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If it was the maximum possible speed, then it must have involved very unlikely events that took billions of years to happen maybe just once, and that's evidence of a Great Filter in our past.

If it wasn't the maximum possible speed, then there should be many planets where intelligence evolved much earlier in the Universe's lifetime, and the fact we don't see aliens is evidence of a Great Filter in the future.

jacob_cannell9y20

evidence of a Great Filter in our past.

Most of the space of possible great filters in the past have been ruled out. Rare planets is out. Tectonics is out. Rare bio origins is out. The mediocrity of earth's temporal rank rules out past disaster scenarios, ala Bostrom/Tegmark's article.

and the fact we don't see aliens is evidence of a Great Filter in the future.

Mediocrity of temporal rank rules out any great filter in the future that has anything to do with other civs, because in scenarios where that is the filter, surviving observers necessarily ... (read more)

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35 Astrobiology, Astronomy, and the Fermi Paradox II: Space & Time Revisited

by [anonymous]

10th Mar 2016

2 min read

35

After a 6+ month hiatus driven by grad school and personal projects, I am finally able to continue my sequence on astrobiology. I was flabbergasted by the positive response my last post got, and despite my status as a biologist with a hobby rather than an astronomer I decided to take a more rigorously mathematical approach to figuring out our biosphere's position in space and time rather than talking in generalizations and impressions.

Post is here: http://thegreatatuin.blogspot.com/2016/03/space-and-time-revisited.html. Seeing as this post is an elaboration on the last one, I am posting a link rather than reproducing the text.

To summarize, I found some actual rigorous observational fits to the star formation rate in the universe over time and projected them into the future. These fits show the Sun as forming after 79% of all stars that will ever exist, and that 90% of all stars that will ever exist already exist. This makes sense in the light of recent work on 'galaxy quenching' - a process by which galaxies more or less completely shut off star formation through a number of processes - indicating that the majority of gas in the universe probably won't form stars if trends that have held for most of the history of the universe continue to hold. It relies heavily on analysis I began in comments on this site a few months ago.

I then lift two distinct metallicity normalizations from a paper that was making the rounds here a while back ("On The History and Future of Cosmic Planet Formation"), in an attempt to deal with the fact that that is a measurement of STAR formation, not terrestrial-planet-with-a-biosphere formation. Depending on which metallicity normalization you use (and how willing you are to take a couple naive assumptions I make in order to slot the math that is too complicated for me to comment on on top of my star formation numbers) the Earth shows up as forming after either 72% or 51% of all terrestrial planets.

These numbers are remarkable in how boring they are. We find ourselves in an utterly typical position in planet-order, even if I am wrong by quite a bit. We are not early. Of interest to many here, explanations of the so called Fermi paradox must go elsewhere, into the genesis of intelligent systems being exceedingly rare or the genesis of intelligent systems not implying interstellar spread.

Now that I seem to have a life again, I will be getting back to my original plan next, talking about our own solar system.

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[-]jacob_cannell9y50

I take this as another sign favoring transcension over expansion, and also weird-universes.

The standard dev model is expansion - habitable planets lead to life leads to intelligence leads to tech civs which then expand outward.

If the standard model was correct, barring any wierd late filter, then the first civ to form in each galaxy would colonize the rest and thus preclude other civs from forming.

Given that the strong mediocrity principle holds - habitable planets are the norm, life is probably the norm, enormous expected number of bio worlds, etc, if the standard model is correct than most observers will find themselves on an unusually early planet - because the elder civs prevent late civs from forming.

But that isn't the case, so that model is wrong. In general it looks like a filter is hard to support, given how strongly all the evidence has lined up for mediocrity, and the inherent complexity penalty.

Transcension remains as a viable alternative. Instead of expanding outward, each civ progresses to a tech singularity and implodes inward, perhaps by creating new baby universes, and perhaps using that to alter the distribution over the multiverse, and thus gaining the ability to effectively alter physics (as current models of baby universe creation suggest the parent universe has some programming level control over the physics of the seed). This would allow exponential growth to continue, which is enormously better than expansion which only provides polynomial growth. So everyone does this if it's possible. Furthermore, if it's possible anywhere in the multiverse, then those pockets expand faster, and thus they was and will dominate everywhere. So if that's true the multiverse has/will be edited/restructured/shaped by (tiny, compressed, cold, invisible) gods.

