Habitable Planets

Today we’ll discuss worldbuilding habitable planets, what makes them habitable in the first place, and why an atmosphere of sulfuric acid is PROBABLY not suitable for life.

Hey everyone, my name is Matthew, at least that’s what this planet’s inhabitants call me, and this post is part of a series where I will be going through a science-adjacent worldbuilding process step-by-step. Last time, we launched the universe into existence with its own big bang, and built the galaxy our soon-to-be created world will reside in.

For today’s discussion, we’re going to be worldbuilding… well, the world itself. We’re going to create our own fictional planet that we’ll be using as the base for all of our other worldbuilding moving forward, looking at aspects like planetary composition, atmospheres, and even how planets are formed in the first place.

Let’s zoom in on one of the stars in the outer galaxy. Meet Primis. Primis is a class B star, 6 times the mass of our real-life sun, located within the Parva Cattus nebula. Primis formed roughly 3.1 billion years after the big bang, or 1.5 billion years after the creation of the supermassive white hole at the centre of the galaxy. Don’t get too attached though, because after its very short 100-million-year life span, it’s about to go supernova. Woops!

Primis isn’t what we’re particularly interested in though, we’ll worldbuild the solar system we’re interested in soon. For now though, what we ARE interested in is one of the very recently constructed small planets it had juuust within its orbit has just been flung out into the galaxy as a rogue planet. Let’s call it… Locus, which is Latin for ‘The Place’, because this will be the place where all our worldbuilding will occur.

From here, we’re going to determine some values that we will need moving forward. I’m not going to discuss any of the maths, but I’ll show it on screen so that you can follow along yourself, and there’ll also be a link to a spreadsheet in the description that will do all the maths for you. First, we need to choose a mass and radius for Locus. Let’s say that it’s 1.279 times the mass of earth, and 1.137 times earth’s radius. From these two values alone, we’re able to determine a lot of information about Locus compared to earth, such as its gravity, circumference, surface area, volume, and density, as well as its density in grams per cubic centimetre.

We mentioned earlier that Locus is now a ‘rogue planet’, which is a free-floating or wandering planet that has no host planetary system. It might be still within a galaxy due to the gravitational pull of the central black holes, but it isn’t bound to a star or brown dwarf. Locus is going to spend the next billion years as a rogue planet staying within the nebula it formed in, which we’ll call the Parva Cattus nebula or “Small Cat” nebula. Eventually, it’s going to be picked up  by the gravitational pull of the solar system that it’s going to settle into. This is not an overly uncommon story across our real-life universe, though it’s overwhelmingly more common for planets to form alongside the stars that they surround in their solar system. If this is what you are planning for your planet, know that most gas giants form first pretty much alongside their star, and then rocky planets form within 100 million years after their star.

Importantly, we need to determine the composition of Locus, both as a physical planetary body, as well as its atmosphere. Locus is what is called a Terrestrial planet, which is one of 17 types of planets if we are sorting them by composition. A Terrestrial planet is an astronomical body composed primarily of metals and silicate rocks. They are almost always the planets closest to the star that they have formed around, and in our own real-life solar system we have four examples; Mercury, Venus, Earth, and Mars.

I’m not going to go through the entire list of planet types, but some other well-known planetary compositions are desert planets like Arrakis in Dune, gas giants like Jupiter and Saturn, ice giants like Uranus and Neptune, and ocean planets like Manaan in Knights of the Old Republic.

If you wanted to, you could really go crazy and give a planet a composition of whatever you wanted, but if your planet is going to support life like ours is (SPOILERS?) then you’ll want to keep it as close to earth as possible. In fact, even tweaking the percentages of elements like oxygen and carbon dioxide is extremely dangerous and possibly hazardous towards life. Seeing as the mass and radius of Locus is very similar to earth, it’s unsurprising that it will also have a composition like earth.

The values for earth’s composition are as follows, and we’re going to keep things similar to avoid chemical difficulties down the road. We are however going to make a veeery slight adjustment that compositionally doesn’t change Locus too much, but we’re going to increase the amount of hydrogen and oxygen present on the planet by a tiny bit. On earth, dihydrogen oxide, more commonly known as water, is one of the trace compounds and makes up just 0.02% of its mass, which sounds ridiculous seeing as earth’s surface is over 70% water! But that’s just it; earth’s SURFACE is 70% water; the rest is not. We’re going to increase the amount of water on Locus’ surface, not so much that it’s a total water world, but enough to bring its water coverage to about 85-90%. I’m not even sure that would change the number of 0.02% considering how small of that number is compared to the rest of the planet’s mass, but for simplicity let’s say that 0.03% of Locus’ mass will end up being water.

