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Planets Around an Ancient Star

The idea of ‘deep time’ exerts an abiding fascination. H.G. Wells took us forward to a remote futurity when his time traveler looked out on a beach dominated by a red and swollen Sun. But of course deep time goes in the other direction as well. I can remember wanting to become a paleontologist when I discovered books about the world of the dinosaurs, my mind reeling from the idea that the world these creatures lived in was as remote as any distant star. Paleontology was a grade-school ambition I never followed up on, but the Triassic and Jurassic eras still have a hold on my imagination.

In a SETI context, deep time presents challenges galore. Charles Lineweaver’s work offers up the prospect that the average Earth-like planet in our galactic neighborhood may well be far older than our own — Lineweaver calculates something like an average of 1.8 billion years older. Would a civilization around such a star, if one could survive without destroying itself for so long, have anything it wanted to say to us? Would it have evolved to a level where it had merged so completely with its environment that we might not be able to recognize its artifacts even if we saw them?

All these musings were triggered by the news from the Max-Planck-Institut für Astronomie (Heidelberg) that European astronomers have discovered a planetary system that is truly ancient. The star in question is HIP 11952, about 375 light years from Earth in the constellation Cetus. Not surprisingly, given its estimated age of 12.8 billion years, it is extremely metal poor, but two giant planets are now known to orbit it. Says Johny Setiawan (MPIA), who led this work:

“This is an archaeological find in our own backyard. These planets probably formed when our Galaxy itself was still a baby.”

The planets around HIP 11952, which have orbital periods of 7 and 290 days, thus present us with a challenge. Elements heavier than hydrogen and helium are what astronomers consider ‘metals,’ and we’ve seen through our ongoing analysis of exoplanets that stars with higher metal content are more likely to have planets. We also have a mechanism for metal formation in which heavier elements are produced inside stars and blown into space through the explosion of supernovae. That a star as old as HIP 11952 should have planets would seem to be a rare thing indeed, but we do have another metal-poor system around HIP 13044, discovered back in 2010.

There is no doubt about the low metallicity of HIP 11952, which has an iron abundance approximately 1 percent that of the Sun. This is a population II star, one of the class that includes the oldest and most metal-poor stars that have yet been observed. Our own solar neighborhood is comparatively metal-rich and contains stars younger than 10 billion years. It’s an interesting speculation (followed up in this expanded news release from MPIA) that HIP 11952 may be a remnant from another galaxy that was absorbed by the Milky Way billions of years ago.

Image: Population II stars tend to have orbits that take them outside the plane of the galactic disk, and are often associated with globular clusters and the galactic halo, a roughly spherical distribution which contains the oldest stars in the Galaxy. Because of their age, most of them consist of almost pure hydrogen and helium, unenriched by earlier generations of stars. Credit: UCSD Center for Astrophysics & Space Sciences.

Are there, then, more planets around metal-poor stars than we have previously thought? We need to find more planetary systems in this age bracket to learn more, but Anna Pasquali (Heidelberg University), a co-author of the paper on this work, says “The discovery of the planets of HIP 11952 shows that planets have been forming throughout the life of our Universe,” a thought that reminds us to be careful about drawing hasty conclusions about planet formation. Don’t be surprised, either, if the idea doesn’t once again bring Dr. Fermi knocking at the door.

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  • Curt Wohleber April 3, 2012, 15:01

    I think it would be fascinating to see what 10+ billion of years of biological evolution might yield, compared to the 3.8 billion years we’ve had on Earth. If intelligence and technology isn’t the inevitable result–and I doubt it is–what sort of strange, complex adaptations and reproductive strategies might appear with much more time for natural selection to experiment? Interstellar explorers might not run into many advanced civilizations, but what about unintelligent but advanced biological systems? They might be as challenging in their own way as aliens with nanotechnology and death rays.

  • Daniel Suggs April 3, 2012, 20:02

    Would a low metal system be capable of producing a rocky planet, or would it be limited to gas giants? What sort of life could form in a mostly hydrogen/helium environment? I’m just guessing from a carbon based viewpoint, but I predict a pretty sterile system.