Barring transcension wierdness, another possibility is that the multiverse is somehow anthropic tuned for about 1 civ per galaxy, and galaxy size is cotuned for this, as it provides a nice sized niche for evolution, similar to the effect of continents/island distributions on the earth scale. Of course, this still requires a filter, which has a high complexity penalty.

[-][anonymous]9y60

I raise my standard point, that there is a huge insufficiently explored possibility space in which lack of interstellar expansion is neither a choice nor indicative of destruction/failure to form in the first place, but merely something that is not practically possible with anything reliably self replicating in the messy real world. Perhaps we must revisit the assumption of increasing mastery over the physical world not having an upper bound below that point.

[-]jacob_cannell9y00

It's not binary of course, there's a feasibility spectrum that varies with speed. On the low end there is a natural speed for slow colonization which requires very little energy/effort, which is colonization roughly at the speed of star orbits around the galaxy. That would take hundreds of millions of years, but it could use gravitational assists and we already have the tech. Indeed, biology itself could perhaps manage slow colonization.

Given that the galaxy is already 54 galactic-years old, if life is actually as plentiful as mediocrity suggests, then the 'too hard' explanation can't contain much probability mass - as the early civs should have arose quite some time ago.

I find it more likely that the elder civs already have explored, and that the galaxy is already 'colonized'. It is unlikely that advanced civs are stellavores. The high value matter/energy or real estate is probably a tiny portion of the total, and is probably far from stars, as stellar environments are too noisy/hot for advanced computation. We have little hope of finding them until after our own maturation to some post-singularity state.

[-]DanArmak9y00

The high value matter/energy or real estate is probably a tiny portion of the total, and is probably far from stars, as stellar environments are too noisy/hot for advanced computation.

Can you expand on this?

All computation requires matter/energy. If a civ wants to increase its amount of computation, then eventually it will need to use up that huge majority of matter that resides in stars. I think it was the Significant Digits hpmor fanfic where Harry remarked that the stars were huge piles of valuable materials that had inconveniently caught fire and needed to be put out. Of course, it's still necessary to have a practical way of star lifting.

One alternative is that advanced civs find a way to use dark matter instead, or some other physics we don't really understand yet.

[-]jacob_cannell9y60

The high value matter/energy or real estate is probably a tiny portion of the total, and is probably far from stars, as stellar environments are too noisy/hot for advanced computation.

Can you expand on this?

See this post.

Extrapolating from current physics to ultimate computational intelligences, the most important constraint is temperature/noise, not energy. A hypothetical optimal SI would consume almost no energy, and it's computational capability would be inversely proportional to it's temperature. So at the limits you have something very small, dense, cold, and dark, approaching a black hole.

Passive shielding appears to be feasible, but said feasibility decreases non-linearly with proximity to stars.

So think of the computational potential of space-time as a function of position in the galaxy. The computational potential varies inversely with temperature. The potential near a star is abysmal. The most valuable real estate is far out in the interstellar medium, potentially on rogue planets or even smaller cold bodies, where passive shielding can help reduce temperatures down to very low levels.

So to an advanced civ, the matter in our solar system is perhaps worthless - the energy cost of pulling the matter far enough away from the star and cooling it is greater than it's computational value.

All computation requires matter/energy.

Computation requires matter to store/represent information, but doesn't require consumption of that matter. Likewise computation also requires energy, but does not require consumption of that energy.

At the limits you have a hypothetical perfect reversible quantum computer, which never erases any bits. Instead, unwanted bits are recycled internally and used for RNG. This requires a perfect balance of erasure with random bit consumption, but that seems possible in theory for general approximate inference algorithms of the types SI is likely to be based on.

that the stars were huge piles of valuable materials that had inconveniently caught fire and needed to be put out.

This is probably incorrect. From the perspective of advanced civs, the stars are huge piles of worthless trash. They are the history of life rather than it's future, the oceans from which advanced post-bio civs emerge.

[-][anonymous]9y00

This idea implies a degree of coordination that does not happen in actual ecologies we have seen. Thus we get trees extravagantly sucking up mineral nutrients and building massive scaffolds to hold their photosynthetic structures over their competition, and weeds that voraciously multiply and compete with each other to take up every bit of sunlight and soil they can that the bigger things can't establish themselves in, rather than a thin scum of microbial mats that efficiently intercepts energy. You are implying a climax community without any other seres, and large amounts of material that while not being used efficiently are not used at all.

