Fiction loves dropping our favorite characters on to single-biome planets, that is a world with only one predominant ecosystem whether it be all tundra, all jungle or all under water. The science, sadly, doesn’t always match up. Earth’s diversity of landscapes and ecosystems is no accident, but rather the result of the evolution of a living world. If a planet is to be defined by a single biome, then it needs to hold up to scrutiny.
Today we’ll take a look at some of pop culture’s most well-known single biome worlds and see just how realistic they are. To fill in some missing information, we’ll be drawing upon the Star Wars Expanded Universe — that is, the collected knowledge of all officially licensed background information outside of the six feature films, much of which has been catalogued in Wookieepedia. After all, Luke Skywalker traveled from the desert planet of Tatooine, to the ice planet of Hoth, to the murky swamp world of Dagobah.
There’s nothing unrealistic about the concept of a desert planet — Mercury and even Mars can be labeled as such — but the concept of a living desert planet requires a little more thought. Luke’s home world of Tatooine is no dead world; apart from its human immigrant inhabitants, it has native flora and fauna, including banthas, womp rats, and at least one sarlacc.
So how does a world that consists of one big desert manage to support life? Tatooine raises some questions to which we may not have satisfactory answers. We know the planet has a non-toxic atmosphere with enough oxygen to support life, because we see humans milling about without any special precautions like helmets or breathing masks. But where does this oxygen come from?
If Earth suddenly lost its sources of oxygen replenishment, it would not take long for all the oxygen in the atmosphere to be depleted, because oxygen is a highly reactive gas that combines readily with other elements. What keeps our atmosphere at a constant 20% oxygen level is the carbon-oxygen cycle — animals breathe in oxygen and expel carbon dioxide, while plants breathe in carbon dioxide and expel oxygen through photosynthesis.
What, then, is replenishing Tatooine’s oxygen? Viewed from space, Earth is covered in greenery, and we don’t even see the blue-green algae that fill the oceans and contribute to the carbon-oxygen cycle. Viewed from space, Tatooine is ball of sand with no visible greenery or oceans. Where is the photosynthetic life needed to maintain the carbon-oxygen cycle?
Further, is there even enough water on the planet to support life? The Expanded Universe tells us that less than 1% of Tatooine’s surface is covered in water, found in rare oases, and we don’t even see Luke living near one of these oases. Before we can point to groundwater reservoirs, we need to consider that the principal occupation of the locals is moisture farming — drawing water out of the air.
Tatooine’s average humidity level is 5.4%. As a point of comparison, the U.S. city with the lowest annual humidity is Las Vegas, in the Mojave Desert, and it averages between 21% and 39% humidity.
How much water can Tatooine’s air contain? Let’s assume a high average temperature of 100°F to give this planet the best chance of success, since hotter air is able to “hold” more water. At 5.4% humidity, every cubic meter of air can contain 2.5 ml of water. To fill a single cup of water, you would need to process 95 cubic meters of air — roughly the equivalent of a 20 ft x 20 ft room. If the residents of Tatooine are resorting to drawing water out of such dry air, one can only imagine how difficult groundwater is to come by.
Going from one extreme of the temperature spectrum to the other, ice planets are also conceptually realistic — Jupiter’s moon Europa being a great example. Covered in water ice, Europa may have a liquid ocean beneath its icy crust, with tidal forces from Jupiter generating the heat that keeps the ocean from freezing over completely.
Similarly, Hoth, the location of the Rebel Alliance’s’ Echo Base, is completely covered in water ice. Since it is said to be geologically active in the Expanded Universe, it’s safe to assume that liquid water does exist at least in places beneath the icy surface. With oceans come the algae that can arguably maintain Hoth’s carbon-oxygen cycle and, consequently, a stable atmosphere.
While neither water nor oxygen pose a big problem to explaining Hoth, its indigenous species do raise some questions about the feasibility of complex life on the planet. Apart from the carnivorous wampas and omnivorous tauntauns seen in the movie, the Expanded Universe tells us herbivorous hogs, rodents, and lichen complete the food chain.
On Earth, lichen form an important part of the food chain in the tundra, with many animals relying on them during the winter months. However, when plants are available to eat during the warmer months, we see that animals will graze on lichen much less, allowing for the lichen to recover. In a climate that is perpetually winter, though, can lichen sustain an entire food chain?
A study was once conducted that showed that after an area of lichen was overgrazed, it took 22 years for the lichen to recover to just 10% of its former state. Given this slow recovery time, it is difficult to believe that a multi-tiered food chain with a large apex predator like a wampa and consequently high energy requirements can sustain itself on Hoth.
Examples: Dagobah, Endor
A place where a lack of plant life is certainly not a problem would be a swamp planet like Dagobah, where Luke met Yoda. Swamps are great environments for the propagation of diverse life: water, nutrients, plants… what more can you ask for? We’ll assume that Dagobah isn’t bathed in perpetual twilight, as the film would suggest, else concerns about the availability of sunlight to power photosynthesis come into play.
At first blush, our solar system has no example of a swamp planet, but it turns out that we need look no further than our own world, some 300 million years ago: the Carboniferous period. It was a time of global warmth, vast swaths of forest, and extensive coal swamps — after which the period is named, since coal is fossilized carbon.
