I want the world to have an extremely long age of discovery, such that even if they were to use a ship to travel in a straight line across their planet's ocean for hundreds of years they'd still be discovering new lands and peoples and so on and so forth.

In an effort to accomplish this I've lowered the overall strength of gravity such that a solar mass in our universe is just a planetary mass in the setting's universe, with a planetary radius to match, so you can see where I'm going with this. Essentially the goal is to create an Earth-like planet(with Earth-like gravity) the size of a star(let's say ours, Sol).

Problem is I don't know if I can get away with weakening gravity to such an extent, whether or not life as we know it would still be able to evolve in such a universe, whether or not things would still work somewhat normally with such a weak gravitational constant.

Can I get away with lowering gravity so that the usual solar masses in our universe are simply planetary masses in the setting's universe?

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    $\begingroup$ Real history did have a very long period of continuous discovery; the period which is normally called the Age of Discovery was just a particularly notable episode in this journey. If you take for example the inhabitants of central Italy, they were discovered by the Greeks around the 8th or 7th century BCE; came into contact with the Gauls and the Macedonians in the 4th century BCE; clashed with the Carthaginians in the 3rd; visited northern and central Europe in the 1st century BCE; established contact with India in the 1st century CE; were discovered by the Huns in the 4th century CE; ... $\endgroup$
    – AlexP
    Oct 3 at 13:24
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    $\begingroup$ ... Had a lot of adventures with the Germanic peoples from the 3rd to the 7th century CE; sparred with the Arabs from the 8th to the 12th century CE; were informed of the existence of the Americas, Australia etc. from the 15th to the 18th century CE. All in all, the inhabitants of Rome hardly had any century from the 7th BCE to the 18th CE, 2,500 years, when they did not contact a new people or learn new geography. And this goes for whatever civilization you chose. Geographical discovery never ceased from the beginning of the Bronze Age to the beginning of the 20th century. $\endgroup$
    – AlexP
    Oct 3 at 13:30
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    $\begingroup$ Life might evolve in such a universe, but most of the biochemistry of Earth itself would become incompatible. For instance, by changing the mass ever so slightly of hydrogen, water itself becomes toxic (someone correct me if I'm wrong about heavy water... it's not the slightly-more-or-slightly less polar-ness of the molecule, but just the extra mass that does it). If you need an incredibly large world, go with a Niven Ring or an Alderson Disk. Sure, you're still screwing with physics a bit, but easier to handwave. $\endgroup$
    – John O
    Oct 3 at 13:51
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    $\begingroup$ Your radius would then be 1000x that of Earth (roundly), area one million times (same as Niven's original Ringworld) and volume 10^9 times (Americans would call that a billion, British a thousand million). I think that's a lot bigger than you need... $\endgroup$
    – Zeiss Ikon
    Oct 3 at 14:31
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    $\begingroup$ @JohnO: We're not changing the inertial mass of hydrogen relative to electrostatic forces, only gravitational weight. I don't think a change to G would affect chemistry directly, as it's already negligible compared to electromagnetic forces (like 40 orders of magnitude for forces between a proton and electron). (There'd be indirect changes in terms of cosmology and production of heavier elements, and thus their availability for life to evolve, if there were even stars to supply energy.) $\endgroup$ Oct 3 at 22:45

9 Answers 9


Your Universe is Fantastic!

I would like to advance this Stack's core belief to you. From the Help Center we read:

World building includes geography, culture and creatures for the world, not to mention magic and planetary physics, in short, everything from the physics underlying your reality to the entire universe you want to build.

Far too often we think of ourselves as Physics Lite. We start to believe that there are only two kinds of imaginary worlds. One that's entirely defined by magic and the other that's just Earth by another name. Even you're tempted to believe your imaginary world can only exist within the rigid and boring context of known science.


In your universe, brilliant stars are the size of planets and habitable planets are the size of stars!

You have no obligation whatsoever to explain, rationalize, or justify those world rules. That is the nature of YOUR universe. It's cool beyond measure! Every discoverer in human history would wish for a world as filled with mystery as yours.

Ignore everyone who says you can't do that and go do that.

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch
    Oct 6 at 6:48

From a physics standpoint, the usual answer given is that if you change any of the universal constants by even a tiny bit (8th or 9th decimal place, say), you produce a universe in which life cannot evolve. Mess with the gravitational constant, and either stars don't form, or they're tiny and have a lifetime of a few decades instead of millions to trillions of years -- or perhaps they always collapse directly into black holes when they burn all their fusion fuel, meaning no second-generation stars and no rocky planets.

What your world needs is a healthy dose of handwaving.

Perhaps your world is a hollow spherical shell, only a few times as massive as the Earth (just enough to give that 1 G at surface, however large it is, and no one will ever know what keeps it from collapsing). Perhaps your universe has an extra dimension, so the surface area of a (hyper)sphere goes as the cube of radius, instead of the square. Perhaps there's a constant haze high in the atmosphere that makes navigation much more difficult (maybe impossible before magnetic compasses are discovered). Perhaps extreme storms mean only one ship in a thousand that ventures more than a couple days out of sight of land is ever heard from again.

Maybe new land rises from the ocean on an ongoing basis (and if you miss it for twenty years, it'll be covered with at least plant life and birds by the time you do find it).

You're doing a lot of handwaving to lower the gravitational constant anyway -- enough to get labeled as fantasy (as was the case with Prince Valentine's Castle and its sequels, which also had an unreasonably large world). Why not go all the way and avoid having to try to explain stuff, when the explanation is much harder to swallow than "This is how it is and no one knows why."