Things that reproduce themselves effectively become more common regardless of efficiency, and even multicellular organisms built of exquisite coordination get cancer.

[-]jacob_cannell9y00

Given that physics is the same across space, the math/physics/tech of different civs will end up being the same, more or less. I wouldn't call that coordination.

To extend your analogy, plants don't grow in the center of the earth - and this has nothing to do with coordination. Likewise, no human tribes colonized the ocean depths, and this has nothing to do with coordination.

[-][anonymous]9y20

I suspect you misunderstand my objection and that I may have used only half of the appropriate analogy

A universe in which your proposed ubiquitous low-matter low-energy interstellar computers exist is one in which space-based self-replication and manufacturing is a thing that happens. This implies the existence of a whole slew of 'ecological niches'. Indeed, the sort that is generally thought of in these circles (more-or-less industrially turning large amounts of matter near stars into stuff that intercepts light and uses the resultant energy for something or other) is rather simpler, is more similar to the demonstrated cases of terrestrial biology / human industry, and has more matter and energy available than what you propose. The low temperature low energy devices would be more akin to crazy deep extremophile lithotrophic bacteria or deep sea fish on Earth, living slow metabolisms and at low densities and matter/energy fluxes, while things in star systems would be akin to photosynthetic plants and algae at the surface, living at high densities at high flux.

In any situation other than perfect coordination, that which replicates itself more rapidly becomes more common. You will have adaptation and evolution. It doesn't matter if more computation can be done in one place than another - in terms of sheer matter and energy, that which uses high energy fluxes and large amounts of matter will replicate to large numbers and be dominant in terms of amount of stuff and effect on the physical universe. Other stuff could still exist, but most stuff would be of this faster heavier type. Niches will be filled. And a stellar system niche is not akin to the deep ocean if an interstellar niche is compared to the surface of the Earth, if anything it's the opposite. The deep sea niche may be where you see all kinds of fascinating bioluminescence and long distance signaling epiphenomena that these organisms care about and of a sort you dont see at the surface, but in terms of biomass the surface niche dominates. Furthermore, competition amongst different things mean they often do things inefficiently so as to gain advantages over each other - those that do become more common faster.

[-]jacob_cannell9y00

The low temperature low energy devices would be more akin to crazy deep extremophile lithotrophic bacteria or deep sea fish on Earth, living slow metabolisms and at low densities and matter/energy fluxes,

Hmm I think you misunderstood my model. At the limits of computation, you approach the maximal computational density - the maximum computational capacity per unit mass - only at zero temperature. The stuff you are talking about - anything that operates at any non-zero temp - has infinitely less compute capability than the zero-temp stuff.

So your model and analogy is off - the low temp devices are like gods - incomprehensibly faster and more powerful, and bio life and warm tech is like plants, bacteria, or perhaps rocks - not even comparable, not even in the same basic category of 'thing'.

In any situation other than perfect coordination, that which replicates itself more rapidly becomes more common.

Of course. But it depends on what the best way to replicate is. If new universe creation is feasible (and it appears to be, from what we know of physics), then civs advance rather quickly to post-singularity godhood and start creating new universes. Among other things, this allows exponential growth/replication which is vastly superior to puny polynomial growth you can get by physical interstellar colonization. (it also probably allows for true immortality, and perhaps actual magic - altering physics) And even if that tech is hard/expensive, colonization does not entail anything big, hot, or dumb. Realistic colonization would simply result in many small, compact, cold civ objects. Also see the other thread.

[-]woodchopper9y00

What you are saying doesn't follow from the premises, and is about as accurate as me saying that magic exists and Harry Potter casts a spell on too-advanced civilisations.

[-]turchin9y10

I take it as strong evidence for Rare earth.

Another interpretation may be that early universe is full of x-ray bursts, and later is full of aliens preventing newborns.

[-]jacob_cannell9y00

I take it as strong evidence for Rare earth.

It's the exact opposite.

If the earth was rare, this rarity would show up in the earth's rank along many measurement dimensions. Rarity requires selection pressure - a filter - which alters the distribution. We don't see that at all. Instead we see no filtering, no unusual rank in the dimensions we can measure. The exact opposite is far more likely true - the earth is common.

For instance, say that the earth was rare in orbiting a rare type of star. Then we would see that the sun would have unusual rank along many dimensions. Instead it is normal/typical - in brightness, age, type, planets, etc.

