Hey gang. I’ve been spending the last few days building a new world for the rest of The Ark series and it got me thinking back to a wonderful project I was involved in called Eighth Day Genesis. It was meant to be a world-building textbook of sorts, and I had the honor of writing the first chapter. If you’re an author or GM who loves really digging in and getting the details right, it’s a great resource well worth the investment.
Here’s the article I wrote for the project. It’s on the long side, so grab a cup of coffee and have a seat. Hopefully it will give you some tools and insights on how to make your own imaginary worlds shine:
Building Worlds in a Hostile Universe
By Patrick S. Tomlinson
So you want to build a world? Excellent. The current record is six days, see if you can beat it. I have faith in you. But wait! Where are you going to put your world once it’s finished? Just like the suburbs, not all galactic neighborhoods are created equal. Some are pretty rough places to crash. Some are so vanilla and boring that nobody would choose to live there. Let me be your real-estate agent to the stars… literally.
Choosing a Galactic Neighborhood:
Just like with the decision to build a house, the first thing you should consider when building your world is location, location, location. If your story is taking place entirely “dirtside,” then your planet’s place in the galaxy may never come up, but there are some interesting things you may want to consider that can help drive the story regardless.
Most of us have heard of the concept of a solar system’s “Goldilocks Zone,” commonly defined as the orbital area around a star that is at just the right temperature that liquid water can exist without freezing or boiling away. We’ll talk more about this zone later, but what many people do not realize is that galaxies have their own Goldilocks Zones where conditions are more favorable for life.
Just like inside a solar system, your world can be too close or too far from the galactic center to give life much of a chance. Not surprisingly, our own Sol system sits smack dab between these zones. This is not to say that life would be impossible outside this neighborhood, but it would definitely face new challenges. Let’s start with the galactic boondocks.
The sticks of any galaxy possess several unique characteristics that could impact your world and how your story develops. But they all revolve around one element; scarcity. The further from the galactic core one travels, the thinner the density of stars becomes. By the simple law of averages, there will be fewer planets, and thereby fewer chances for life to evolve in the outskirts. While this is obvious, what may not be so obvious is that fewer stars, especially very large ones, also mean fewer heavy elements.
As you likely know, all of the elements, save hydrogen, helium, and small amounts of lithium, are formed inside the core of stars. What you may not know, however, is that a small to medium sized star can’t manufacture elements past iron on the periodic table.
This is because iron is a star killer. At the heart of a star, elements fuse together, releasing energy and fueling the furnace. In young stars, this fuel is hydrogen almost exclusively, but as they age, other elements are introduced to the fire. Each new element can be fused into the next, releasing progressively less energy, until the star reaches iron. The problem is, when you fuse iron, the process actually absorbs energy, rather than releasing it. Instead of gasoline, iron acts like a bucket of cold water thrown onto a camp fire, snuffing it out in a matter of seconds.
If your star is about twice the size of ours or smaller, the story ends with iron. It is only when you get to stars large enough to collapse into supernova can all of the other elements be created. In the outer rim of the galaxy, gasses are less abundant, which means the stars that do form trend on the small side. The interstellar medium this far out, therefore, will not be nearly as rich in heavier elements as it is closer to the core.
Fewer heavy elements mean less material available for rocky planet formation, and therefore even fewer Earthlike planets. Among the terrestrial worlds that do manage to form this far out, the CHNOPS elements may be abundant, (the six elements critical to life as we know it: Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorus, and Sulfur) but the elements of civilization and industry might be scant indeed. Your characters may live on a world where metals like nickel, copper, zinc, and lead are as rare as silver and gold here on Earth. Plutonium and Uranium would be almost unheard of, making nuclear fission impossible. In the near absence of such materials, building a technologically advanced society would be very difficult. Of course, so would building nuclear weapons, so there’s that.
On the other side of the habitable zone is the galaxy’s inner core. Here, overabundance is the issue. Stellar density increases the closer one gets to the core. More stars have the potential to bring more than just beautiful nighttime viewing.
The core would bring much higher levels of high-energy radiation. Somewhat counter-intuitively, somewhat higher radiation levels might not be all bad for life on some worlds. The bedrock mechanism of evolution is mutation. On Earth, most mutations start when a stray high-energy particle crashes headlong into a DNA strand, altering a bit of code. Most of the time, the result isn’t good for the organism. Every now and then, however, the change is actually beneficial. With slightly elevated radiation levels, evolution on your world could be supercharged. But outside that narrow window, things would become difficult for complex life, with higher rates of cancer and genetic damage overcoming the increased rate of evolution.
