World Building for Science Fiction

Planetary formation and the evolution of life


Yeah, yeah, governments, weird customs. and what do the girl Squidiods wear to the Royal Squidiod Ball . . . all well and good, but first you have to have a place to put them.

So let’s take a look around and see what’s realistic in the way of planets.

It all started with the Big Bang 13.7 billion years ago.

Things were pretty messy for awhile, but by 9 billion years ago, things were settling down to reasonably recognizable situations. The stars were mostly in galaxies, and there’d been enough supernovae to seed the universe with heavier elements, so planets were forming.

Everything else out there, that your characters are going to be living on, exploring, or running from is probably all less than nine billion years old.

The Sun coalesced and the planets of the solar system aggregated around 4.56 billion years ago.

So lets make a totally unfounded assumption: Earth is average. In an average planetary system.

I mean, really, it could be quite extraordinary and unique. But the safe bet is that it’s not all that unusual. So with that in mind . . .

Let’s look at the solar system. Rocky worlds, gas giants, icy worlds. And lots of smaller rocks and ice orbiting mostly in certain areas.

We’ve got four rocky planets, ranging in size from less than 5000 km to 12,700 km. There are six large moons, but only three of them have a high enough density that they are probably mostly rocky rather than mostly icy. So let’s call it seven big rocks. Tending to be close to the sun. Seems like the SF writer is safe in assuming lots of rocky planets happen, and finding several in a single system is no big deal. Three out of seven (Earth, Moon, Mars) are within the “habitable zone” so that’s common enough. And the planet just needs to be big enough to hang onto a good sized atmosphere (Earth and Venus). Find one in the habitable zone and you’ve got a good start on a new home.

Gas Giants. Three. Further from the sun. Yep. Rev up your plans for Cloud City.

Icy worlds. They tend to be small, even the largest are called minor planets, and not too surprisingly, farthest from the sun. There are lots of them. Don’t leave them out of your fictional planetary systems.

The smaller chunks are found in the main Asteroid belt, the Kuiper belt and the Oort cloud. The gravitational effects of the larger planets probably both “herd” them into these regions, and occasionally toss one out. If you want to write about mining in space, you’ll find just about anything you want to base your story on. Do keep in mind that they’re spread out over a huge distance. It’s not like Star Wars.

So, lets look at the search for extra solar planets and see if we can tell if our system is typical.

Wiki claims that as of a few weeks ago, they’ve discovered 2062 planets in 1310 planetary systems. The Kepler space telescope’s running an average of more than one planet per star. 1 in 5 Sun-like stars have a “Earth sized” planet in the habitable zone. Earth sized, in their survey was one to two Earth radii. And that’s just what they can detect from here.

So. This is looking very promising. There is nothing weird, that you would have to explain away, in writing about earth-like planets around sun-like stars. Unlike the solar system, gas giants in the inner planetary system appear to be reasonably common.

Now, let’s focus in on the Earth. The main differences between it and the other rocky worlds is its favorable distance from the Sun, a humongous big moon that nearly qualifies us as a double planet, a nice thin atmosphere, and a strong magnetic field to protect us from the more energetic output of the sun. I suspect the magnetic field is going to be fairly important in the development of advanced lifeforms, but only going out and taking a look will answer that question. The Moon was possibly formed when the proto-Earth was apparently struck by, well, another proto-planet, knocking off a great deal of material as they merged. This also probably knocked off a lot of the early atmosphere. So we aren’t a hot hell like Venus. Maybe. It’s all theory. It’s one of the few things I can see as being rare, in planetary formation. Some day we’ll find out if it’s crucial.

So. The Earth formed 4.5 billion years ago. As soon as there was liquid water, there was life. OK, not “presto!” but it happened very quickly on these sorts of time scales. So quickly that we have to consider that the step between complex chemistry and life is easily made. If your imagined planet has liquid water, you don’t have to explain *why* it has life, you would have to explain why it does *not*.

So various microscopic single celled things ran around, eating each other or maybe metabolizing sulfur or some such. No doubt gaining complexity, and mutating and evolving all over the oceans until one of them started this interesting chemical process using sunlight and CO2. That nasty by product, free oxygen, was quickly snapped up by the dissolved iron in the seas. It started raining rust, in the oceans 3.7 billion years ago. These days geologists call these rock beds “Banded Iron Formations.” The rest of us call it iron ore and find it very handy. Two billion years later, all the iron was used up, and the oxygen started accumulating in the oceans and atmosphere.