[-][anonymous]9y40

This overstates the case. If planets like Earth were very rare in ways that didn't change much with time you'd still see a time that was typical. One can imagine some things we have a sample size of one for being rare in ways that don't have anything to do with star order - origins of eukaryotes, plate tectonics, oxygenic photosynthesis...

This being said, I think the sheer DEGREE of rare Earth being implied by turchin and others is still very unlikely, even though there's a whole lot that we have little information on. It remains a not fully excluded possibility, but there are a hell of a lot of others.

[-]jacob_cannell9y00

If planets like Earth were very rare in ways that didn't change much with time you'd still see a time that was typical

The time measurement is not the only rank measurement we have. We also can compare the sun vs other stars, and it is mediocre across measurements.

Rarity requires an (intrinsically unlikely, ala solomonoff) mechanism - something unusual that happened at some point in the developmental process, and most such mechanisms would entangle with multiple measurements.

At this point in time we can pretty much rule out all mechanisms operating at the stellar scale, it would have to be something far more local.

Tectonics as rare has been disproven recently. Europa was recently shown to have active tectonics, possibly pluto, and probably mars at least at some point.

For later evolutionary development stuff, it will be awhile before we have any data for rank measurements. But given how every other measurement so far has come up as mediocre . . ..

We can learn alot actually from exploring europa, mars, and other spots that could/should have some evidence for at least simple life. That can help fit at least a simple low complexity model for typical planetary development.

[-][anonymous]9y00

We can learn alot actually from exploring europa, mars, and other spots that could/should have some evidence for at least simple life. That can help fit at least a simple low complexity model for typical planetary development.

Dear gods yes. We are finally at the point where we can start asking the intelligent questions. We have learned so much about these places and about life on Earth that we forget how little we do know.

[-]turchin9y00

If earth is really very rare, it may imply some natural process which kills habitable planets very often. We were just lucky to survive until now, but nothing garantee future survival that said natural disaster will not happened very soon.

For example, the real reason for "rareness" may be intrinsic instability of earthlike planets atmosphere which tend to become either iceball or greenhouse (Mars or Venus). In this case we overestimate stability of our athmosphere and small push could move it into another state. Think about runaway global warming connected with clathrate gun https://en.wikipedia.org/wiki/Clathrate_gun_hypothesis

So rare earth is rising probability of human extinction soon. It doesn't "save" us from technological great filter of self-extermination, which may be another solution of Fermi paradox, if not rare Earth.

[-]DanArmak9y20

Life on Earth has existed for billions of years without experiencing a terminal snowball or greenhouse scenario. It also recovered from several snowball periods once they ended.

So the fact that intelligence took this long to evolve - 4-5 billions of years after biogenesis, and 600-700 million years after the first multicellular animals - must be important.

If it were the case that the Great Filter was the short average lifespan of habitable planets before they became iceballs or greenhouses, then we should expect to appear much much earlier in our planet's history.

[-]turchin9y20

We don't know if speed of evolution was maximum possible speed or not.

We could derive some limitations on ice ball probability from the fact that we are late, using the same line of reasoning as was used by Bostrom and Tegmark in their article : http://arxiv.org/pdf/astro-ph/0512204.pdf

[-]DanArmak9y00

If it was the maximum possible speed, then it must have involved very unlikely events that took billions of years to happen maybe just once, and that's evidence of a Great Filter in our past.

[-]jacob_cannell9y20

evidence of a Great Filter in our past.

and the fact we don't see aliens is evidence of a Great Filter in the future.

Furthermore, natural disasters are already ruled out as a past filter, and thus as a future filter as well.

So all that remains is this narrow space of possibilities that relate to the timescale of evolution, where earth is rare in that evolution runs unusually fast here. Given that there are many billions of planets in the galaxy in habitable zones, earth has to be 10^10 rare or so, which seems pretty unlikely at this point.

Also, 'seeing aliens' depends on our model of what aliens should look like - which really is just our model for the future of post-biological civs. Our observations currently can only rule out the stellavore expansionist model. The transcend model predicts small, cold, compact civs that would be very difficult to detect directly.

That being said, if aliens exist, the evidence may already be here, we just haven't interpreted it correctly.

[-]The_Jaded_One9y10

Rare bio origins is out.

Really? Why?