  • James D. Stilwell April 3, 2012, 20:05

    Clarke in his masterpiece “The City and the Stars” wrote of a pure mind that lived in space, taking millions of years to mature….and travelled between the stars…and did not understand the Territorial Imperative…This mind had a twin that did not mature correctly…called the Mad Mind…
    Nevertheless, hope as well as danger awaits us among the stars…
    Such is life…
    Keep going….
    JDS

  • Abelard Lindsey April 3, 2012, 23:03

    If intelligence and technology isn’t the inevitable result–and I doubt it is–what sort of strange, complex adaptations and reproductive strategies might appear with much more time for natural selection to experiment?

    The Chtorran ecosystem?

  • Interstellar Bill April 4, 2012, 2:24

    The latest theory of the formation of planetary systems is by
    Michael Wilkinson, at arXiv December 2008
    http://arxiv.org/abs/0802.4099 Please do study it.

    Planets form first as an interstellar cloud collapses, with the biggest and fastest-growing one becoming the primary star, and the rest moving chaotically. Gas alone will form only a very large star.

    In Wilkinson’s theory of stellar formation, a disc finally does form, but only after the planets get started, so they can collide with the disc depending upon their orbital inclinations. The results can vary wildly, depending upon the initial distribution of angular momentum (the more of that the bigger the system) as well as eccentricities and orbital inclinations. The entire stellar mass spectrum from brown dwarfs on up, as well as multiple stars systems, comes naturally out of this model.

    The earliest stars would have formed the same way, just with fewer planets in the contracting cloud. Even the oldest stars surviving today would have formed this way, just with fewer planets.

    The very first stars, formed solely of hydrogen, would have collapsed with no disc, having no planetary triggers, so that only truly gigantic masses (100+ Msun) would have formed. None of those very first stars remain today, of course, due to their ultra-brief lifetimes. They would have gone supernova so soon that the very first hundreds or thousands of them to form would rapidly have seeded the Galaxy with solid elements.

    Only after that happened could ‘normal’ stars form, by Wilkinson’s mechanism, hence the heretofore suprising existence of such early planets.

    The Planetary-Formation Paradigm Shift is under way.
    This is good news since it means that nearly every star will have planets and asteroids. The only problem may be a reduced abundance of elements heavier than iron, definitely putting a techno-crimp on any settlers. They’d have to either mine the stellar wind or fashion ultra-large accelerators to synthesize the elements directly by fusion.

  • Martyn Fogg April 4, 2012, 4:06

    With a metallicity this low, it seems highly likely that these giant planets formed via protostellar disk fragmentation and gravitational collapse, rather than via accretion of gas onto multi-Earth mass solid cores. Their orbits could be explained by type II migration within that disk. The presence of terrestrial planets would seem very unlikely in a system such as this. There are just two few heavy elements from which to build them.

    I would bet with reasonable confidence that HIP 11952 is not a life bearing system. But I would also say that we now have pretty good evidence that both core-accretion and gravitational collapse mechanisms of giant planet formation operate in nature.

  • Anthony Mugan April 4, 2012, 8:23

    Presumably planets that old would have to be essentially hydrogen plus a small proportion of helium and tiny traces of heavier elements? Presumably not of astrobiological interest, but absolutely amazing that they exist at all. I’ve read some suggestions that such early planets may form as the role of viscosity in the early universe has been underestimated previously but that is well outside my area of expertise so I’m not sure how secure that idea is and what the general view is on the extent to which this is a major challenge for models of planetary formation or not…?
    If Linweaver’s model that the average age of terrestrial type planets is 1.8by older than earth is broadly right then that is deep time enough for Mr Fermi to reappear in the discussion. I think you make a very important point, Paul, by noting that it is almost beyond imagination what a civilisation that is hundreds of millions or even more than a billion years old would have evolved into. Any emerging primitive culture, which may well be as far behind them technically as single celled organisms are behind us, will in the end have to join their club, unless we are incredibly lucky and actually are the first civilisation to emerge across the billions of planets and billions of years available…

  • tom baty April 4, 2012, 11:00

    Some very big IF’S. IF.. the average age of a civilization is 1.8 billion older then us, or perhaps much older, then, IF true, perhaps advanced civilizations may be much more common then we expect. The ambigious “L” in the Drake equation has a lot to do with the number of civilations that might exist out there. (This has been discussed numerous times in prior discussions here on Centauri Dreams) IF.. this is true it may explain why we have not heard from, or been contacted.Why would a 1.8 billion year old civilization have any interest in us at all when there far more interesting contacts to be made. Emerging techological societies could be so common that they would look at us and say “oh, there is yet another one…so what…why bother” I know that is brusing to our egos, but why should the universe be concerned about our egos. Lots of unanswered IF”s make for a very interesting discussion, but with few real answers at this point in SETI research.