Nasty toxic stuff, that oxygen. Some cells handled it by doubling their cell walls, other cells moved in to that buffer space to take advantage and became symbionts, and then indistinguishable from parts of a single complex cell. Eukaryotes. It’s what you’re made of.

Another billion years and you get multicellular organisms.

Add them up and you get 83% of the age of the Earth having nothing but single celled organisms. And the first 60% the air wasn’t even breathable.
For the Science Fiction writer, these stats are rather dismaying. “Wait, I have the right size planet, the right distance from the right sized star and you say there’s a better than 80% chance my space marines will find nothing but stinking algae washing up on the beaches to fight? What kind of story is that going to make?”
Fear not! After we get to the multicellular level things speed up. I blame it all on sex, myself. Once you start mixing and matching genes between critters, changes can happen more rapidly.

You’ve got at least a 10% chance of finding land plants. And insects have been around for about as long.

Land animals—They only really got going in the last 5% of the Earth’s history. They went for size pretty fast, as in, dinosaurs. After that they got a bit smaller. Except in the oceans.

Intelligent life—This is definitely the realm of the imagination. There are so many extinctions in Earth’s history . . . you could easily argue that they drove evolution *or* set it back several times. But, even counting Austrolopithicus, you’re looking at intelligent life existing on Earth for less than 0.1% of the time.

Life of a planet

So where does this leave the SF writer?

With lots of planets. With lots of primitive life. Some with breathable air, some just getting there, but a good resource for colonists with domes. With a few having interestingly complex life, that can be saved, exploited, replaced, poisonous, edible, medicinal, dangerous . . . A few worlds with large dangerous critters . . . and ever fewer with intelligent life. But . . .

It happened here, and we’re *average*. Right?

Just a few words like “rare” and “incredibly lucky find” or perhaps “older than the Earth, it had evolved advanced . . . ” and even a geologist like myself will be happy to follow you wherever the adventure leads.

Jump forward a couple of thousand years, and those “algae worlds” have been terraformed, the humans have forgotten their origins and rediscovered space travel and are aggressively attacking other old colony worlds. Or need to be actively recruited to form a united defense against some real aliens. Or their former pets are tracking down the Myth of the Old Earth . . .

The possibilities are endless, and all you need to do is avoid the *definitely* impossible.


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86 responses to “World Building for Science Fiction

  1. Draven

    The thing is, Venus is earth-sized and *just* outside of the habitable zone. Barely. Some even say it *is* in the habitable zone, but because there was never life, there was nothing to lock up the carbon, etc.

  2. Reality Observer

    I love the sound of a potential customer… So much like the background I’m picking away at. (Or is that “plucking?”)

    There is one more factor (at least from my research), which makes that proto-planet collision of ours rather more critical. The theory goes that it also thinned out the crustal material (silicates, etc.) rather drastically, making our very tectonically active planet possible. (OK, just had to add “tectonically” to the dictionary. Maybe I’ll find out what the heck its suggestion of “tectonic ally” is supposed to mean someday…) Tectonics is very important to atmosphere and hydrosphere composition.

    Reason that most of my colonies are on worlds circling massive primaries. Then, of course, I have to keep them out of the radiation belts. Sigh…

    • I’ve wondered if the tidal flexing hasn’t kept Earth’s interior a bit more malleable than it otherwise would have been. And whether this had any relevance in concentrating iron in the core, which is critical to the magnetic field.

  3. Pingback: World Building for Science Fiction | madgeniusclub – SciFi Picks

  4. Laura M

    If anyone is ever in DC the Air and Space Museum has a show in the planetarium of the formation of the moon. You sit back in those nice lounge-y chairs and watch the movie on the ceiling. It’s stunning. Highly recommended.

  5. Actually, even the completely impossible is okay if you set it in a special universe, or maybe have the Old Ones jiggling the odds, or…

    The improbable we accept easily, the impossible takes a couple of extra strokes to get that gnat going down…

    • That’s the difference between SF and Fantasy. In Science Fiction, the “weird stuff” is explained away as a tech breakthrough. In Fantasy . . . well, even there you can evoke “Parallel Universe” so the border is quite fuzzy and shifting. So perhaps the main difference is the author attempting a scientific rational, versus a straightforward acceptance of Old Ones, magic, and elves.

