When building a fictional world, one of the very first things to consider is what kind
of animals, plants, and people are going to inhabit it. Are the lifeforms the same as
or similar to those on found earth, or are they entirely different? What’s the climate like?
What kinds of biomes and ecosystems are present? Depending on the type and style of world you’re
building, the answers to these questions can vary dramatically, but for this series, we’re
starting totally from scratch. We’re going to evolve a scientifically feasible alien
biosphere using the principles of evolution.
Of course, this will all be entirely speculative, since so far, we only know of one instance
of life occurring in the universe, but any alien life would be subjected to the same
laws of nature as life on earth, so by studying our own biosphere, we can make some predictions
of what aliens might be like.
But before we can make an alien planet, we have to make, well… the alien planet…
as in all the non-living stuff. We’ve got to lay out some groundwork about what this
planet is like, since this will have a heavy impact on how its resident lifeforms will
evolve.
Now, planet-building can be a huge rabbit-hole in and of itself, and it’s kind of out of
my wheelhouse, which is why I’ve brought Artifexian along to help.
Hello!
So, where do we begin?
Best place to start is with a rough brief. What are you looking for?
Well, since I’m going to be evolving the biosphere from the very beginning of this
planet’s natural history, we might want to start out with something similar to the
conditions of ancient earth, but maybe tweak a few things to subtly influence the way life
develops; keeping in mind of course, that the further we stray from earth-like conditions,
the more complications are introduced, and the more speculative things will get when
designing the biosphere, so we should probably err on the side of too earth-like. Sound good?
Perfect! That’s very doable. I already have a bunch of neat ideas. But first, let’s
start with your star. Or stars …?
I think just one will suit our purposes just fine. Let’s not get crazy…at least not
yet.
Alright, one star it is. Let’s set the star’s luminosity to 75% that of Sun. Doing so mimics
the output of our Sun during the Archean Eon—2.5 - 4 billion years ago. With the luminosity
set, we can derive a number of stellar features, most notably your stars habitable zone—the
region around your star where liquid water can exist on the surface of a planet. No water,
no life so knowing this zone is important. If we have your planet orbit a distance of
0.85 AU, its average temperature will be about 20ºC which is hotter than Earth but nothing
crazy. This will also mean that its year will be roughly 298 Earth days long. Happy so far?
Yep, works for me.
With all that in the bag we can move onto the planet itself. To construct your planet
we need to figure out its mass, radius, surface gravity and density. All of which are related
like so.
Ooh!? Can we make it a low gravity planet? I quite like the idea of working with megaflora
and megafauna.
Definitely. We could keep your planet the same size as earth, again nice and familiar
but drop the gravity to 80% Earth gravity.
Hmmm, could we go lower?
Not really. Any lower and the density will be such that your planet won’t have an iron
core. No iron core, no magnetic field, no life.
Alright then, 80% gravity it is.
In terms of day length and axial tilt, we know that an Earth day was much shorter in
the past and that the earth’s axial tilt varies over time. So, let’s make your day
about 20 hours long and give your planet 22º of axial tilt. This lower axial tilt means
that overall your planet will be subject to slightly reduced seasonal variation. You ok
with that?
Yep, as long as there’s at least some seasonal variation, I’m happy. Now, what about my
atmosphere.
I suggest emulating Earth’s atmosphere as closely as possible. Messing with the atmosphere
can be … well … messy.
Some might say interesting …
Oh, boy, I don’t like where this is going. I mean, 1.5 billion years ago, oxygen was
super low and CO2 super high. So, riffing off that I can do something like this. But
that’s really not very …
Ah c’mon, surely we can do something more interesting than that! How about we say 1%
of the atmospheric composition is taken up by hydrogen sulphide?
You … you want to add hydrogen sulphide?!
Sure, I mean it exists in small quantities on earth, including as a metabolic byproduct,
and some organisms live off the stuff, so how about we throw in a tiny bit extra, just
to mix things up a little.
Ok, if you insist but just be aware that the oxygen is below what is recommended for carbon
based, humanoid lifeforms. The CO2 is far, far, far above what is recommended or safe.
And the hydrogen sulfide … well … it’s not recommended at all.
Don’t worry. I’ve totally got this… I think.
I hope so. Anyways, what’s left? A moon.
Yes, let’s just stick with one moon for simplicity, and can we make it so that it
creates fairly large tides?
Sure, one way of doing this is to place your moon close to your planet. Let’s say it
orbits about 50 earth radii out which will give it an orbital period of 22.97 earth days.
Also, let’s make your moon 50% more massive than our Moon. The increased mass and closeness
will give you tides about 2.7 times greater than those we experience on Earth. Now, obviously
this will vary wildly depending on local geography, nevertheless it’s a good ballpark.
Looks great! Speaking of local geography, I think the last thing I’ll need is some
landmasses. Since I’m going to explore a big chunk of the planet’s natural history,
I’ll need to know how the continents are going to drift over a period of a couple hundred
million years or so. As we’ll see, the shape of the land has a pretty huge effect on how
life develops.
Sure, what are you thinking?
Well, I thought it would be cool to start off with a pangea-like supercontinent.
Aha.
And then have it break apart into two continents
Yup.
Only for them to rejoin later.
Sure.
And maybe an island continent could also break off at some point too.
Done! You happy with
this timescale?
Yes. That looks perfect!
Cool, so there you go: a star, a planet, a moon, a totally non-controversial atmosphere,
and some landmasses.
Excellent! Thanks very much for all your help, Artifexian, it’s been a pleasure having
you.
Not a problem, the pleasure was all mine.
And if anyone is looking for more on how to construct their own alien planet, and if you’ve
somehow never heard of artifexian before, go and check out his channel for all the info
you could ever need. As for my planet, I think we’ve got just
about all the bases covered, so next up… let there be life!