  • philw1776 April 4, 2012, 11:02

    The question is…how long can a planet support the equivalent of multi-cellular motile life forms? Two factors work against 10 billion years. First, the F, G and early K stars evove rapidly enough such that planets once comfortable get too hot as the star evolves, and secondly less understood, possibly necessary continental drift that recycles carbon, etc. may stop after > 5 billion years of planetary core cooling.

    So there may be very few if any ecosystems with vigorous 10 billion years of evolution. A likely scenario has life simplifying itself along lines like early Earth life to adapt to changing conditions. As we currently understand it, evolution proceeds without purpose and has no “time’s arrow” direction. It simply adapts.

  • Ole Burde April 4, 2012, 15:05

    This system has been through a lot , literally ! what would the probability of a starsystem getting realy close to another star be , during 13 bill years ? The observed lack of heavy
    elements is probably only verifyed for the star itself , and not necesarlily for the planets . One or both of them could have started their existence somewhere else , perhabs as lonely freeflyers .

  • ljk April 4, 2012, 18:08

    This also assumes that advanced beings would even want to hang around on just one planet for billions of years with a whole Universe out there. Unless they all go underground and get lost in their virtual worlds and don’t care about anything else.

  • Eniac April 5, 2012, 6:38

    Why would a 1.8 billion year old civilization have any interest in us at all when there far more interesting contacts to be made.

    Because, presumably, there are lots of scientists in that civilization, and they each have different interests. Over billions of years, and millions of civilizations, that amounts to a lot of interest.

    You may carelessly step over the blades of grass in the park, but somewhere, probably right in your city, there is a catalogue with specimens of that very grass, carefully numbered and annotated. You may not know or care how many species of ant there are, but someone does, and each is documented in voluminous scientific literature.

    You think we would escape this sort of scrutiny if there really were advanced civilizations capable of examining us? Out of disinterest? Guess again.

  • Ronald April 5, 2012, 6:55

    As Martyn Fogg also points out, this suggests two mechanisms for planet formation: core accretion, which depends on elements > He (metallicity) and results in planets ‘as we know them’, and gravitational collapse which results in stars all the way down via brown dwarfs to really big gas giant planets in the purest sense, i.e. only consisting of gas without a rocky core.

    This would also answer Daniel Suggs’s relevant question: these metal-poor old systems most probably do not possess rocky planets nor even gas giant as-we-know-them with rocky cores.

    Ref. more advanced evolution on very old planets: I think it is a classical error to think that evolution would always and inevitably lead to something ‘higher’. Evolution is the result of natural selection and adaptation and can go in any direction that the environment in broadest sense ‘forces’ life to go. In other words, it is very well possible that a very old planets possesses only primitive single-celled life, if conditions remained very stable and fovorable to that life without changes (i.e. selective pressure) forcing it to different evolutionary pathways.

    Summarizing: old age may allow time for complex changes, but does itself not imply those at all.

    philw1776 raises an extremely relevant point: for solartype stars (roughly late F through G to early K) evolutionary fun is over at least for complex life, if not any life, after a limited number of gys, for the combined reasons of stellar aging and planetary aging (ending of plate tectonics, atmosphere loss). It seems, therefore, that, with regard to potential biological lifespan, there has to be a kind of optimum somewhere in the middle, for instance a largish earthlike planet with prolonged plate tectonics and rather dense atmosphere orbiting a slighty dimmer sunlike (roughly G5-8) star. Guesswork.

  • Ronald April 5, 2012, 9:56

    @Eniac: I fully agree with you.

    This is what I often argue when people mention a supposed alien disinterest in us lowly humans (a kind of collective inferiority complex): if there is an advanced alien intelligence somewhere out there, I am convinced of at least one thing: they will be highly interested in us, at least for study.

    First, because intelligent life is most probably very rare and therefore very special anyway.
    Second, because, no matter how unadvanced we may still be, we are a budding civilization and that is how they must have started themselves.
    Thirdly, to a true scientist, there will be no such thing as an uninteresting planet or lifeform, as to a passionate ornithologist there is no such thing as a boring bird.