[-]jacob_cannell9y50

We keep finding earlier and earlier fossil evidence for life on earth, which has finally shrunk the time window for abiogenesis on earth down to near zero.

The late heavy bombardment sterilized earth repeatedly until about 4.1 billion years ago, and our earliest fossil evidence for life is also now (probably) 4.1 billion years old. Thus life probably either evolved from inorganics near instantly, or more likely, it was already present in the comet/dust cloud from the earth's formation. (panspermia)

With panspermia, abiogenesis may be rare, but the effect is similar to abiogenesis being common.

[-]The_Jaded_One9y20

I think Robin Hanson has a mathematical model kicking around that shows that, given anthropic selection bias, early life on earth is not evidence that life is an easy step.

I think the argument is that if you need (say) five hard steps in sequence to happen for technological civilization to arise, and each step succeeds very rarely, then if you look at the set of all planets where the first step succeeded, you will see that it is unlikely to happen early.

However, if you look at the set of planets where ALL five steps happened, you always tend to find that the first step happened early! Why? Well, because those were the only ones where there was even a chance for the other four steps to happen.

Anthopics then comes in and says that we are guaranteed to find ourselves on a planet where all five steps happened, so seeing the first step happen quickly isn't really evidence of anything in particular.

[-]jacob_cannell9y00

"Anthropic selection bias" just filters out observations that aren't compatible with our evidence. The idea that "anthropic selection bias" somehow equalizes the probability of any models which explain the evidence is provably wrong. Just wrong. (There are legitimate uses of anthropic selection bias effects, but they come up in exotic scenarios such as simulations.)

If you start from the perspective of an ideal bayesian reasoner - ala Solomonoff, you only consider theories/models that are compatible with your observations anyway.

So there are models where abiogenesis is 'easy' (which is really too vague - so let's define that as a high transition probability per unit time, over a wide range of planetary parameters.)

There are also models where abiogenesis is 'hard' - low probability per unit time, and generally more 'sparse' over the range of planetary parameters.

By Baye's Rule, we have: P(H|E) = P(E|H)P(H) / P(E)

We are comparing two hypothesises, H1, and H2, so we can ignore P(E) - the prior of the evidence, and we have:

P(H1|E) )= P(E|H1) P(H1)

P(H2|E) )= P(E|H2) P(H2)

)= here means 'proportional'

Assume for argument's sake that the model priors are the same. The posterior then just depends on the likelihood - P(E|H1) - the probability of observing the evidence, given that the hypothesis is true.

By definition, the model which predicts abiogenesis is rare has a lower likelihood.

One way of thinking about this: Abiogenesis could be rare or common. There are entire sets of universes where it is rare, and entire sets of universes where it is common. Absent any other specific evidence, it is obviously more likely that we live in a universe where it is more common, as those regions of the multiverse have more total observers like us.

Now it could be that abiogenesis is rare, but reaching that conclusion would require integrating evidence from more than earth - enough to overcome the low initial probability of rarity.

[-]The_Jaded_One9y00

The posterior then just depends on the likelihood - P(E|H1) - the probability of observing the evidence, given that the hypothesis is true. By definition, the model which predicts abiogenesis is rare has a lower likelihood.

We are in a vast, seemingly-empty universe. Models which predict the universe should be full of life should be penalised with a lower likelihood.

Abiogenesis could be rare or common ... it is obviously more likely that we live in a universe where it is more common, as those regions of the multiverse have more total observers like us.

Those regions of the multiverse contain mainly observers who see universes teeming with other intelligent life, and probably very few observers who find themselves alone in a hubble volume.

But this is all a bit off-topic now because we are ignoring the issue I was responding to: the evidence from the timing of the origin of life on earth

[-]jacob_cannell9y00

We are in a vast, seemingly-empty universe. Models which predict the universe should be full of life should be penalised with a lower likelihood.

The only models which we can rule out are those which predict the universe is full of life which leads to long lasting civs which expand physically, use lots of energy, and rearrange on stellar scales. That's an enormous number of conjunctions/assumptions about future civs. Models where the universe is full of life, but life leads to tech singularities which end physical expansion (transcension) perfectly predict our observations, as do models where civs die out, as do models where life/civs are rare, and so on. . ..