It’s also important to state that we don’t know the exact surface temperature of Locus at this stage, but as a rogue planet its temperature is likely to be very cold. With no warming from a host star, Locus’ temperature as a terrestrial planet is mostly dependant on how much its atmosphere can keep it from simply being at the background temperature of space, which is 2.7 kelvins, or -270 degrees Celsius.

This frigid temperature actually works in our favour in terms of maintaining elements within the atmosphere, because the colder the planet, the more likely compositional elements will stay within the atmosphere due to being unable to reach the planet’s escape velocity. Escape velocity is the minimum speed needed for a non-propelled object to escape the gravitational influence of a primary body, which in this case, is the planet of Locus itself. This is the equation to determine whether an element will stick around within your planet’s atmosphere based on the planet’s mass, radius, and temperature relative to earth, but there are resources online that you can use to work it out for you. Specifically, there is a spreadsheet created by Artifexian that is easy to read and does the calculations for you. If you somehow don’t know who Artifexian is I am absolutely amazed, and I wholly recommend checking out his channel, he is a guru on science-adjacent worldbuilding. I’ll put a link to his video regarding earth-like atmospheres in the description, and you can find the spreadsheet I’m talking about in that video’s description. Again, full credit to Artifexian for that.

For Locus, if its average surface temperature is that of Pluto (40 kelvin, -233 celsius) then it would be able to keep all elements within its atmosphere except hydrogen. If we lower the temperature juuust a little bit below Pluto’s temperature, by say 10 degrees kelvin (-243 celsius) then even hydrogen can stay within the atmosphere.  

So, as Locus hurdles through space as a rogue planet, let’s say its average surface temperature sits at 30 degrees kelvin (-243 celsius), which is below Pluto’s temperature (40 kelvin, -233 celsius) but above the background temperature of space (2.7 kelvin, -270 celsius). At this temperature, many elements that would normally be a liquid or gas on earth, would be a solid on Locus. In fact, at this temperature, only three elements are not a solid; helium, hydrogen, and neon, all three of which exist as gases, though the temperature is very close to the temperature that neon exists as a liquid (27 kelvin, -246 celsius). At this stage, Locus might have some liquid neon in small pools on the colder parts of its surface, but not in great enough quantities to be even remotely significant. Seeing as hydrogen and helium can’t escape its atmosphere, and that they’re the only two elements that exist in gaseous form, Locus’ atmosphere at this stage would be almost exclusively hydrogen and helium, with trace amounts of neon. All the oxygen and nitrogen that would be present in an earth-like atmosphere is currently frozen or combined with other elements such as silicon to form minerals and other compounds.

You might be saying that Locus sounds like a frozen hellscape at this point, and you’d be absolutely right. Don’t worry, a lot of this is going to change once it’s warmed up by a star. Before that happens though, let’s give it an interesting point of history, around 100 million years before it’s picked up by the solar system it’ll settle into. Let’s say that a spacefaring species used Locus as a mining and research colony for a short period of time. Not so long that it has changed the overall composition of Locus, but long enough that there’s an abandoned settlement. This has some pretty strong worldbuilding implications, and already opens up a box of potential storytelling elements. Obviously, we’ve just stated that there are spacefaring civilisations already in existence, so we’ve just confirmed life outside of Locus, and they’re likely to be billions of years more advanced than Locus, assuming they survive such a period of time. They have to at least be advanced enough to create a biosphere on Locus, or perhaps they are so alien compared to life as we know it that they are able to survive the extreme conditions Locus presents. If you’re interested in creating some ancient Lovecraftian Cthulhu monsters, this is a great way to introduce them to your planet, so that they’re already there before other life evolves. Maybe their colony even seeded life on Locus, or at least left the biological material necessary for life to start. This is the panspermia hypothesis of the creation of life, but that’s something we’ll get into more when we start to establish life on Locus. For now, we’ve just set up the POTENTIAL for panspermia, if that’s the route we choose to take. Let’s conclude that Locus had an unspecified spacefaring race establish a single minor settlement that has been long since abandoned, and that is likely to be 99% destroyed by the time life evolves on Locus.

So, to recap, our planet, Locus, is a terrestrial planet that is slightly larger than earth, with a slightly higher than earth amount of water. While its composition is similar to that of earth, it’s currently a frozen wasteland that is hardly suited for supporting life and will remain so while it’s still a rogue planet, until it gets picked up by another host star. It did however have a group of ominous visitors for a short period of its history, though those visitors have long since left with almost no traces.

Join me next time when we will have Locus be pulled into the solar system that it will reside in, establishing its star (or starS!). And until next time… stay awesome!