  • Ronald April 5, 2012, 10:09

    I should have added to my previous: this indeed makes Fermi’s footsteps even louder (i.e. the fact that an advanced civilization will definitely be highly interested in us). But of course we would have no idea of the ways that such an advanced civilization would study us. They may choose to do this totally unnoticed and non-interventional. Even we ourselves will likely possess (or at least have the technical ability to possess) very powerful means of remote observation, such as (solar) gravitational telescopes, within this millennium.
    About the technical abilities of a civilization that is actually able to visit us for close-up study we can hardly even guess, such a civilization approaching the seeming magic that A.C. Clarke referred to.

  • Anthony Mugan April 5, 2012, 13:45

    @eniac and Ronald

    Entirely agree with the combination of your comments. The logic is that we would be studied but probably be very hard pressed to notice or make much sense of it if we did stumble across a few clues.

  • ljk April 5, 2012, 13:55

    A sufficiently advanced ETI may indeed be interested in Earth and its life forms, but they may not need to actually travel to Sol to learn about us.

    Already we lowly humans are conceiving of space telescopes that can image continents and take other measurements of distant exoworlds. We only lack a combination of the will, interest, and money from the majority of people who are in the positions to make such projects happen.

    Hopefully a more advanced species means a more enlightened one as well, devoting their time and lives to exploring and understanding the Cosmos rather than warfare, power grabs, deities, celebrities, and reality programs.

  • Rob Henry April 5, 2012, 19:40

    Interstellar Bill says of early planets “The only problem may be a reduced abundance of elements heavier than iron”. In its context it implied that population III stars disproportionately release element from their neucleosynthesis relative to those produced in supernovas. Knowing very little about population III stars, I am intrigued as to whether he is correct.

  • Eniac April 5, 2012, 20:15

    As we currently understand it, evolution proceeds without purpose and has no “time’s arrow” direction. It simply adapts.

    I think it is a classical error to think that evolution would always and inevitably lead to something ‘higher’. Evolution is the result of natural selection and adaptation and can go in any direction that the environment in broadest sense ‘forces’ life to go.

    This is a surprisingly common misunderstanding of the path of evolution. Evolution adapts, but it also advances. Even after billions of years, there are still always new niches to colonize, or non-trivial advances to be made in utilizing existing ones. Once there was no photosynthesis, then it arrived, to stay forever. Once there were no Eukaryotes, then they appeared, spread, and never went away. Once there were no multicellular organisms, now they are ubiquitous and not going away anytime soon. Once there was no life on land, but then it arrived and it never went back. Once there were no animals, then they arrived, and never disappeared again.

    If that is not a “time’s arrow”, I do not know what is.

  • Rob Henry April 5, 2012, 20:42

    Ever since the similarity was pointed out to me by a friend 30 years ago, I have often thought of evolution as uncannily similar to chemistry. It is near impossible to predict the actions of a few molecules, but in huge numbers their actions should really become more and more predictable.

    In chemistry rules start to emerge that are dominated by enthalpy and entropy. In evolution the enthalpy equivalent is the selection for ever more efficient creatures. The entropy is selection for ever simpler creatures (or more accurately, those whose construction requires the least information), and the heat is the mutation rate.

    As you can imagine, theory from such a system would allow nothing that looks like progress, or at least very little of it. This seems nicely reflected in our fossil record at the base of the Cambrian. Almost every modern phylum of animals seems to have appeared around that time, and developed a body similarly sophisticated to modern forms within a few million years. Even better for our analogy this change, and major transitions before it, seemed fit the idea that they were triggered by a sudden dramatic change in the environment, better than they fit being the outcome of continuing evolutionary progress.

    Then something strange happened – post Cambrian animal brain structures and immune systems continued to gain net complexity with time in defiance of the above rules. In our case, they seem to be the one bit of evolution where competition was sufficient to lead to continuing advancement. If any of you can come up with why a different factor would be forced to join this list on a different planet I would love to hear. It must be something that is open to continuous improvement, and have little possibility of settling around locally perfect designs.

  • Rob Henry April 5, 2012, 20:58

    Eniac, you are seeing biological, geological, and atmospheric changes. How can you confidently separate one set of changes from the other. Perhaps, for most traits, life can’t progress unless its environment changes, and even then only briefly.

