can life survive Oort-like conditions to allow such transfers? We’re also hampered by our lack of knowledge about the Oort Cloud itself and, indeed, the local interstellar medium beyond the Kuiper Belt. Zubrin draws his best estimates from the current peer-reviewed papers, as he must, but it’s clear that to tighten this kind of argument, we’re going to need data from future explorations of the Oort. In such ways does a speculative astrobiology sharpen its focus, within a process of scientific inquiry that is by necessity multi-generational.

The paper is Zubrin, “Exchange of material between solar systems by random stellar encounters,” published online by the International Journal of Astrobiology 18 June 2019 (abstract).


Comments on this entry are closed.

  • Ashley Baldwin July 3, 2019, 13:19

    Nemesis…Genesis , I get confused with all these Premises.

    • Michael Spencer July 4, 2019, 10:09

      An “upvote” mechanism is sorely needed.

  • Alex Tolley July 3, 2019, 15:41

    The problems I have with Zubrin’s paper is that it makes an assumption (microbes in the Oort clouds) and a slight of hand (random encounters become periodic).

    The suggestion that there is a store of microbes in the Oort clouds of stars is based purely on speculation by Zubrin. He has previously suggested that microbes can be blown from the HZ by sunlight for some to be stored as spores in icy bodies. It is an interesting hypothesis, but there is zero evidence of that so far. I certainly think it is worth investigating whether microbes are being propelled away from Earth – an easy experiment that a lunar based sensor should detect a 28 day periodicity of spores impacting the trap. (I consider this a worthwhile experiment for any lunar science mission, with the trap either on the lunar nearside, or in orbit.) However, at this point, the idea that microbes are being blown off Earth, survive the journey to the Oort cloud, and are either frozen or in spore form embedded in an Oort object is pure speculation at this point.

    The second issue is that the whole idea of Nemesis was predicated on the apparent periodicity of the 5 extinction events. While there has been some question as to whether this periodicity is real, there is no question that FFTs on fossil data does show up an approximate 26my spike ( IIRC, also a weaker ~50my signal). Zubrin argues that this could be due to close encounters with an average frequency of about 25my. However, a random encounter would not be periodic, so I find this hard to countenance. It seems to me that Zubrin is playing some sleight of hand when he calculates the average encounter frequency and then argues later that it is periodic. I also recall that the source of the asteroid (not comet) causing the KT event was from a disruption of asteroids around Jupiter and that the KT event occurred long after this disruption.

    Which finally leads me to his last sleight of hand, which is to convert Oort cloud, icy objects, into the cause of the asteroid impacts. They may disrupting asteroid orbits, but the asteroids will be in the home system and will not bear life. The comets may bring life (if they are carrying microbes), but this has little bearing on the encounter frequency. Finally, it is not at all clear that extinction events are caused by impacts. The KT even has a smoking gun. The Permian extinction was caused by the volcanic emissions that might possibly have been set off by an impact, although we have no evidence to support this.

    Overall, I find this paper very speculative, making connections to events that may not exist, and calculations to support the connections that are tailored to support the hypothesis.

    • Ron S. July 4, 2019, 13:26

      Nicely argued.

      The item that really triggered me was: “a random encounter would not be periodic”. Indeed, periodicity is used as a test to validate a RNG. This is a blatant elementary error by Zubrin.

    • James Franklin July 8, 2019, 10:26

      Alex, I think you are misunderstanding a lot of what Robert is saying and is understood about comets, asteroids, periodicity of impacts events and extinction events.

      In the original Nemesis hypothesis the 26Myr periodicity was simply an average and not a hard figure, the impact rates used originally varied from 3.3 million to 57 million years with the apparent rms value (peak cratering rates) occurring at around the 26Myr rate mentioned. There was no talk of the 26Myr rate being a hard and fast number.

      With regards to the separation between asteroids and comets, this is simply an illusion for the most part. Many short period comets act more like asteroids and many asteroids have been seen to act comet like, forming coma like tails when at perihelion. It is likely that as our knowledge improves we will discover that the two types are object are interchangeable in the majority of cases. Of course there will always be exceptions, but as most bodies contain volatile materials, as that object approaches a source of heat (obvious example being a star) the volatiles start to sublimate from the object.