But to call Carboniferous Earth a swamp planet would be an oversimplification, as it was not the only biome at the time. Swamps and forests couldn’t cover the entire landmass — not if topography has anything to say about it. Swamps will form in areas of low-elevation that are saturated with water. While forests can exist at much higher elevations, we do eventually hit the tree line — the point at which trees can no longer tolerate the environmental conditions, be it due to lack of moisture or heat in the thinner air. In fact, it becomes difficult to rationalize why any geologically active, living planet would not have a diversity of biomes. Mountains will form via volcanic activity and the collision of tectonic plates, and mountains affect climate by blocking wind, rain, and moisture.
What if Dagobah were not geologically active? What if it was flat, with no mountains forming at all? That would imply the planet has no liquid outer core to drive this geologic activity. If Earth’s outer core would freeze into a solid — which could be our eventual fate as the planet continues to cool — then the magnetic field generated by the action of the flowing magma would die off. Without our magnetic field to deflect them, cosmic rays would strip away our upper atmosphere, including the ozone layer that protects us from UV radiation. Through DNA damage, the radiation would ultimately wipe out most life on Earth.
Were a forest planet to exist, it would look much more like Endor, the Ewok homeworld. While Endor is known as the Forest Moon and is appropriately covered by massive pines and redwoods, the expanded universe does specify that the moon also contains vast dry deserts, grassy plains, small oceans, secluded lakes, and mountain ranges. This calls into question whether you can truly label Endor a single-biome world.
The Wookie homeworld of Kashyyk is described as lush and tree-filled, giving the initial impression of a forest planet. But with 60% of its surface covered with water, the dominant biome is actually ocean, as in the case of our own planet, whose surface is 70% covered with water. Kashyyk and Earth — even Carboniferous Earth — would be more appropriately labeled ocean planets if you truly had to pigeonhole them into a single biome.
However, a true ocean world is actually a distinct possibility. Take a planet like Earth, add some additional ice on the polar caps, crank up the temperature, and voila — the whole planet becomes submerged in one great ocean. Kamino, the water world in which the clone army was created, fits this exact description, and is possibly the most plausible single-biome world in the Star Wars universe. Complex marine life continues to thrive; geological processes keep the planet alive; algae maintain a breathable atmosphere.
As a final case, we’ll consider an ecumenopolis — a world in which urban sprawl has continued to grow until the entire surface is overrun with what amounts to one big city. Coruscant, depicted at great length in the terrible prequel films, is one such world-spanning metropolis.
According to the Expanded Universe, the population of Coruscant has historically risen to up to several trillion. On a planet with no farmland to grow crops or raise livestock, how do you feed such an enormous population?
Current estimates state that at least 1.2 acres of farmland per person are required in order to maintain American dietary standards. Assuming a population of 5 trillion people, that’s a requirement of 6 trillion acres of farmland needed to sustain Coruscant’s population.
Earth’s surface area is 128 billion acres in size; if we assume Earth-sized planets with 100% of their surface covered in farmland can exist in abundance, then it would take almost 50 of these planets to feed Coruscant. But 100% farmland is unrealistic. On Earth, only 30% of the surface area consists of land, and only 33% of that land is suitable for farming, which amounts to 12 billion acres. It would take 500 Earths to feed Coruscant.
Assuming you can have a multitude of solar systems serving as farms for a single planet, how much food would have to be delivered to Coruscant on a daily basis? Americans typically eat up to 5 lb. of food a day — so 25 trillion lb. of food would need to be delivered daily. Let’s put that into context. Darth Vader’s flagship Super Star Destroyer, measuring 19 km in length and dwarfing the regular Star Destroyers, which are less than a tenth of its size, has a cargo capacity of 550 million lb. It would take over 45,000 of this largest warship ever built (at the time) to deliver the food required to sustain Coruscant’s population — every day.
I didn’t mention water yet because the expanded universe tells us that water distribution on the planet is a self-contained system, likely requiring recycling technology of sorts. Why can’t the same be said for food? Because “foodstuffs” is listed as one of Coruscant’s major imports. Even if Coruscant is able to produce 75% of its own food requirements, importing the other 25%, that’s still 12,000 Super Star Destroyers worth of food coming from dozens of planets.
Even if we get past the food crisis, a heat issue also presents itself. Coruscant’s energy demands would be so high that the heat given off as a result would render the world uninhabitable. We won’t get into the math on this one, since a physicist with a sense of humor was kind enough to do that already, but estimates suggest the temperature could rise to 440°F — effectively an oven.
One can argue that the technology on such a planet is so advanced that we can hand wave away these considerations. Well, while Arthur C. Clarke did say, “Any sufficiently advanced technology is indistinguishable from magic,” one thing that technology has yet to do is defy the natural laws of the universe. Until that happens, basic thermodynamics tells us that Coruscant would not be able to dissipate the heat fast enough.
What we observe is that the more we try to rationalize single-biome planets, the more they end up looking like Earth — or Leia’s homeworld of Alderaan. A geologically active planet with a breathable atmosphere and sustainable life will naturally evolve multiple biomes. Sure, a single biome may be prevalent, but at that point, it is no longer a truly single-biome world. Even an ocean planet is much more diverse than its simple label would suggest, with an undersea world rich in diverse ecosystems.
While poking holes in the science of Star Wars may seem a little unfair, given the spirit of the original trilogy was science fantasy rather than hard science fiction, I offer a single word in response: