# How to explain a livable planet the size of Jupiter in a fantasy universe

I am creating a story with wizards and a war between the medieval and the magician, but I liked the idea that this world was huge, and the different people lived a colossal distance apart, not just days, but almost years.

Therefore, this world would have to be something like the size of Jupiter. However, as I did some research, I found out that very large planets end up collecting too much gas and also have other factors that make them unsuitable for life as we know it.

That being the case, what can I use to make my huge world plausible? Should I just use the excuse of it being a fantasy world, or do I have to drastically reduce the size of the planet?

• All depends. The disk world is a disk planet that rides on the back of elephants standing on the back of a turtle swimming through space. So why is it that big? – bowlturner Dec 25 '14 at 23:14
• Ringworld, or a Dyson sphere. – Beska Dec 26 '14 at 13:43
• I find it tends to ruin fantasy stories for me when the author tries to bring hard-science into it because that then only leads to more inconsistencies. I wouldn't even bother bringing it up. – GrandmasterB Dec 26 '14 at 17:06
• Even assuming a fast speed of 50 miles per day, every day, it would take you almost a year and a half to travel around the Earth. You may only need a planet a few times larger than the Earth, or even just an Earth with less ocean so the continents are larger. – 2012rcampion Dec 26 '14 at 22:46

In a magical/fantasy universe then it's easy enough as you say to just explain it away using magic. There's no need even to go into details, but you can either say the laws of the universe work differently, or use a less direct explanation like saying for example that the planet is hollow (and held together/supported by the magic).

In a true fantasy universe you can have the planet any shape you like, including flat. :)

• Never explain something that don't need explaining. At most, mention the oddness and handwave the cause and consequences. – MakorDal Feb 26 '16 at 9:39

If you are concerned about the size of your planet, you should consider why it needs to be that size.

With low technology levels, it does in fact take years to cross the Earth. Crossing the Earth in days is only a recent invention. You can increase the distance covered by reducing the tech levels.

After all, technology has brought the world closer. Inverting it would result in the reverse.

• You can also have the terrain and other conditions reduce travel speeds. Many types of terrain either slow travel, require going around, or require limited resources and preparations to cross. A planet the size of earth with more mountains and bogs and jagged hills and large rivers and canyons, and with smaller oceans and more serpentine seas, could be many times larger than our own in terms of the number of separate places, and the travel limes between them. – Dronz Dec 25 '14 at 20:25
• Also, travel can be seasonal. And they could have a poor schedule: wait for spring to cross mountain pass, then wait half a year to cross a sea, etc. – JDługosz Dec 26 '14 at 1:27

Similar to March Ho's idea, you can make the world Earth-sized but in the shape of a maze. Separate the paths by extremely tall mountains or oceans of acid, making it impossible to travel effeciently. To someone living on the planet, it will seem as though the circumference is far larger than it really is.

For example, in the map below, green represents inhabitable land, while black represents impassable terrain. Assuming the map is of an Earth-sized world, the circumference will seem to be over 300,000 miles.

• It doesn't have to be nearly that extreme, nor nearly as deliberate, to have that effect. – Dronz Dec 25 '14 at 20:26
• Riverworld has 10 or 20 million miles of linear living space. – JDługosz Dec 26 '14 at 1:26
• Yup. There's some particularly strong mineral on the world that makes mountains able to be taller. The plates also move far faster meaning more crumpling. The result is a world of very high mountain ridges--put the tops high enough that survival is impossible and you get something like this. You could also have places that are uncrossable in winter and the path alternates hemispheres. – Loren Pechtel Dec 26 '14 at 2:47

For example you can assume that the gravitation constant $G$ is smaller.

Actually I think that chemical composition can be always tuned, but if a planet should be similar to Earth, gravitational acceleration $g$ on the surface should be similar. Thus, from

$g = \frac{G M}{r^2}$

and

$M = V \rho = \frac{4}{3} \pi r^3 \rho$

we get

$G = \frac{g r^2}{M} = \frac{3 g}{4 \pi r \rho}$.