      The idea that the stars encounter each other at random is also somewhat inaccurate. Estimates of the stars within around 500 light years show the solar system comes to within 1 light year of another star every 300,000 years, and the last closest approach occurred only some 70,000 years ago.

      It is highly probable that encounters with extra-solar objects occurs far more often than we realise or are aware of . Just as stars increase in frequency as their mass decreases, it is likely that brown dwarves and unattached giant planets are equally as common as the lowest mass stars. Detecting such objects is currently fraught with all manner of problems with current technology, but for every stellar encounter, we likely have several sub-stellar encounters.

      How often such encounters produce disruptions that result in impacts on Earth is open to reasonable speculation, but it is safe to say the solar system likely does exchange material with these other systems. It is even conceivable that there are objects in the solar system that may have been captured from other stellar/sub-stellar system, and we may have lost some bodies to these systems too.

      Now, does biological material get exchanged between systems, well we have very little understanding of what life truly needs to get going, for all we know life is far more ubiquitous than we currently think, perhaps there is like in oceans on Europa, Ganymede, Callisto, Titan, Triton and even diminutive Pluto (based on the evidence of a potential sub-surface ocean from new Horizons).

      I am not convinced of panspermia at all, and the exchange of life from one system to another I find difficult, but it is not impossible and it is potentially more likely than we may currently understand.

      We should always keep an open mind.

      • Alex Tolley July 8, 2019, 15:14

        My understanding of the Nemesis hypothesis is that it was based on extinction rates gleaned from the fossil record. Impact rates had nothing to do with this. At a SETI meeting many years ago, Muller presented his research showing that there was a periodic rate when using an FFT on the fossil data. Whether that was a good analysis I leave to others.

        Asteroids and comets. Your argument seems like muddying the waters to me. Are you really claiming that Oort cloud objects which Zubrin posits as the source of impactors can be rocky, rather than icy? If so, please cite your evidence.

        Again, are you claiming stellar close encounters are periodic? That seems highly unlikely unless there is some uniformity resulting from both their distributions in space and their velocities. What evidence do you have for this?

        I don’t think anyone is arguing against stellar encounters. Such encounters have been discussed before on CD. What I take from Zubrin’s paper is that Zubrin is arguing that these encounters can account for the known extinction events and could also be spreading life between the stars. Zubrin argues that extinctions by impacts are most probably due to an ejection of material from the encountered star rather than a perturbation of material in our solar system. While it may be the case that the extra solar material could comprise the most important impactors, albeit as I understand it icy comets, there is no evidence that all the major 5 extinctions are caused by impacts. (The KT impact is the only definite coincidental one with the end of the Cretaceous, and we know that the current ongoing 6th is caused by human agency). I am more than happy to consider that some material in our solar system is extra solar, although I would like to see evidence of this. The only evidence I have seen so far is the visitor ‘Oumuamua, which appears not to be passing through due to a stellar encounter, but from an ejection from its host star. Maybe a lunar research station will find examples. As for panspermia, there is as yet no evidence for it at all. As I said, it is worthy of doing experiments, but until evidence is accumulated, it is purely speculation.

  • Robin Datta July 3, 2019, 15:56

    An interesting scenario for panspermia.
    To “seed” another world, a functionally complete organism would be needed (in contrast to viruses, which are entirely dependent on fully functional, live organisms). Such organisms, even in a spore, cyst or other quiescent mode would be metabolically active, to however slight a degree. An Oort cloud could provide a deep freeze to completely arrest metabolic activity while maintaining intact the biochemical machinery.

  • Antonio July 3, 2019, 18:19

    The problem with this and other theories is that the 26 million year cycle simply doesn’t exist. A more careful look at the fossil record made it dissapear some years after being proposed, but for some reason people keep creating new theories trying to explain this non-existent cycle or similar ones.


  • John walker July 3, 2019, 18:30

    “… the potential to transfer biological material is evident”

    No, sorry not to me it isn’t.

    There is no evidence that either short or long period comets are transporters of biological substance. In situ development of life in Oort Cloud like environments would be a breath taking discovery. But, it is pure conjecture.

