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.

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.