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  • Writer's pictureRaelyn Teague

Worldbuilding Planets |Worldbuilding 4|

Last episode we took a look at making a sun or two to keep your characters toasty during the day, but who cares about that if your characters are floating aimlessly through space? Maybe your entire novel takes place on a spaceship your characters never get to leave, but most of you probably want an actual planet for your characters to explore. So that’s what we’re going to take a look at today.

First, my usual disclaimers. This is an overview of what planets are and how they work, but if you want an in-depth look at how to make colonizing Jupiter an option, you’ll need to do further research. Again, the majority of my focus will be on how an Earth-like world works, but if you want to get creative, I encourage you to think of how changing one of Earth’s rules or features might make for an interesting setting in your novel. I also encourage you to think through what the consequences of changing that one thing might be, because each change you make is likely to have some kind of ripple effect. That could lead to a lot of fun worldbuilding, or it might give you a headache and a novel with a setting that doesn’t make sense. So think it through. (Not that every aspect of your worldbuilding has to be completely plausible. It’s okay to have a whimsical world that doesn’t follow the rules of physics if that fits the tone you’re going for.)

With that out of the way, lets take a look at:

Earth. Our World. Our home. The probable setting for a near-future apocalypse brought on by climate change.


Every story needs a setting, and for the majority of novels surviving to this day, that setting is somewhere on Earth. Heck, even when we set our stories in made-up worlds, they’re usually an Earth-like substitute.

But do they have to be? Do we even want to make them different? How could we make them different?

The short version is no, maybe, and in so many ways it’ll make your brain hurt.

Let’s break this down into smaller chunks, shall we? First, what even is a planet?


There are a few contested definitions used to differentiate planets from other celestial bodies. According to the International Astronomical Union (IAU), there are three requirements a celestial body must meet in order to qualify as a planet.

First, they must be in orbit around a sun.

They also must have cleared the neighbourhood around their orbit. This basically means they have become the dominant celestial object in their orbit and no longer compete for space with other major bodies. They’ve moved all that stuff out of their path. This, by the way, is the reason Pluto is no longer considered a planet. It hasn’t yet cleared its neighbourhood.

Finally, the planet must be round, or at least nearly round in shape.

Sorry to any Flat Earther’s out there (well, not sorry actually) or to any sci-fi writers who wanted to build a frisbee-shaped world on the backs of some elephants à la Terry Prachett’s Discworld, but technically, it’s not a planet if it’s not round.

This isn’t exactly discrimination against celestial bodies with disc shapes, lumpy shapes, or any other shape. It’s more about the mass of the planet. Any celestial body with enough mass to develop its own gravitational pull will eventually form into a sphere or near-sphere.

This means speculative fiction writers out there don’t have to feel like unoriginal hacks because they defaulted to having a round planet. If you want to have a world with gravity, an atmosphere that doesn’t float away into space, and any number of things we take for granted, you’re going to want a spherical world. At least you will if scientific plausibility is your goal, but I remind you, it’s okay if it’s not.

I fully encourage anyone out there who wants to set a story on an other-shaped world to have fun with it! The popularity of the Discworld series obviously didn’t suffer because it was set on a, well, disc world. That completely fit with the fun, humorous tone Prachett used. So don’t feel constrained to the globe model.

Just…your world won’t technically be a planet by the IAU’s definition. That’s all.

However, the IAU’s rules are debated, and some astronomers believe whether or not a celestial body should be granted planet status is better defined by another set of parameters: the geophysical definition of a planet.

Using the geophysical definition, a planet can be any celestial body that isn’t so large it has ever undergone nuclear fusion, which would instead make it a star and not a planet, but that is still large enough that its own gravity has formed it into a sphere.

Sorry, folks. Even under this definition Flat Earth still isn’t a planet. However, many other celestial bodies would be considered planets. Not only would Pluto regain its lost title, but our moon and many other celestial bodies would also qualify for planetary status.

You may realize, then, that both the geophysical definition and the IAU’s definition allow for a lot of variation. Even within our own solar system, our planets are so varied in mass, composition, length of orbit. What’s the best setting for your story? For most novels out there, the answer is simple: you’re probably going to want to set your story on a terrestrial, or rocky, planet like our dear Mother Earth.


The standard type of terrestrial planet has a metallic core (mostly made of iron), a silicate mantle, and a secondary (rather than primary) atmosphere. What does that mean? The short version is that secondary atmospheres are much thinner than primary atmospheres and are formed either by the debris left behind after the impact of a comet or by the planet’s own volcanic activity. Volcanoes release a number of gasses that then become part of the atmosphere.

If a terrestrial planet has tectonic activity, like Earth does, or an erosive liquid, like water, that’s how they get to have all sorts of cool geographical features like mountains and hills and valleys.

Looking to get a little more exotic with your invented world without going too far off the beaten path? Although, for now, they’re only theoretical, there are also three other kinds of terrestrial planets apart from silicate planets like our own. There are carbon planets (metal core made mainly of carbon), coreless planets (a silicate planet with no core), and iron planets (planets are almost entirely made of iron).


If you look at carbon, coreless, or iron planets and think you want to get more exotic still, there are lots of other kinds of planets to choose from. It would be madness for me to try to go thoroughly through each one in a single episode, which I don’t think will be necessary for most people’s purposes anyway, so here are just a few different kinds of planets that may interest you.