In addition to the obvious dangers posed by radiation, the density at the core brings other issues life would have to contend with. Our solar system is surrounded by a shell of trillions of comets and debris known as the Kuiper Belt and the Oort cloud. This region starts just past the orbit of Pluto, extending perhaps as far as an entire light year into deep space. It is expected that most solar systems have a similar feature. Normally, objects in the Oort cloud are of little risk to life on Earth. However, every now and then, either through collisions or gravitational disturbances, a comet is knocked loose from its stable orbit and plunges towards the inner system.
In the core, the tight proximity of other stars means that gravitational interactions between different solar systems will be far stronger than they are in our neck of the woods. This could lead to dramatically higher orbital instability in the Oort clouds of core systems, meaning higher levels of comet and asteroid bombardment of any planets. Ask a dinosaur how that worked out for them.
Picking Good Neighbors:
So you’ve settled on the right stellar cul-de-sac for your planet. Good, but before you pack the moving starship, maybe you should meet the neighbors. Just fifteen short years ago, exoplanets were and unproven theory, and believed by many astronomers to be a rare breed.
Today, we know better. As of this writing, over seven hundred exoplanets have been detected, with another thousand potentials waiting to be confirmed. However, while planets are plentiful outside of our solar system, most of them truly deserve the name ‘alien’.
Our solar system isn’t unique, which is great for sci-fi lovers, but its arrangement may be fairly unusual, presenting even more complications for life. A large portion of the planets we’ve found are Jupiter-range gas giants, simply because their large size makes the easier to detect. What surprised astronomers was the diverse range of orbits these giant occupied.
Many of them are what are known as ‘Hot Jupiters’, gas giants that orbit unbelievably close to their parent star, sometimes close enough that they complete an orbit in only a few days. Under our current understanding of planet formation, gas giants condense far from their star. Therefore, these hot Jupiters are believed to have migrated on a decaying orbit towards their star until finally stabilizing closer in. On their downward spiral, these monsters would have either destroyed and absorbed any rocky planets they came across, or ejected them from the system, dooming them to float untethered through deep space. It is nearly impossible that any system with a hot Jupiter could also be home to a terrestrial world in a habitable orbit.
However, while some gas giants are world-devouring monsters, others act as guardian angels. Such is the case with our own Jupiter and Saturn. Their stable orbits far from Earth, coupled with their huge masses mean that any asteroids or comets with ill intentions first have to run the gamut of the outer system. Jupiter’s immense gravity has absorbed countless impactors, most famously illustrated when the comet Shoemaker-Levy 9 crashed into Jupiter’s atmosphere in July, 1994. The resulting impact scars were larger than Earth herself.
It is impossible to know how many bullets Jupiter has jumped in front of for us, but the number is probably very high. A solar system without stable gas giants would be like living on the fifty yard marker of a shooting range. A world without such a shield would have a rich history of asteroid cataclysms.
Speaking of asteroids, let’s clear up one thing real quick. Asteroid belts are not like in the movies, okay? So sparsely populated is the asteroid belt, that when NASA sent Pioneer 10 and 11, Voyager 1 and 2, Galileo, Cassini, and New Horizons into the deep solar system, they didn’t have to make a single course correction to avoid a collision.
While there are millions of objects in the belt, they are very dispersed. The total mass of all objects in the belt is less than one percent that of Earth. They are the remnants of a failed planet whose formation was interrupted by orbital resonance with Jupiter, (acting the bully this time). Much denser, and the belt would have had enough material to overcome the gravitational disruptions from Jupiter and form another rocky planet.
So while we can all agree that the asteroid scene in ESB was really awesome, it was also really impossible, (the ring scene in EP: II was better, but had its own problems).
Another potential danger is the discovery that many exoplanets circle their parent stars along highly elliptical orbits, which bring them scorchingly close, then sling them far from the heat of the star. Any terrestrial planets on such a path would bake in sterilizing heat and radiation, before freezing solid, with only brief periods spent inside the system’s habitable zone. Any gas giants on such a path would make it impossible for any other worlds to maintain stable orbits.
But those are neighbors on the next block. What about the one right next door? Moons can have huge influence over their host worlds, for good or bad. Of the eight planets in our solar system, (sorry Pluto, take it up with Neil Degrasse Tyson) only two are moonless; Mercury, and Venus. Yet even among all the dozens and dozens of moons swarming around the rest of the planets, ours is unique, which was very lucky.
Earth’s moon is strange in several ways, but most prominently is its size relative to Earth. Mar’s twin moons are just large rocks, probably asteroids captured by the red planet’s gravity well after being knocked loose from the asteroid belt. Jupiter has four large moons, yet these bodies are all miniscule in comparison to Jupiter’s bulk. The same is true of Saturn, Uranus, and Neptune.