  • Eniac April 5, 2012, 21:37

    Three more great, irreversible evolutionary advances, two of which have not yet happened: The rise of intelligence, the advent of mechanical life, and the colonization of space.

    Intelligence is a particularly jarring advance, because it replaces natural selection by the far more efficient method of intelligent design. This means the end of evolution as we know it, for good.

  • Eniac April 6, 2012, 21:06

    Rob,

    Eniac, you are seeing biological, geological, and atmospheric changes. How can you confidently separate one set of changes from the other.

    You can not. Doesn’t matter here, though, because the irreversible, “time-arrow” nature of biological innovation is common to all of them.

  • Eniac April 7, 2012, 11:55

    In chemistry rules start to emerge that are dominated by enthalpy and entropy. In evolution the enthalpy equivalent is the selection for ever more efficient creatures. The entropy is selection for ever simpler creatures (or more accurately, those whose construction requires the least information), and the heat is the mutation rate.

    It is a seductive analogy, but I think it is quite flawed. For one, the mutation rate is not an outside force, but part of the design of an organism. It is quite tightly regulated by evolution to produce the maximum opportunity for positive selection. Too few mutations, progress is slowed down, too many result in error catastrophe. There is a tight relationship between genome size and mutation rate that demonstrates this.

    I think that the defining characteristic of Eukaryotes actually is the fact that they were the first organisms to achieve the replication fidelity needed to have arbitrarily large genomes under the above relationship, where the error rate can be tuned for optimal mutation/selection rather than be limited by physical/chemical factors. This led to a situation where complexity is no longer much of a liability, at least not in terms of genome replication. The advent of Eukaryotes was one of those rare innovations that change the very nature of evolution itself, along with sexual reproduction and the rise of intelligence.

  • Rob Henry April 8, 2012, 2:41

    Eniac, I realise that my chemical analogy was simplistic, but you go a little too far when you say the “tight relationship between genome size and mutation rate” proves that evolution by itself finds its optimal level. You also have to remember that the more proof reading there is during DNA replication, the more energy is expended, and the slower the strand can be replicated. It is my belief that that (combined with your aforementioned error catastrophe) is a more important reason for this correlation, and I suspect the advantage of being able to mutate faster will prove only marginally useful to an animals line.

    Actually, thinking of evolution rates and mutation Richard Dawkins* states as true (in The Ancestors Tale) a trend that I have also noticed. That animals that have changed very little morphologically from their fossil record ancestors, tend to have shown much higher rates of genomic mutation, than ones that have changed more.

    *I realise that Richard Dawkins’ mathematical skills are too poor for him to really understand these models, but his friends seem so good at directing him that he almost never gets these sort of things wrong

  • Ronald April 8, 2012, 16:18

    Slightly off-topic, but this is just in:

    The solar type star HD 10180 appears to have 9 planets, according to HARPS data, a record so far:

    http://arxiv.org/pdf/1204.1254v1.pdf

    Again, mainly super-earths and Neptunes, and again, the system is very compact: 7 are within 0.5 AU. But there may still be room for another planet between f (at 0.5 AU) and g (at 1.4 AU).

  • Eniac April 9, 2012, 0:00

    Rob,

    Eniac, I realise that my chemical analogy was simplistic, but you go a little too far when you say the “tight relationship between genome size and mutation rate” proves that evolution by itself finds its optimal level.

    I said demonstrates, not proves, because you are right, the relationship is not proof. Then again, I think it is almost self-evident that the mutation rate has to be controlled to be “just right”, because “too high” leads to extinction by error catastrophe, and “too low” leads to extinction by inability to adapt and compete.

    I suspect the advantage of being able to mutate faster will prove only marginally useful to an animals line.

    Not in the long run, I don’t think. Stagnation and loss of adaptation will lead to extinction just as surely as the error catastrophe. Even in the most static of environments, there are always other organisms ready to eat your lunch if you don’t evolve as fast as they do.