When classifying planets according to mass, on the “really big” side of the spectrum you’ve got your giants. You’ve got gas giants like Jupiter and ice giants like Neptune. You might also come across terms like “super-Earth,” which is a planet with a mass more than Earth’s but less than the giants, or “sub-Earth,” which is likewise a planet much smaller than Earth.

Classifying planets according to composition gives us even more kinds of planets. Many, many more. Again, here are just a few different kinds of planet compositions you might want to consider for your worldbuilding.

You have the carbon, coreless, and iron planets I already mentioned. Gas giants or gas dwarfs. Ice giants. But you also have a number of theoretical planets that may provide an interesting setting for you fantasy or sci-fi writers out there, such as a desert planet.

A desert planet is a terrestrial planet with, and this may shock you, deserts! Which is another way to say this kind of planet has very little water. How would beings on this planet adapt to life on this world? Could make for an interesting story.

On the flip side of desert planets, ocean planets are for anyone out there who wants to have a water world, as much of this planet’s mass would be comprised of water. We’ve seen stories about water worlds before, but there’s so much more that could be done with a setting like this. Maybe this is where mermaids come from, and the reason we have mermaid mythology on Earth is because the aliens who built the pyramids brought stories of the other planets they’d been to.

And for the last planet type I’m going to bring up (because it’s my favourite), we have lava planets. Lava planets are believed to have surfaces covered mainly by, well, lava. I have no idea how life would exist on a planet like this, but I definitely want to see someone else’s take on it.

Okay. So now maybe you’ve chosen the kind of planet you want to have, but there are still things left to consider, such as…


When designing your planet, you’ll want to make sure you take careful consideration of your planet’s orbit. This isn’t just so you know how many days to count down until your world’s version of Christmas or Halloween comes around, though if you’re designing your own cultures with holidays and calendars of their own, that might be reason enough. But how far your planet is from the sun, whether their orbit is circular or more eccentric, or whether their orbit is influenced by other objects in space will probably affect your seasons (and therefore food production) and cultures.

The safest route for authors who want an Earth-like planet is to have a circular orbit within the solar system’s habitable zone. Again, you’re not an unoriginal hack if this is the route you choose. This is the route I usually choose, and it’s the route most writers choose. You’re in good company.

If, however, you want to give your planet a more extreme orbit, such as an elliptical orbit, it will provide you with many interesting challenges. How do beings on your world survive getting closer and then so far from their sun each year?

Or maybe, instead of a planet that circles the sun, you want to set your novel on a rogue planet. Rather than a star, these planets orbit the galaxy itself and would come with an array of fascinating worldbuilding challenges and potential story ideas.

Maybe you’d rather set your novel on a double planet. No, this is not a double-sized planet, but rather it is two separate planets that orbit each other as they go around the sun. Is the other planet also habitable? Do the beings on each planet know there’s life on the other? Can they travel from one planet to the other in your universe?

Speaking of more than one planet orbiting the sun together, you also have the possibility of trojan planets. In astronomical terms, trojans are usually asteroids that orbit the sun on the same path as another celestial body, following the path either a bit ahead of or a bit behind said body. Earth itself has a trojan asteroid that follows us around the sun. Having a stable trojan relationship is dependent in part on the masses of the sun, the trojan, and the celestial body it shares its path with, which means that most known trojans are asteroids or co-orbital moons and not full-sized planets, but for any speculative fiction writers out there, I see no reason why you couldn’t make a trojan planet work for you.


I’m going to speak more on axial tilt when I get to my video on climate, but I thought I should put a pin in it in this video so you can start thinking about it. You don’t have to know the exact degree to which your planet tilts, but a less or more extreme angle will absolutely affect what the days and nights look like on your world and how extreme the climate is. So keep this in the back of your head while you’re brainstorming.


Now that you hopefully have some ideas for the planet your characters will inhabit, what about the other planets in your solar system?

As I mentioned with stars in the last episode, this is another aspect of worldbuilding I won’t blame you for skipping if you’re pressed for time or your characters never travel to any planet but their own. Even if you don’t want to create the entire solar system your world fits into, consider whether or not you might want to have a planet or two entering your night sky. A few months ago we had the Great Conjunction of Jupiter and Saturn. That was pretty cool. Maybe you have a doomsday cult in your novel that sees the aligning of two planets as a sign they need to bring about the apocalypse. Because doomsday cultists are cheery like that.


For this week’s challenge I want you to take a look at our fair Earth. Think about one thing you’ve probably taken for granted about our planet. Gravity. Oxygen. The mix of land and sea. The fact that Earth is round. As a thought experiment, choose one of those things you’ve taken for granted and change it. Gravity is now half what you’re used to, or maybe the oxygen levels are twice what you’re used to. Try to think of at least three things that would be different about life on Earth as a result of that change.

If you picked a hard thing, you may find yourself doing a little research, but hopefully this will get you questioning the world around you and thinking of ways to make the world in your story interesting and thought-provoking.

That’s it for this week. We’ve still got a lot more worldbuilding to cover, so I hope you’re enjoying it so far. I will see you in the next episode.


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