So we need $G \approx 6.0 \cdot 10^{-12}$ m$^3$ kg$^{−1}$ s$^{−2}$ instead of 6.67 m$^3$ kg$^{−1}$ s$^{−2}$ — a little bit more than ten times weaker because the planet must be a little bit more than ten times bigger than Earth and besides the same and the equation gives inverse proportionality.

Than escape velocity is, from

$v_e = \sqrt{\frac{2 G M}{r}} = \sqrt{\frac{8}{3} \pi G \rho} r$,

37000 m/s and this probably determines chemical composition — more light gases should stay. We have to assume that the planet is similar to Earth due to an improbable but not physically impossible accident or because it was created artificially, maybe using magic.

We can also focus on correct escape velocity and composition. We determine gravity constant to obtain escape velocity like for Earth:

$G = \frac{3 v_e^2}{8 \pi r^2 \rho} \approx 5.4 \cdot 10^{-13}$ m$^3$ kg$^{−1}$ s$^{−2}$.

Than $g$ will be 0.86 m/s$^2$, so everything would be lighter, but besides, similarity to Earth should be more plausible.

Data used:

• Jupiter radius is $r \approx 70000$ km.
• Earth mean density is $\rho \approx 5500$ kg/m$^3$.
• Gravity of Earth is $g \approx 9.8$ m/s$^2$.
• Escape velocity of Earth is $v_e \approx 11000$ m/s.

This method should stay in the range of soft sci-fi, not hard, but also not totally crazy fantasy.

• Earth's core is iron, which is a pretty dense metal. If the OP's planet had somehow formed with a lighter substance for the core, as small as possible, and maybe even a porous material above that, I wonder if he could achieve Earth gravity. Also, it may be reasonable to assume that short stocky humanoids could exist in 3 or 4 times Earth gravity. (Good luck to dragons though!) – joeytwiddle Dec 26 '14 at 7:20

How about a "reverse" Dyson Sphere? An artifically created sphere-shaped world the size of Jupiter where the inhabitants live on the surface. The interior of the sphere may be hollow or contain just enough mass to ensure Earthlike gravity on the surface. Better yet, the interior of the sphere contains an artificial sun, somehow suspended in the center of the sphere, that allows inhabitants to live in a zero G environment (according to the Shell Theorem gravity is nullified at any point inside a sphere).

• A Dyson sphere is unstable, though. You'd have to handwave that (might be ok for a fantasy setting) or to provide some background explanation (such wizards using tidbits of forgotten advanced technology, the same advanced technology that allowed building the world — perhaps with repair machinery still operating inside). – Gilles Dec 27 '14 at 15:48
• @Gilles - Yes, building a shell (or a ring) around a star ultimately isn't a good idea without some hand-waving or resorting to magic. A collision with the sun is inevitable. Dyson's solution is disconnect the surface of the sphere into smaller pieces - a Dyson swarm. Each piece orbits the sun separately, perhaps with slightly different orbital radii to avoid collisions on intersecting orbits. – RobertF Dec 29 '14 at 22:13
• Ok, I found the correct term for a "reverse Dyson sphere" - it's called a Bubbleworld. For example, taking an Earth mass planet and "inflating" it uniformly to the size of Jupiter by injecting gas would create a thin-shelled Bubbleworld with approximately 1 G surface gravity. – RobertF Dec 29 '14 at 22:19

I would personally suggest that you make all landforms islands or similar, and getting from one land form to another extremely difficult due to the presence of water. To a relatively primitive species, they would seen impenetrable. If, however, you would like to say that their magic should be advanced enough to cross a body of water, then you could resort to the age old technique of superstition. Anything from an ancient monster to the unknown could be lying on the other side according to them.

The Americas, for example, obviously exist, but due to the Atlantic and Pacific ocean, it took the people from the 'mainland' un unimaginible amount of years to build proper boats, cross over and finally reach the other side. And even that required the us to feel the need to reach India. Your species might not want or need to explore further than where they already are.

If you can cope with having a world in the size of Uranus instead, there would be some interesting topics that can add to your story.

1) The gravity on the surface is interestingly low. It's actually a bit lower than on Earth. It may differ on the poles though since the rotational speed is contributing to the actual surface gravity.

2) There are areas that are relatively calm and some that have a hysterious weather. This of course depends on the atmospheric pressure. If you have a thinner atmosphere then you will have a calmer weather.

3) The rotational speed is very fast and makes an Uranus day shorter than Earth's. It's possible to modify this for a story but if it differ too much the gravity will either be too high (or negative, which will use it's excess energy to sling particles out from the planet at the equator until the gravity returns to zero there).

4) The orbital period of Uranus is 84 years. If the planet "moves" toward the sun it will of course be shorter, but it is also already hotter since it have a large greenhouse effect so it doesn't have to be moved so much. It is also more unlike that a planet of Uranus size will form too close to the sun.

5) The atmospheric pressure is very, very high. You may have a planet with much lesser atmosphere if you want it habitable. In universe, this would be a realistic option if the planet doesn't collect too much gaseous substance from its creation and on.

6) Its surface contains more ice and the atmosphere contains more helium, hydrogen and methane than its Earthlike counterpart. It is because a larger planet that have as big gravity field as Uranus doesn't lose light atoms. Earth loses atoms like hydrogen and helium, therefore as time passes we lose our water into space (as Mars already have done) and so alike is Helium very uncommon here since it doesn't bond to a heavier molecule. The effect of the Uranuslike planet is that it will have more water (Since hydrogen bonds with oxygen), or ice. It will also have a much higher greenhouse effect (with the methane as the biggest contributor)

7) Uranus have many moons and they are relatively large. They can be colonized for a specific purpose or inhabited in a story.

8) The composition of the inside is entirely different from Earth. Even if the outside have a solid cap of ice there may be deep oceans behind the surface thanks to the large amount of water it has collected. The planet will also have much larger tectonic activity (continents that moves) since it doesn't have an all solid interior.

The summary is: I see a possibility to make a habitable planet out of a planet at the size of Uranus. There are a lot of interesting topics surrounding this size of planets that doesn't appear to planets in the size of Earth. You may imagine a larger one too, but it may probably not be surrounded with so much interesting topics since because when it is too big (We aren't certain where exactly the limit is) the interior pulls itself so hard that the interior composition changes, the surface comes closer to the core and the surface gravity will be extreme.

If you find this parameters thrilling, you may read more about Uranus and make your modifications to make an exciting world from a real stellar example!

Assume that your planet has few if any metals, in particular no iron core. Then you can have a somewhat larger planet (someone else can do the math to figure out just how much bigger). See for instance Jack Vance's "Big Planet" and other stories set on the same world.

Interesting topic. Assuming it is a fantasy world so what if the planet would be formed from two different matters. Like dark and visible, or gravity and antigravity. Probably some nonsense, but it's a fantasy :)

One trapped inside during formation would create fast rotating core, perhaps some sort of antimatter and antigravity origin; surrounded by heavy gases. The outer part would consist of visible matter pushed by gravity to the centre but bouncing of antigravity core thus hovering in the space would create outer crust rotating by different speed or perhaps even in the opposite direction.

The outer crust would rotate slower to allow for day/night time.

The planet could have a giant gas ring permanently exchanging gases between planet and the ring dividing the planet in north and south as massive storms are happening at the equator where the ring is touching the ground. The ring majestically reflecting the "sun" rays would create gigantic space mirror most visible during sunrise and sunset.

In the equator also you would have a series of fast changing sink holes connecting outer atmosphere with inner atmosphere beneath the crust. That also will be the only ground connection between north and south. Sweet :)

I'm sure I'm coming into this a bit too late but I'd like to build on the comment made by @2012rcampaign & to a lesser degree the answer provided by @abcde.