    When considering the possibility of Earth impact based detritus being the source for the seeding of other stellar systems via lithopanspermia, I am sceptical. Simulations have demonstrated that even seeding as far up the solar gravity well and then down the Saturnian onto Enceladus would be an exceedingly rare event.

    Panspermia aside, my take on Zubrins math is that it suggests that planetary systems in M Class systems are exposed to extinction level impact events at such a high frequency that the evolution of complex ecosystems would be virtually impossible.
    Just when you thought the habitability of M Class systems had enough going against it. Now this.

    • Bruce D. Mayfield July 4, 2019, 15:35

      Agree completely. How can something with no confirmed evidence be “evident”?

  • Thomas R Mazanec July 4, 2019, 7:33

    Gliese 710 or HIP 89825 may do this in 1,283,000 AD. It is a K-type star.

  • kzb July 4, 2019, 8:50

    There is an open-access version here:


    The local star density is greater than 0.003/cu pc. More like 0.01 stars/cu pc ?

    • Jed July 5, 2019, 16:43

      More like 0.1 stars per cubic parsec. There are 63 known stars within 5 parsecs (523.6 cubic parsecs). 63/523.6 = 0.12.

      I think what they meant to say is 0.003 stars per cubic light year, which is correct.

      • kzb July 8, 2019, 8:24

        Sorry everyone my mistake. Didn’t notice the unit was cubic light years in the article. 0.12 per cubic parsec works out to 0.0035 per cubic light year. Wikipedia gives the local star density as 0.004 per cubic LY. However why are brown dwarfs not included in the calculation?

  • Michael July 4, 2019, 16:08

    If a star is in the red giant phase the stellar wind velocity is lower and much larger as well as having lower light damaging effect due to a lower temperature. This process could potentially work on a low gravity ice moon around a gas giant dissipating and releasing organisms that have made a home there.

  • Al Jackson July 5, 2019, 6:20

    Robert Zubrin is not very well informed about comet showers. His article makes no mention that Jack Hills proposed the Oort Cloud stellar encounter shower process in 1981 , Jack G. Hills (1981). “Comet showers and the steady-state infall of comets from the Oort Cloud”. Astronomical Journal. 86: 1730–1740. Nor does Zubrin seem aware of the perturbations of the Oort cloud by Giant Molecular Clouds, Julio A. Fernández (2000). “Long-Period Comets and the Oort Cloud”. Earth, Moon, and Planets. 89 (1–4): 325–343. GMC’s can have the effect to striping Oort clouds from stellar systems , this must be accounted for. Also , one must be careful about stellar encounters with the solar system , if too close, they will break long established orbital resonances between the planets. …… which means there is a limit on how close.
    Antonio, the question of periodicity in the ocean fossil record is still in contention. It is not a settled matter:
    Melott, A.L.; Bambach, R.K. (2011). “A ubiquitous ~62-Myr periodic fluctuation superimposed on general trends in fossil biodiversity. I. Documentation”. Paleobiology. 37: 92–112.

  • Robert July 5, 2019, 12:13

    Wickramasinghe has been expounding a form of these ideas about microbes in space for decades. But more importantly, he has some data. Arguably not the greatest data but still it’s a start.

    • Alex Tolley July 5, 2019, 14:58

      He and Hoyle made all sorts of assertions about life in space including the idea that flu viruses came from space. Wickramasinghe was also involved in the “red rain” mystery in India, making claims the substance was cellular. I would ignore these ideas as speculations rather than anything substantial.

  • Michael Fidler July 5, 2019, 14:42

    A couple of interesting points:
    1. We take for granted that the Big Bang is the defacto universe, but there is evidence for a small bangs steady state universe.
    2. The idea of the 14 billion years but even more so the infinite steady little bang universe could result with interstellar ecosystem.
    3. The comets that now enter our solar system may have deep frozen or may have fossil life that existed when they formed in the interstellar ecosystem.
    4. Rouge planets are 1000’s of times more common then even red dwarfs and could have an even greater effect on the Oort cloud.
    5. Rouge planets form in the same clouds that form our and other solar systems from open star clusters.
    6. The rouge planets may enter the Oort clouds in groups or gangs because of there grouping in the original star clusters.