Our moon is a massive body by contrast, so much so that a small group of astronomers prefer to think of us as a twin planet system. It is also very dense, second only to Io. The Moon’s large size gives us more than just the tides, its gentle tug helps to stabilize the Earth’s rotation, preventing our axis from wobbling more than a few degrees, keeping our seasons and weather patterns stable and predictable, (larger ‘Super Earths’ may have enough mass to maintain a stable rotation on their axis, but they have other issues we’ll talk about shortly). And as the Moon’s cratered surface can attest, it has taken more than a few hits in our defense.
But the relationship wasn’t always so rosy. The Moon has been moving slowly away from Earth since its formation four and a half billion years ago, at the rate of about an inch and a half per year. As it goes, the Earth’s rotation slows ever so slightly. In the distant past, however, the Moon was much closer, the Earth’s day was much quicker, (only six hours!) and the tidal effects of the moon’s gravity were absolutely devastating.
In the early days of Earth’s oceans, the moon was so close and its gravity so powerful that the tides swelled not the handful of feet we see today, but hundreds of feet. These immense walls of water swept inland dozens of miles, every day. Beachfront property would be a hard sell on such a world. So, while our moon today is Earth’s greatest partner, things could have been very different.
Moons aren’t limited to just a supporting role in sci-fi, however. Star Wars, Firefly, and Avatar all prominently featured moons filled with complex life, even whole civilizations, (yes, the Ewoks were a civilization, stop whining). But not so fast, life on a moon has hidden dangers to consider.
Of all the dozens of moons we know about from our own solar system, none of them are even a significant fraction of Earth’s size. The largest in both diameter and mass is Jupiter’s Ganymede, which also has the distinction of being the only known moon with a dipolar magnetosphere powered by a liquid metallic core. Yet even mighty Ganymede is only two and a half percent as massive as Earth, with only fifteen percent the surface gravity. This is not to say that much larger moons are impossible elsewhere in the universe, but it appears such bruisers would be rare.
So, your moon-men will probably be living in very low gravity. Low gravity typically means a very thin atmosphere. The exception, (there’s always an exception) is Titan, Saturn’s famous moon. Its atmosphere is actually denser than our own. However, this has more to do with how far from the sun Titan is, which protects its atmosphere from being stripped away by the weakened solar wind. However, bring your long-johns, because this far out, it’s about three-hundred degrees below zero.
Low mass also typically means a metallic core that has already cooled and solidified, which means no magnetosphere, or a very weak one, which leaves whatever atmosphere there is vulnerable to the fate of Mars. However this is less of a problem than it might first appear.
Thus far, all of the major moons featured in the movies, such as Yavin IV, Endor, and Pandora, have orbited gas giants. These giants can themselves have very powerful magnetospheres, extending many millions of miles into space and shielding their satellites. Unfortunately, gas giants can also sport massively powerful radiation belts, enough to cook the surface of any moons that orbit too closely. In the case of Jupiter, it actually emits more energy in radiation than it receives in light from the sun. So, lead long-johns for everybody.
Home Sweet Home:
Congratulations, you’ve finally found a good neighborhood, populated with neighbors who aren’t completely crazy and or violent. Now it’s time to pick a plot and draw up some blueprints.
Let’s pause to consider how big of a yard you want. As mentioned previously, solar systems all have a Goldilocks zone around their parent stars, the area in which a planet could potentially have liquid surface water. Our system actually has three rocky planets inside this zone, Venus at the extreme inside edge, Earth snuggly in the middle, and Mars at the extreme outer edge.
“Wait!” you’ll say. “Venus is way too hot, and Mars is way too cold.” True, but this has as much to do with their size and the composition of their atmospheres as their distance from the sun. As best as we can determine, Mars once had a thick atmosphere and water lakes, rivers, and even shallow seas. But, as previously discussed, its small mass meant that its molten iron core cooled and solidified billions of years ago, switching off the magnetic field protecting its atmosphere. Venus had the opposite problem, way too much atmosphere with way too much CO2, leading to a runaway greenhouse effect. If Mars had formed with the mass of Venus, John Carter wouldn’t be nearly so far-fetched.
What kind of star you’re swinging around directly controls where and how big the habitable zone is going to be. Also, each star type is going to bring unique conditions for life to contend with.
Small, red-dwarf type stars are far and away the most numerous in the universe. An advantage of their small size is that they can continue to burn for many tens, even hundreds of billions of years, giving life on any planets a long, long time to get up and running. However, their habitable zones sit in a very tight orbit, which presents two challenges.
First, a terrestrial planet orbiting so close to its star would probably be tidally locked to said star, which is just a fancy way of saying there would be no day/night cycle because the same side will always face inward. It was once believed that this would bake one one side of the planet, while freezing the other side solid, leaving only a small strip of habitable land around the terminator. Today, we may know better. The study of several “Hot Jupiter” planets has shown that strong convection currents in the atmosphere can cool the bright side, while warming the dark side of a tidally locked world. So things may not be so bad in that respect. Instead you’ll just have constant hurricane force winds to deal with.
Secondly, close proximity to the star also brings your world into a zone of strong radiation, solar wind, and even occasional lashings from solar flares. Any life that develops here will need to be pretty hardy, and carry a lot of sunblock. Incidentally, while not impossible, it’s unlikely that such a planet would have moons, as the proximity of the parent star would make maintaining a stable orbit problematic. It probably isn’t a coincidence that the only planets in our system without moons are also the closest in. By the way, no moons and no rotation also means no tides and no seasons.
By contrast, very large stars would feature habitable zones far from dangerous radiation and flare activity, and wide enough to fit multiple worlds. The only real downside is the cops are going to get called to break up the block party early.
The larger the star, the shorter its lifespan. While life on Earth started very early in the planet’s history, perhaps as short as half a billion years, it took several billion years more before anything more advanced than pond scum came about. The very largest stars burn for not billions, but mere millions of years, scantly enough time for planets to form, to say nothing of cooling enough for life to have a shot.
We’re nearing the end. Finally, you can submit your blueprints to the city and start lining up contractors to build your dream home. But what should it look like: an efficient starter house, or a full blown McMansion?
We’ve already talked at some length about the perils of small planets, (weak gravity, thin atmosphere, no magnetic shield) but there could be some upsides, too. If life does get up and running on a lightweight planet, the low gravity would allow plants and animals to grow to stupendous proportions. This has been touched on in sci-fi several times, with the nine foot tall Tharks of Barsoom, and the similarly framed Na’vi of Pandora and their massive Home Tree.
What sci-fi has largely overlooked, (up to this point, because you’re writing sci-fi, right?) is the effect low gravity would have on the landscape. Less gravity means tectonic forces could push mountains higher. Volcanoes would grow closer to the sky. Erosion wouldn’t be able to pull either down nearly as quickly. Counter-intuitive as it may seem, everything would be bigger on a small world.
Now, what about the heavyweights? They have some alluring qualities. More surface area, for starters. More gravity can better hold an atmosphere. A stable axial rotation without need of a large moon is a nice feature. And while higher gravity will mean shorter mountains and a dearth of svelte blue cat-women, erosion would cut deeper valleys, canyons, and rivers.
So, everything’s cool, right? Plop down the extra cash for the upgrade. Slow down a step. Recently, computer simulations have shown that the higher pressure at the center of super earths may keep the core solid. No liquid core means no magnetosphere, just like on smaller worlds. More sunblock for everybody.
After reading the above, you’re probably feeling a little hemmed in. like the universe is out to get us, and that Earth is the one, tiny speck of dirt where life has a chance to thrive. That’s probably a mistake.
My intention in writing this was not to scratch every other type of planet and solar system off your list of potentials. Instead, I wanted to convey just how improbable our planet, and therefore our type of life, may be.
If you need inspiration, look around our planet’s forgotten corners. We find organisms living in complete darkness, under crushing pressures, scalding temperatures, in pools of acid, without oxygen, eating rock and metal, photosynthesizing radiation, and generally carrying on in a fashion that drives biologists into alcohol dependency.
Judging by life’s tenacity and ingenuity here on Earth, I believe we will find organisms clinging to every planet, moon, asteroid, and nebula that hasn’t gone out of its way to completely sterilize every cubic inch of real-estate. It wouldn’t surprise me in the least if we discover some critters swimming around Europa’s that would go nice with butter and lemon juice.
Instead, the lesson you should take away from the rarity and good fortune of our planet is that, as a writer, you’ll need to be creative. The habitable zone for water-based creatures would mean instant death for creatures based on liquid ammonia or methane. Radiation zones and thin atmospheres are meaningless to fish swimming in an ocean buried under ten miles of ice.
Worlds aren’t built for creatures. Creatures are built for worlds. So take what you’ve learned from this article, the good and the bad, and run your aliens through the same evolutionary gauntlet that your ancestors actually went through. Make them face adversity and overcome challenges on their way to civilization. They will be all the stronger, more alien, and yet more believable for your efforts.
See what comes out the other end. The more surprised you are, the more impressed your readers will be.
All done. If you liked that, there are nineteen more great essays to be had in the full textbook, each one focusing on a different aspect of world-building from biology, to religion, even building realistic economic systems. So please consider grabbing yourself a copy. You won’t be disappointed.
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