      • Or you go my route with Science Fantasy, where the magic is all based on scientific principles and the “wizard” can explain how he’s using thermodynamic principles as he harnesses kinetic energy to do work or a “sorcerer” utilizes a creature’s chemical energy to his own ends…

        Yeah, sadly the term Speculative Fiction has such a tarnished name because of its use by the Literary Fiction crowd, because it really does best describe Fantasy and SF genres, we’re all speculating on the “what ifs” in our writing worlds.

      • I think for enjoyable fantasy there must be a constant way for that world to work. It has magic? How does that magic work? What are the laws of magic? What are the limitations? Most importantly, has this all set up prior to the hero pulling out a magic whatzit to get out of a corner the author finds he’s painted himself into?

  6. Nice to see someone encouraging authors to use real science in books. I try to. Sometimes it engages the reader, sometimes it turns them off. There’s a fine line between using real science (with a dash of imagination!) and becoming preachy. Science buffs will walk that line with you; others seem to resent it.
    As for Venus, it may well be livable, one day. My WIP, title NFI (with a spaceship on the cover, foreground, and the Earth and Luna in the middle distance and background, respectively, so readers won’t just dismiss a three letter title! It’s the second book in the New Frontiers Series) will deal with early steps in terraforming Venus. It’s not a major thread in the current plot, but it will become part of book three or book four in this series. It practical terms, even if my system was workable, terraforming Venus will probably take at least a few hundred years. Biological processes start slow and proceed at their own pace.

    • Christopher M. Chupik

      Venus could take more than a few centuries, then again, maybe not. A lot of terraforming scenarios seem predicated on the idea that there won’t be technological advances and breakthroughs during the centuries needed to terraform a planet. It could start slow and move forward at an accelerate pace in the later stages.

      • I figure, like most things, that last 10% will take the longest and be the most crucial. Establishing a healthy ecosystem (don’t want it overrun by rodents or giant insects), stabilizing the weather (got to be some extreme effects from massive changes in the planet’s atmosphere and temperatures), and getting the entire system stable and livable. It’s all the fine detail stuff that always takes the longest on any project.

    • Now I’m curious about how you’re going to cool it down.

  7. One of the ideas driving the development of the Azdhagi was what if the reptiles won? No Chixulub, no Age of Mammals. Instead you had an intelligent pack predator becoming sapient, slipping from bipedalism back to being predominate quadrupeds, and taking over the world.

    As trivia, reptilian and mammalian digestive systems and metabolisms are such that the mammal who moves in is going to have a steep, occasionally near-lethal learning curve about what is edible.

  8. Paul (Drak Bibliophile) Howard

    It’s an older work but IMO still holds up.

    • Jamie

      There are some [exterior] revisions to it, with respect to super earths. Basically, if the super earths that Kepler is finding are greater than 1.6 earth masses, they enter Neptune territory. They won’t be rocky like earth. I got this from an astronomer’s blog,

      • I think Venus demonstrates that the mass threshold for building up thick atmospheres is quite low. Too hot for liquid water, no biological processes turning CO2 into a solid, and you have a hot hell.

        I suspect that the angular momentum of the collapsing gas cloud has a lot to do with the number and position of the gas giants in a system. If my logic is sound, low angular momentum system would collapse into the proto-star sooner and trigger fusion earlier in the planetary formation process. The stellar wind would drive the lighter elements out of the inner system. High angular momentum, slower infall, and greater likelihood of inner system gas giant formation prior to fusion.

        Meh. Something like that.

        • Weird thought: First generation stars were all hydrogen until they started fusing it into helium and other stuff. That rules out rocky planets around first generations stars, but not gas giants of hydrogen.Or is that off the mark? And if some of these gas giants were ejected from their system, couldn’t some pure hydrogen rogue worlds still be out there?

          • Meh, supernovas are rough on the neighborhood. But yes, a hydrogen cloud collapsing into an all hydrogen planet is theoretically possible. I suspect though, that a lot of planetary formation is triggered when the temperature of the cloud falls below the freezing point of water, and dust and more ice start sticking together, gradually building up a gravity well to trap more of whatever it encounters.

            Planetary systems may have been rarer without the nuclei of heavier molecules to coalesce around. Another thing we may, eventually, find out.

            • I really need to brush up on cosmology. I’m assuming stars happened because of uneven distribution of hydrogen, and the record of that is in variations in the background radiation.

              In truth, didn’t think about nucleation at all. Was just thinking along the lines of that poem:

              “For big whorls have little whorls
              That feed on their velocity,
              And little whorls have lesser whorls
              And so on to viscosity.”