    So another example of little evidence that could be a big fire breathing dragon! ;-{○

    • kzb July 8, 2019, 8:27

      Michael do you have a reference for your numbers of rogue planets? I’ve not seen a paper that gives anything like your number?

      • Michael C. Fidler July 10, 2019, 5:57

        Survivability of planetary systems in young and dense star clusters.


        There may be 50 billion free-floating planets in our galaxy.


        The assumption has been that that free floating planets must come from planetary systems. The stellar nursery could just as easily produce them by themselves with the effects from nearby novas, supernovas and large stars in or near the clouds. A good survey of microlensing in such clouds may give us a better idea as to how many actual form independently. There could also be many other ways that they could form, besides the stellar shock waves and ejection from planetary systems. One I can think of is the wake created by stars moving thru dense interstellar clouds.

        • Michael C. Fidler July 13, 2019, 2:41

          The Habitability of Titan and its Ocean.


          So how many free floating Titans may exist in the depths of interstellar space? The number of free floating Titan to Earth size planets could be huge and with large sub oceans they would make for great panspermia. Like the cracking of the cosmic egg the destruction of such an object would produce a frozen test tube of microbes exploding into space!

          Just how many

  • Geoffrey Hillend July 5, 2019, 18:43

    Comets can’t be responsible for all of our mass extinctions without wiping out all larger forms of animal life, the result of too many large impacts. There is no evidence of craters on Earth to support this hypothesis.

    Scientists think volcanoes and other environmental changes are mostly responsible for mass extinctions. There is the separation of the super continent or land masses through plate tectonics and the rising of mountains with exposed basalt carbon dioxide absorbing rocks and cooling climate caused by the absorption of Co2 by the carbon cycle and plant life and chemical changes in the ocean and atmosphere.

    Zubrin also had the Centauri Dreams paper which included “Interstellar Data Transmission by Microbial Storage Drives,” an interesting idea, but he did not explain what was the chemical composition of the small spheres which would hit the atmosphere at least escape velocity or twenty five thousand miles per hour or more. Even if the spheres were made heat resistant tungsten, they might still get red hot which might destroy the microbes and the information. A ballistic coefficient certainly does depend on the weight of an object which might determine its atmospheric drag, but that is of little consequence if every object which has to come from interstellar space that has get hit by the air molecules at escape velocity in order to land on Earth. Zubrin has not tested his assumption that such spheres would not be incinerated and I have no doubt they would still get heated, and I am not saying that I know for sure that the microbes would be damaged. The good news is this idea can be tested scientifically in a laboratory on Earth. Cosmic dust lands on the Earth 5 to 300 metric tons every day, but there is no evidence any microbes, bacteria or viruses which have survived the heat of atmospheric friction.

    • Alex Tolley July 5, 2019, 22:14

      I try to keep my mind open to the possibility of panspermia. There are so many phenomena that prove unexpected when found. It might take just one bacterium surviving planetfall on a prebiotic Earth to be the ancestor we call the last universal common ancestor (LUCA). I wouldn’t even rule out Gold’s idea that life was left unintentionally by visiting aliens. There is no evidence for either and such ideas just push back the origin of life to somewhere else, so they are unsatisfactory. (It should annoy Creationists that there is an alternative explanation for life than “God did it”.)

      Data from experiments is always preferable, and I see no reason why (apart from cost) that we shouldn’t look for life being ejected from Earth, and life entering the solar system. We know bacteria live in the clouds and may even be [one of] the seeds for raindrops. Bacteria are swept up into the stratosphere and beyond. We also know that bacteria can survive in space based on samples taken from outside the ISS. Whether they can reach escape velocity by some means and travel through space is still unknown, primarily because we have not looked.

      It is fortuitous that our probes looking for life on other bodies in the solar system are doing so now, since we now have the tools to be able to analyze life at the molecular level to determine its similarities and differences to terrestrial life. If we found microbes on Mars or the icy moons that was so similar to terrestrial life that it could be added to the tree of life we already have, I would be sorely tempted to accept that as due to panspermia, rather than another genesis/other geneses. If in the far future the same result was found for life on exoplanets, that would be further proof.