But this is all a bit off-topic now because we are ignoring the issue I was responding to: the evidence from the timing of the origin of life on earth

If we find that life arose instantly, that is evidence which we can update our models on, and leads to different likelihoods then finding that life took 2 billion years to evolve on earth. The latter indicates that abiogenesis is an extremely rare chemical event that requires a huge amount of random molecular computations. The former indicates - otherwise.

Imagine creating a bunch of huge simulations that generate universes, and exploring the parameter space until you get something that matches earth's history. The time taken for some evolutionary event reveals information about the rarity of that event.

[-]The_Jaded_One9y00

"anthropic selection bias" somehow equalizes the probability of any models which explain the evidence is provably wrong.

Of course! That would be ridiculous! There are infinitely many models that can explain any given evidence set!!! I have no idea where you got that from though - not from me I hope!

[-]jacob_cannell9y20

So the fact that intelligence took this long to evolve - 4-5 billions of years after biogenesis, and 600-700 million years after the first multicellular animals - must be important.

~5 billion years out of an expected ~10 billion year lifespan for a star like the sun - mediocrity all the way down!

[-]turchin9y20

Sun luminosity is increasing and oceans will boil down in 1 billion years or sooner.

[-][anonymous]9y30

Correct. We are somewhere between 250 megayears and 2 gigayears away from the Earth becoming another Venus depending on whose models you look at (the runaway greenhouse being one of 2 or 3 endpoint outcomes for a terrestrial planet given enough time).

This being said, the whole of earth's history might not be relevant to look at for complex life. Eukaryotes are OLD, gigayears old, but there's a set of paleontologists who think that the Cambrian diversification of macroscopic animals 550+ megayears ago might have been CAUSED by increasing oxygen concentrations which might have something to do with the running down of Earth's geology. More on this in another post; for now I recommend the book "Oxygen: A 4 Billion Year History." If one uses this notion of a windowed subset of time when complex life is possible we could be roughly in the middle of THAT.

[-]turchin9y00

Thanks for the link on the book.

I think that intelligent life tends to find itself not in the middle but near the end of any "a windowed subset of time when complex life is possible". One reason for it is observation selection and some considration about distribution of peroids of stability. The longer periods of stability should be more rare. The large catastrophe may be overdue and could be easily triggered by our actions.

Another reason is more speculative (if anything could be more speculative here). It suggest that large changes in climate helps human evolution as they favour more universal ways of fitness, that is human intelligence. But such unstable environment may be near the end of stability period needed for evolution of complex life. (Think about recent Ice ages in last several millions years that coincide with development of Homo.)

My earlier article on the topic is here (need to be retranslated and rewriten):https://www.scribd.com/doc/8729933/Why-anthropic-principle-stopped-to-defend-us-Observation-selection-and-fragility-of-our-environment

[-]DanArmak9y00

Wouldn't mediocrity imply intelligence evolving many times in the Earth's past, which would imply a Great Filter in our near future? See also my other comment.

[-]jacob_cannell9y40

Depends on what you mean by 'intelligence'.

If you mean tech/culture/language capable, well it isn't surprising that has only happened once, because it is so recent, and the first tech species tends to takeover the planet and preclude others.

If you mean something more like "near human problem solving capability", then that has evolved robustly in multiple separate vertebrate lineages: - corvids, primates, cetaceans, proboscids. It also evolved in an invertebrate lineage (octopi) with a very different brain plan. I think that qualifies as extremely robust, and it suggests that evolution of culturual intelligence is probably inevitable, given enough time/energy/etc.

[-]The_Jaded_One9y00

I expect that we will find a lot of planets without life, and of the rare occasions we find life, it will be algae-like.

[-]jacob_cannell9y00

I assume by 'algea-like', you actually mean cyanobacteria. The problem is that anything that uses photosynthesis creates oxygen, and oxygen eventually depletes the planet's chemical oxygen sinks, which inevitably leads to a Great Oxygenation Event. The latter provides a new powerful source of energy for life, which then leads to something like a cambrian explosion.

The largest uncertainty in these steps is the timeline for oxygenation to deplete the planet's oxygen sinks. This is basically the time it takes cyanobacteria to 'terraform' the planet. It took 200 million years on Earth, but this is presumably dependent on planetary chemical composition and size.

From the known exoplanets, we can already estimate there are on the order a billion-ish earth-size worlds in habitable zones. By the mediocrity principle, it's a priori unlikely that earth's chemistry is 1 in a billion. Especially given that Mar's composition is vaguely similar enough that it was probably an 'almost earth'.

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