    Also, there is another reason besides cost of replication (which may or may not be as burdensome as you imply) that keeps mutation rates from going down too much: Improvements in mutation rate are self limiting, because as fidelity increases, natural selection loses its power to drive them. See this very interesting perspective on the subject:
    http://www.indiana.edu/~lynchlab/PDF/Lynch183.pdf

  • Rob Henry April 10, 2012, 18:52

    Eniac, that article you referenced seems equally transfixed as you and I by the stunning correlation between genomes size and mutation rate. Because this article appears to be more a general analysis of a mystery, rather than an opinionated answer, it demand ones full attention to get the best from it, and I admit that today I’m not in that sort of mood. Until that time, I’d just like to point out the following

    1) the effective population size can often vary over several orders of magnitude, and as such N has far more influence on the total number of new advantageous mutations that can be selected for than changes in the mutation rate.

    2) the lower the mutation rate the closer to the local optimal perfection natural selection can hone a creature to. This could give lower mutation animal of today such a large advantage over higher rate ones that it seems immaterial to contemplate any slight advantage to their competitors line tens of thousands of generations hence.

    3) through-back genes that proved useful in former times will deteriorate much faster and be less amenable to reactivation in high mutation rate lines.

    4) some viruses have the ultimate possible high mutation rate, and so effectively exist as pseudospecies. Despite this, natural selection keeps their range within such a tight cloud that before PCR techniques it was thought that each strain was its consensus, and this varied little, if at all, over any epidemic (rather than find better optimal genome for each individual infected).

    I realise that my previously given reasoning for upwards pressures on mutation rates might not be sufficient to generate such a tight correlation, but I thought yours was even worse. From what I have comprehended of that article it upholds neither of our views on this important subject, and in a day or two I will know if it is much help either.

  • Rob Henry April 10, 2012, 19:26

    I should have also noted that the paper Eniac cites posits that there is absolutely no upwards pressure on mutations rates at all, but I will not know how strongly the think/uphold this till I go through their maths line by line and figure out all the assumptions that these hold.

  • Eniac April 12, 2012, 21:39

    Rob, you make some good points, but I think in your point 2) you are ignoring the effect of competition. Because of competition, your “local optimal perfection” is a moving target. In fact it would be moving about as fast as the fastest evolving of the competing organisms, and put slower ones at a disadvantage.

    This is particularly relevant for those viruses you mention, because they are in competition with something far more sophisticated than merely another evolving organism: They are fighting an immune system that has been optimized to evolve faster than any free-living organism can, for the very purpose of protecting its much more slowly evolving host.

    Come to think of it, perhaps the recombinant immune system is another one of these groundbreaking evolutionary game-changers: It enables organisms with larger genomes and longer generation times to withstand the assault of simpler ones, which would otherwise out-mutate them.

    I agree with you on your interpretation of Lynch’s article, he is not making my point, nor yours. I cited him because I thought it an interesting perspective, maybe a little to counter your argument, but not to support mine.

  • Rob Henry April 15, 2012, 20:10

    Eniac, from all that I have read, your opinion seems highly unorthodoxy. Upon much contemplation for your view that there is upwards pressure on mutation rates due to competition, I am starting to feel that it might be able to be salvaged for the restricted case of direct interactions with the immune system (where you posited that the effect was highest).

    Also, a while back, you mentioned that chemical defence systems might also undergo similar interactions. Your theory and its expansion to include that could explain why cone shells (with the most complex chemical defences of any animal that I know) have an anomalously high mutation rate, whereas complexity is normally inversely related to mutation rate. Also notable is the mystery over how a morphology complex enough structure evolved to allow the bombardier beetle its chemical defences.

  • Rob Henry April 15, 2012, 22:52

    It should be the exception, not the rule, to have the changing landscape of evolving animals provide sufficient dynamism to prevent designs precipitating out of a sea of mutations around local optima. A classic example is the eye, that evolved about 30 different times during the lower Cambrian (often even utilising some of the same precursor genes) and within a few million years (and probably much much less), crystallised into a variety of sophisticated forms, which have thereafter varied remarkably little to this day. For that matter, the body plan of virtually every modern phyla was laid down at around that time, and kept to with sufficient fidelity, that had we had no fossil record, a biologist that was transported back to that time should have little difficulty placing a great number of them in modern phyla. I also hear, that apart from the 2 (or 3?) aforementioned exceptions, there would be no objective criteria that could mark them as more primitive (the argument here that the few traits that we have marked as “primitive” have only been done so after the fact).

    I bags that more evolution on these ancient planets just begets a greater variety of animals that resemble life on Earth but with bigger brains and more complex immune systems (and better chemical defences?).