Essentially, a world the same size or slightly larger than the earth could be made much "larger" from the perspective of a medieval traveler by doing the following 3 things:

1. greatly reducing the amount of surface area covered with large bodies of water
2. more evenly distributing what's left over the surface of the planet
3. dispersing mountain ranges more evenly (and in smaller groups) across the planet

Point 1:
The overall amount of available water would have to be decreased AND I would recommend flattening out the surface relative to the areas of the planet that might (such as on earth) be covered by ocean...

Currently the continents on earth are like very large islands, which we live on the top of, when you consider the depth of the part of our world now covered by oceans. If you flatten this out, so higher land like the continents, would not exist, or would be very rare then this produces much more area that must be traveled.
In the absence of enough lower land, less water would likely be needed to provide enough for the sustaining of life because it could be made to be available across the entire world... where now the water generally runs off into the oceans and is lost until it evaporates and returns (sometimes) to the contents as precipitation. So replace the oceans with many relatively shallow lakes spread fairly evenly across the world.

The presence of many lakes, rivers, vast marshes, & other water features in place of oceans, might provide enough evaporation for a fairly normal water cycle (water evaporates from the surface so it might be possible to have 45% of the surface covered with water without having large concentrations of water, along with the evaporative effects of much larger areas of vegetation).

Point 2:
If there were no large oceans & the general level of the surface of the planet where flattened out, there could be vast areas of very difficult terrain including:
-Very large marshes/swamps
-very tall seeming mountain ranges (the mountains we generally think of are all on top of contents which are nearly mountains themselves from the average depth of the ocean floor... so take away the contents and have a few mountains reach up past where a content would have reached... this would effectively be very tall mountains...)
very broad, long, slow moving rivers (with less of an incline toward an ocean both longer, more broad, & slower could be possible... though none of it guaranteed given that there would/could also be many lakes, swamps, etc...).

Point 3:
Finally you may actually have to more evenly disperse mountains into smaller mountain ranges, which are also spread more evenly across the surface of the planet.

Mountains provide runoff of snow and a condensation point for clouds (precipitation often occurs around mountains as, if I remember correctly, clouds are pushed up by the movement of atmosphere being deflected up by the presence of the slope of the mountains...[colder thinner air, as I understand it, can hold less moisture). If the mountains are more evenly dispersed then having many more bodies of water would be more believable as would having an overall greater 'flatness' to the world.

Sounds like an interesting story, good luck!

In 1981 Robert Silverberg's novel "Lord Valentine's Castle" was nominated for a Hugo (Joan Vinge's "The Snow Queen" won), and won the Locus award. It was the first of many stories set on Majipoor, a world ten times the diameter of Earth, yet having a similar gravity. The explanation given was that the core was far less dense, and that consequently the world was very poor in metals.

It's been over thirty years since I read that novel, so I can't recall if there was any more justification for the large size but normal gravity. Nor do I recall if there was any discussion of lack of tectonics and a molten metal core, and the effect on magnetic fields for the planet. I do recall that characters sometimes travelled for years to go from their origin point to a destination. There were also descriptions of cities with populations in the billions.

The setting has a fantasy feel to it, but the foundations were science fiction, with multiple races of aliens settling the world over thousands of years, displacing the natives.

If this is not enough, many years before these novels, in the mid-1950s, Jack Vance wrote a pair of novels ("Big Planet" and "Showboat World") which take place on a large planet with gravity similar to Earth's, putatively due to low density and a metal poor core.

• Could you explain a bit more? We just don't like answers that are link dependent. It's a problem if the link become inaccessible for some reason. – Vincent Dec 26 '14 at 16:49
• There is no data in the link that is relevant to the question but not already in the answer. More explanation would be nice, but you aren't going to get it from that link. I don't know if Silverberg went beyond larger but less dense (lighter core). – Brythan Dec 27 '14 at 5:29
• I've rewritten the answer. If this is unsatisfactory still, go ahead and delete it. – Don Wakefield Dec 27 '14 at 22:20