One of the planets in my science-fantasy series is small (around the size of Earth's moon). The gravity implications of this are the one part of this the magical origins of my universe are able to hand-wave away so for the sake of this question, let's just assume it has Earthlike gravity. Anyway, this planet is divided into four main geographic zones:

Around the south pole is a gigantic patchwork of marshes and swamps called The Saltu (similar to the Everglades) where all the excess water ends up due to the planet's lack of actual oceans.

North of this is a band of thick, flat jungle cut through by several large river valleys (think Amazon basin) that flow down into The Saltu. This area is mostly referred to as the Atoyatl Basin (since the Atoyatl River is the largest of these rivers and is basically thought of as this planet's equivalent to the Amazon).

North of these jungles, straddling the planet's equator, is where pretty much all the planet's tectonic activity happens. In the eastern hemisphere, the planet's two tectonic plates (remember, it's a very small world) are rifting apart in a similar fashion to what we see in Iceland, and it is in this rift valley that there is a massive freshwater alpine lake called Lake Coxcote (around the size of all of the American Great Lakes combined and around as deep as Lake Baikal). This lake is surrounded by a myriad of landforms creating by this process, ranging from faults to canyons to mountains to volcanoes. This entire area sits on a plateau similar to the Mexican Plateau. The equatorial region of the western hemisphere is where these two plates meet and subduct underneath each other, creating an absolute ton of volcanoes and mountains. As the equatorial regions of both the western and eastern hemispheres are both elevated and mountainous (the west more so than the east though), these areas are referred to as the Equatorial Ranges.

To the north of these areas is a vast desert called Mictlan. Mictlan is the only region known in the entire galaxy where water simply does not exist in liquid form (it only exists in trace amounts in the air that make it over the Equatorial Ranges from the south, and this is never enough to produce rain or even dew). Life does not exist here for obvious reasons, and the land here looks more like the surface of Mars than Earth.

The only parts of this planet that are inhabited are the areas around Lake Coxcote, some of the lower slopes of the Equatorial Ranges, and the northern third or so of the Atoyatl Valley. The rest is completely empty of sentient life (Mictlan has no life at all).

Is this a realistic habitation pattern for a planet with this bizarre geography, and could a planet with this bizarre geography even exist at all?

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    $\begingroup$ Given your statement "the magical origins of my universe", you've clearly handwaved away a boatload of issues and as such you are free to handwave the rest of the problem too, right? You've not said anything about the nature of your magic or its limitations, making reality-checking it rather difficult! $\endgroup$ Mar 8, 2021 at 9:45
  • $\begingroup$ As discussed by Raisus below, gravity is not your only issue. So to answer your question, no it is not realistic. But with magic, it doesn't have to be. You can either handwave the issue, or come up with detailed magical systems that explain why this exists the way it does. $\endgroup$
    – Gwyn
    Mar 8, 2021 at 10:16

3 Answers 3


TL,DR: not without a lot of handwavium

Unfortunately, Gravity is just one of the many things that make a planetary body suitable for life. The main problem is the core at the center of your world. With a planet that small, the core of said planet would either be non-existent or deceased due to entropy (a good example of this is Mars, whose own core has since become inert)

A planetary body's ability to harbor any life at all (even vegetation) is dependent on the ability of the planetary body generating a magnetic field by dynamo action to protect the inhabitants from deadly solar radiation.

It is unlikely that any planet this small would have such an ability, meaning that, over time, the solar radiation is stripping away the atmosphere of the planet and also bombarding its surface with ionizing radiation, killing even the smallest of microbes and bacterium (maybe not Tardigrades, but they're extremophiles anyway)

Note: not even Gas Giants are immune from the atmosphere-stripping effects if close enough to the parent star (see Kelt-9b or HD 209458 b) so it's not a matter of Gravity overcoming this effect either.

Another side-effect of the lack of dynamic core motion (liquid outer core, surrounded by a solid inner core due to pressure) is a ceasing of volcanism or tectonic movements on the surface (again, see Mars)

The geography doesn't make sense either, the sheer lack of water (the lake notwithstanding) means a lack of the water cycle (Evaporation, condensation and precipitation) which also means a lack of rivers and valleys. With no fresh water feeding the lake, it will eventually evaporate away (current examples IRL: The Dead Sea, Lake Chad and the Aral Sea)

"Generally" speaking, a planet that has distinct zones such as the ones you are describing (Deserts, marshes, etc.) are symmetrical across the equator, all-be-it, the only example of this that we know about is Earth, and even then, it's not perfectly symmetrical.

  • $\begingroup$ OP talks about magic. So whilst everything you've written is quite reasonable, most of it seems to have been handwaved away already. $\endgroup$ Mar 8, 2021 at 9:45
  • $\begingroup$ @StarfishPrime - OP specifically says "The gravity implications of this are the one part of this the magical origins of my universe are able to hand-wave away so for the sake of this question, let's just assume it has Earthlike gravity." meaning that only Gravity can be magic'd away using handwavium. I'm just trying to point out that, unless he extends that to the other factors in a planetary body, it's unlikely to be possible $\endgroup$
    – Raisus
    Mar 8, 2021 at 9:49

The plate tectonics may be a problem

The earth's moon does not have active volcanoes for a simple reason, its mantle. The Moon's Mantle, too cool to move easily, has no convection and no active tectonic plate motions, due the Moon's smaller size and cooler interior than Earth. Over billions of years the moon cooled until it is like today.

However, if you want to magic it away there are some semi plausible explanations. (Currents of magic moving through the mantle heating it or etcetera.)

The same gravity magic that allows the atmosphere could be involved. If it went through cycles of more or less gravity the compression would keep your planet warm. These cycles could be local edies that move over the surface and extend into the mantle. This could create local bubbles of increased or decreased gravity.


Is this a realistic habitation pattern for a planet with this bizarre geography?

Only if your planet is very sparsely populated in general and likely of pre-Iron Age development. Otherwise, with the exception of your waterless region, everywhere that could support habitation would. Other than the desert, your habitats are flush with surface fresh water and so it follows they'd be relatively flush with people.

Just the European share of the Roman Empire is estimated to have had a population between 20 and 30 million; the entire world population around 1 CE was around 230 million. On your moon-sized world that would be about 6 people per square km (230,000,000 / 3.793×107 km2). Denser still if there's a significant portion hostile to life, but the rest is not.

Could a planet with this bizarre geography even exist at all?

This should have a second question and I will flag it as such, but to answer: No, not at all.

When it comes to generalized climate patterns on terrestrial planets they come in two species: free-spinning planets have bands mirrored at the equator, and tidally locked planets have stretched bull's eye (a so-called lobster pattern) oriented around their stellar axis. In both cases there is a thermal gradient connected to the surface relative to the sun. Your planet seems to have non-mirrored bands with no relation to the sun; roughly tropical, temperate, and arid, from south to north. Even if the arid waterless region were a substellar point on a tidally locked planet, you have a jungle around the antistellar point and a temperate forest between them.

Your tectonics are also highly suspect.


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