              Which probably isn’t applicable at all. Was just thinking of whorls concentrating hydrogen enough to draw in more, but it might not be enough.

              BTW, on ejected planets, was thinking about theories that planets can be tossed out of solar systems through interactions with other planets as things get sorted out.

              • Yeah, I’m more than a little out of date. IIRC, the prevailing theory involves and smaller universe, so denser to start with, and either gravitational collapse of a massive gas cloud or shock waves compressing gas clouds enough to form gravity wells deep enough to hold H2 and collect more, or both.

    • The Other Sean

      Less troublesome than To Serve Man, certainly. 😉

  9. In building worlds, one of the peeves I have is ignoring any sense of realistic travel times between worlds. Sure in the good old days Flash, Buck, or Dan just jumped into their rockets, lit the candle and a short while later crashed into Venus, Mars or Mongo. But the average Hohmann transfer time going one way between Earth and Mars is about 9 months. So any intrepid space explorer that needs to dash off to Mars in a hurry to rescue a scantily clad space princess needs to file his flight plan a couple of years in advance. And being packed like spam in a can for months on end is, let’s face it, boring.

    So I tried to think of a way to get the travel times down to a more manageable level by making smaller solar systems.

    I tried to flesh out this idea in some blog post that you might find interesting.

    Designing a Miniature Solar System – Part 1

    Designing a Miniature Solar System – Part 2

    Designing a Miniature Solar System – Part 3

  10. Jamie

    I love science fantasy, and adventure SF, and am glad to see them coming back. For a while it seemed Mundane SF was everywhere I turned. I am not a scientist at all, so I’ve gathered some resources for myself for world-building. In case there’s a limit on links in a post, I’m only going to list the Google search terms/names.

    -Prof. Elizabeth Ann Viau’s is a project she designed for a class at California State U. Starting with the first lesson unit (solar system), she gives a step by step planet design using “friendly” math to determine distance, planet’s year, etc. How large is its moon in the sky? What color will the moon be (based on what its made of)? A writer who uses her lessons will avoid pitfalls, like putting a desert next to a rain forest 🙂 She doesn’t mind if you copy the site; so I took her lessons and turned them into a Scrivener template for myself.

    -Astrographer’s bookshelf has a roundup. There’s a commonly-cited world cookbook included, too, for world builders.

    For up-to-the minute numbers,

    -Ravi Kumar Kopparapu, an astrophysicist who has determined a revised limit for the Goldilocks zone of earth-like worlds: between .99 and 1.688 (1.7) AU to avoid greenhouse water loss. This is to be used in the insolation formula (I=Luminosity of the star /Square of the Distance in AU) to find the planet’s zone for the chosen star.

    -Joni Clark and Paul Mason, astrophysicists who have researched new figures for the habitable zones for binary stars. The article I found was at

    -The first Google result for Project Rho is the one you want. Plot bunnies galore: the circumstances for having a “Mos Eisely hub of wretched scum and villainy” on a section of your space station, a tourist season to Mars if you’re using Hohmann transfers, and much more.

    I haven’t really found any resources on designing plausible alien physiology, though. I want my own Avariel (reference for Baldur’s Gate 2 fans).

    • A writer who uses her lessons will avoid pitfalls, like putting a desert next to a rain forest

      And now I want to make a world where there’s a lush rainforest on the leeward side of a tall mountain range, and a bone-dry desert in the rain shadow. 😉

      • Mary

        I’m a fantasist. I don’t need the mountain range. 0:) Just a suitably motivated wizard.

      • Jamie

        Heh. I was thinking of an essay I read long ago, which I think was an open letter to Christopher Paolini from … a fantasy writer? A fantasy fan? Apparently Paolini had randomly placed different biomes in his books, which is forgivable for a teenager; the essayist suggested he learn where they should properly go. If nothing else, I figure it could make an interesting clue about a wizard’s power if an environment is “wrong” for its location 🙂

      • Christopher M. Chupik

        Or make it a plot point:

        “Why is there a desert a few miles from the jungle?”
        “Drunken wizards.”

      • Oh, you mean Washington State?

        Seriously, there’s a reason we refer to the Wet Side and the Dry Side.

    • I need to write some straight up SF. Space Opera probably, not MilSF.

  11. Won’t get into details with it here, because it’s long and verges into other things, but this gets into an itch of late: The whole infinite monkeys with infinite typewriters typing the works of Shakespeare thing. Of course, it deals with infinities and not finite time, but what would be the odds of randomly typing the works of Shakespeare? Assuming a 30 key typewriter, and that Sonnet 18 alone has 625 characters, the odds are about 1 to 1.5878 x 10^923. The universe has only been around about 4.3549 x 10^17 seconds.

    That’s just one sonnet. What are the odds of everything we see on earth arising, including 30 key typewriters?

    My point is that life might be very rare, even given a reasonable estimate stars with habitable zones. There’s about 400 billion stars in our galaxy, One estimate puts 1 in 5 stars with planets in a habitable zone. That’s 80 billion in our galaxy. But a habitable zone doesn’t mean a habitable planet – I’m looking at you, Mars and Venus – so the actual number is likely less.

    This gets into things that taxes what little I know about probability, not to mention we don’t know the odds of randomly spontaneously generating life. If it’s sorta kinda something like certain compounds forming crystals, then the odds are much less than purely random, but we still don’t know by how much.

    What we do know is that the odds seem stacked against life, not only by our own planet’s extinctions, but by things like gamma ray bursts. Then our life not only has to come into being, but become intelligent.

    Depressing, I know. I love SF stories with aliens, strange lifeforms, and authors that explore the boundaries of what’s possible. But if it’s all random, it might not be likely.

    Here’s something, though. Our sun may be a third generation star. If that’s the case, and if our parent second generation star had life, then other stars birthed in that stellar nursery might have life, too. SETI might want to take a look at identifying our sister stars and listening hard.

    Thoughts (as long as they steer clear of a religious discussion)?

    • “Life” simply defined, happened so fast that I give it a high probability of happening anywhere you’ve got liquid water. The so-called habitable zone will be tested before we leave the solar system. Europa, one of the moons I counted as a rocky planet, may have liquid water under the surface ice.

      It’s the advanced life that took so long to evolve. Individual mutations may have been random, but survival rates depended on their effects on the organism. Survival–and successful reproduction–drive evolution.

      • Paul (Drak Bibliophile) Howard

        In discussions about “where are the Intelligent aliens”, I’ve pointed out several “unknowns”.

        1) We don’t know how common life is.

        2) We don’t know how common multicellular life.

        3) We don’t know how common Intelligent Life is (which likely has to be multicellular life).

        4) We don’t know how common tool-making aliens are (intelligent wolves likely won’t be tool-makers).

        5) We don’t know how many tool-making aliens will form large societies.

        6) We don’t know how common the development of science is. There may be cultural factors that would prevent that development which would likely limit the aliens to steam-powered technology (at the best).

        Of course, when we get into the idea of space-traveling aliens, there are other unknowns such as the likelihood of political/cultural factors that would encourage the development of space travel.

      • We assume it’s easy because there’s things like evidence of life in 4.1 billion year old zircon and that’s only about 430 million years after that Mars-sized object slammed into the Earth (now wondering how long it would take the Earth to cool after that event). But what if we “won the lottery” and it’s harder than it looks?

        One of the things in the back of my mind is the apparent lack of continuing developing life. It seems the Earth wasn’t as hellish at that time as first thought, and had water and land areas – and of course little free oxygen, iron in the water, and whatever else the cyanobacteria was using.

        I don’t know and don’t have an argument, and wish I knew more probability to analyze multiple “trials”. For I know it was more than one chance per estimated 80 billion stars with habitable zones in our galaxy; it would be every location on every world where life could occur and the number of possible trials times the number of possible habitable planets.

        Doesn’t help that no one seems to be sure how life could spontaniously occur, and theories seem to shift. Once it was sea bed smokers. Don’t know what it is now. Just really, really wish we had harder numbers.

  12. “… better than 80% chance my space marines will find nothing but stinking algae washing up on the beaches to fight? What kind of story is that going to make?”

    See Kim Stanly Robinson’s Aurora for a nice answer to this question. 

  13. I found Stephen L. Gillett’s book _World Building_ from the Writers’ Digest sci-fi series to be a useful volume. Stanley Schmidt and Ben Bova’s _Aliens and Alien Societies_ was also pretty good. Both have nice lists of books, web-sites, novels ans stories, and other stuff in the back.

    • Gillett’s book allowed me to make a sot of “planet planner” spreadsheet that allowed me to plug one value into a variable and calculate others for a given planet and star. One thing it taught me is that smaller stars like Tau Ceti have habitable zones so close to the star that planets in the zone would be tidally locked, and thus ecologically peculiar. I recommend the book highly.

    • Jamie

      Thank you, I could not remember the names of those books and I was at work so I couldn’t just look at my bookcase. I bought them years ago, and I definitely found Gillett’s book particularly helpful. It’s now in Kindle form … I thought of the Aliens book as a jumping off place. I checked the references in the back last night, and now it occurs to me to look at the also-boughts on Amazon. I may just yet get my Avariel 🙂

  14. Mary

    In the latest Wearing the Cape book, some discussion is made of the Hyperion Theory, which is that life is not native to our universe but came from elsewhere, springing from an inability to find life anywhere else.

  15. Since I forgot the usual blatant self promotion, I’ll just leave this here and ruin my reputation as a serious scientist. 0:)

  16. mrsizer

    One of the things I enjoyed about the Lensmen series was that there were alien aliens, not just funny coloured people.

    I’m trying to figure out how create the technology of a water breathing race. Obviously fire is out, which rules out zillions of other things. I’ve been thinking of making them telekinetic, but that seems a bit cheaty. It’s not really relevant to anything I want to write – but I do need some aliens and I’d like something different.

    • Christopher M. Chupik

      Nope. Wrong. Sorry. The Received Wisdom of our wise and benevolent Social Justice class is that all aliens before the modern age were just thinly-veiled stand-ins for oppressed colonial peoples, promoting racism and heteronormative dominance.

    • Paul (Drak Bibliophile) Howard

      Chris Nuttall had an interesting alien species. The Males could dwell on land but the Females were much larger and while air breathers had to remain in the Ocean.

      Your “water breathers” likely won’t develop technology.

      An Ocean dwelling species that were air breathers but were able to live on land (for short periods) might develop high technology.

      • Draven

        I think that water breathers would depend on complex chemical reactions instead of fire.They could have learned such things watching deep smokers.

        • Toss them in a place like the Gulf of Mexico where you have a lot of tectonic activity, but reasonably shallow water, and you’d get a good chance for some interesting societies. You’re shallow enough for energy from the sun and you get the geothermal stuff as well. I could see cities being based around vents and rift zones. (This is what I’m doing with a fantasy race. I haven’t thought it through for an alien species.)

    • Lenses. Initially small ones ground from quartz and suspended above the water. Came about when someone noticed the effect of jewelry in the shallows. They can heat a small point in shallow water to boiling. Pump out water from an enclosure with a clear top and you have a small solar furnace. The problem comes if the aliens don’t have appendages to grasp things with.

    • Maybe their world evolved a coastal seaweed with fiberoptic capabilities and they’ve evolved a ceramic/lattice construction material backed on their own carapace structure: Space Nautilus!

  17. What Does a Martian Look Like?: The Science of Extraterrestrial Life (2004) by Jack Cohen and Ian Stewart

    Copies readily available used at nominal prices+postage

    • Hubble – one of the few government projects in recent memory that’s been worth the $$ and then some. 🙂 (And not just for the really cool images that you can buy posters of.)

  18. Peter

    If explorers landed on a planet with distinct life forms, we’ll call it Planet 770 (to pull a name out of the proverbial hat), would they be inclined to classify it as having intelligent life?

  19. Seriously, it’s a world, man. You should treat it with more respect,

    Call it “Gerrold,” or something…

  20. Pingback: What are the Odds? | Cheek's Bay

  21. I know, I’m a little late on this, but there was a presentation when I was in college that discussed mass extinctions. The lady giving the talk had found some interesting things. After a mass extinction event (such as the extinction of the dinosaurs) average recovery time (as defined by return to the same number of species previously populating the world) was 10-12 million years. After lesser events it was much longer (20 million years if I remember correctly, it’s been around 8 years so. There’s the salt.) I haven’t heard anything else on the topic since, but it may be that on worlds with larger numbers of mass extinctions, things would truck along faster due to the rapid speciation that happens after an extinction event.

    • Laura M

      Admittedly, this is a tangent, but your comments brought this to mind–the way Taylor Anderson never said that the asteroid didn’t hit in his parallel universe was awesome. WWII destroyers get swept into a parallel universe where there are intelligent (but mean) lizard people who must have evolved from dinosaurs. It’s very obvious to the modern reader that the asteroid didn’t hit, but it must have just about killed him not to have one of his characters say it. But, being WWII types, they wouldn’t know that. It was a lovely example of self-control coupled with a reader cookie.