      Let’s just do the experiments. Given their likely scarcity, it is unfortunate that only a physical trap can be used. However, should solar sails ever become common, they might prove possible traps for microbes, or their material deployed as traps.

  • Geoffrey Hillend July 5, 2019, 19:11

    “Computer simulations of atmospheric heating and retry show that micrometeorites of plus or minus 100 micrometers enter Earth’s atmosphere without heating.” Flynn, 1989, Atmospheric Entry Heating of Micrometeorites. Zubrin’s idea is scientifically sound. It might still be a problem getting the spheres to Earth over long distances because deep space is saturated with high energy cosmic rays, x rays and gamma rays that can go right through a small coating of soot. High energy gamma from supernovas, active galactic galaxy cores etc can go through lead and concrete.

  • The Exoplanets Channel July 6, 2019, 7:07

    Great article!

  • Geoffrey Hillend July 6, 2019, 17:10

    I am not against the idea of panspermia, but I have to think it to be very improbable with very low odds for it happening. There are just too many obstacles for it to happen as a regular occurrence. There are the cosmic rays I mentioned, the relativistic particles which include most of the table of elements from hydrogen to iron. Some of these have far more energy than the collisions in the Large Hadron collider in addition to the x rays and gamma rays, beta particles etc. In order for bacteria to get into space it has to be the result of a large meteor impact which has the energy yield to blast a rock into escape velocity of Earth and the solar system. A collision in space might help it also. The rock has to be big enough to protect the bacteria from cosmic rays. It has to travel a long way through interstellar space travel to the right solar system or hit a planet like Earth with a magnetic field in the life belt, the odds against this are high. It has to survive impact which is possible if there is a shallow grazing angle of re-entry.

    Like the advocates of panspermia, I do believe that intelligent life is common in our galaxy based on that fact of our galaxy contains more than one hundred billion stars and most of them have planets. It seems to me far fetched that they all planets which have life were the result of panspermia due to the odds against a hit. If that were true, we should have found some ET bacteria or viruses which are harder in cosmic dust, meteorites, etc but we haven’t so panspermia can’t be common. Also it is very easy for life to occur on a planet like Earth that has the all the necessary conditions for long term survival of life, the safe, fertile environment which is proven by the Miller Urey experiment which contains the chemicals and atmosphere of early Earth. H2O, CH4, NH3, and H2 were put into a flask with a spark to simulate lightning which synthesized the organic compounds which are the building blocks of life, the amino acids.

    It is certainly possible that intelligent ET’s could physically and deliberately put bacteria on another world to seed it to start life there, but I don’t think that will ever happen because it does not fit into the psychological profile and scientific principles and ethics of how ET’s with interstellar travel would behave. I think they follow a prime directive to not interfere with other worlds or contaminate them. The leading edge view of scientists is any ET’s with interstellar travel would be at least a million years more advanced than us and their ethics, and psychological knowledge, etc is superior to ours. I predict that the only exoplanets we find intelligent life will have the exact same requirement for the long term adaption of life as our Earth. Nature and evolution work through necessity and that makes it’s environment scientifically predictable which should be the same everywhere in the universe. I think the panspermia idea is projection of our unconscious human behavior onto a theory of nature and evolution. Nature never has to leave it’s home planet it began to survive because it has adapted to the environment through evolution. Animals and plants are in harmony each other and with their environment. It is humanity which is not in harmony with nature through pollution, primitive green house gas technology, etc. Through civilization and development of the abstract mind we have lost touch with the Earth and nature.

  • Eric S July 8, 2019, 12:54

    Why must the Oort objects contain the biomatter? Since the premise is that the captured objects in-fall quickly, while the other star is close by, wouldn’t it be more likely that those impacts eject life, which then rides the “short” distance to the other star?

    (I realize that dynamically, it’s hard to believe the ejected rocks have enough excess velocity to catch the passing star, but that seems more likely than samples of inner system life getting ejected to take up stable Oort orbits and then after a very long time falling into another system & being viable.)