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    $\begingroup$ Perhaps there's a constant haze high in the atmosphere that makes navigation much more difficult - Because you can't see the stars? Beware of cultural side-effects of that; cf. Krikkit :P Although a world so huge in area, like Ringworld size, would probably have the same effect of needing to think of your civilization as a speck in a vast ocean. $\endgroup$ Oct 3 at 22:41

Size is relative. To a human, crossing a meter-wide stream is a skip and a jump; to an ant colony, it's a major engineering project. To a human, crossing an ocean is a major engineering project; to a humpback whale, it's a yearly tradition.

The Earth has plenty of animals much smaller than humans. Roughly speaking, pygmy marmosets are 10 times smaller, African pygmy mice are 30 times smaller, Vijayan's night frogs are 120 times smaller, and some jumping spiders are 300 times smaller. On a planet the size of the Earth, gregarious, tool-making creatures the size of African pygmy mice might take about 30 times as long to explore the globe.

Size isn't directly proportional to range or speed. Some tiny creatures, like monarch butterflies, can travel on continental scales much more easily than humans do. However, disproportionality can also work in your favor. Imagine a crew of pygmy marmosets crossing the Ka'ie'ie Waho Channel to get from Kaua'i to O'ahu. Proportionally, we could compare them to a crew of humans crossing the Bay of Bengal to get from Sri Lanka to the Andaman Islands. Their challenges, however, would not be to scale! To them, meter-high waves would be towering walls of water, eight times the height of the tallest sailors. Smaller species tend to have faster metabolisms, so they'd have to pack much more food and water relative to their size.

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    $\begingroup$ Faster metabolism also means shorter lifetime, generally. Thus they will have to travel for generations. Also you could probably double the surface size of the Earth without increasing the gravity by playing with metallic composition, less iron more silicon... Combine that with smaller animals, shorter life-time you get what the op wants. $\endgroup$ Oct 4 at 7:26

I don't understand the negative answers to this question. I think it can be done just fine, and I don't really even see a problem with it. As you said you can just make the gravitational constant smaller, and you will then be able to have a planet that's much bigger than the Earth but with the same surface gravity as Earth.

As one answer points out, if you really wanted to be hard-science about this you would run into problems with cosmology, in terms of how stars and planets form, how quickly stars burn, etc. But that could easily be handwaved by just saying nuclear physics works differently in your world. (Or by just not mentioning it if it isn't important to the setting.)

So let's just say planets and stars can form ok in your universe, and we have our planet that's 100 times the radius of Earth (with 10000 times the surface area), and its surface is made of the same kinds of stuff the Earth is. Here are some consequences of that:

  • Your planet will be much more flying-saucer-like in shape than Earth. This is because, presumably, you want the planet to rotate about once per 24 hours. This means that there's a centrifugal force of about $\left(\frac{2\pi}{\text{24 hours}}\right)^2 \times 100\times \text{Earth's radius}$, or about 0.34g, pulling outward from the equator. So the equator will bulge out quite a lot, resulting in an oblate spheroid instead of a sphere. (The Earth is also approximately an oblate spheroid, but Earth is much closer to spherical than your planet will be.) Because of the mass of the bulge, the people on the surface of your planet will not feel as if they are lighter at the equators than at the poles - the gravity will just be 1g everywhere. This effect increases as you make your planet bigger, and there's a limit to how big you can make it before the planet stops being stable at all. But 100 times Earth's radius seems to be just about ok.

enter image description here CC BY-SA image of oblate spheroid by Tomruen from Wikipedia

  • Aside from being a lot more squished and saucer-like than Earth, your planet will be much smoother and closer to its ideal oblate spheroid shape. This is because the heights of mountains and continental bulges and so on are limited by material properties, and making the planet bigger won't change those. But this is probably exactly what you want - mountains about the size of Earth's mountains, but just a lot more of them.

  • If there is a moon it will be much rounder as well, so it will appear much more like a smooth sphere, with details only visible through a telescope.

  • The year will be really long, and so will the month if there is a moon. This is for two reasons. Firstly, because the gravitational constant $G$ is smaller, an orbit of the same size will take longer to complete. Secondly, the sun and the moon will presumably also be around 100 times bigger than Earth's sun and moon, which means they will have to be about 100 times further away in order to look about the same size in the sky. This also means their orbits will take longer to complete. A year on your planet will be at least 10000 Earth years in duration and probably much longer. This means that one pole of your planet will probably have been in permanent darkness throughout all of recorded history, and the other in permanent daylight. Most likely the pole that's in darkness will be covered in ice but the other one won't - it might actually be quite warm there, just because it's had a long time to warm up. The equator will still be warmer though, because the sun rises higher into the sky. (Note though that since stellar physics probably has to work differently in your world, you could just say the sun and/or moon are much smaller than I've assumed, and therefore closer, which would make the year shorter. If you really want a year to be about one Earth year, you could have the sun orbit the planet.)

  • Plate tectonics might be weird. I'm not sure how it would work on a planet that size. But maybe you can just say the mantle is about the same thickness as Earth's mantle, with the planet having a giant core underneath that. Then you might be able to have continents about the same size as Earth's continents, just a lot more of them. Alternatively you could try to have a much larger mantle and correspondingly larger continents, but I'm not sure how well that would work, due to the aforementioned limits on material properties.

  • Climate and weather will probably be weird, and not just because of the long year or the weird shape of your planet. If you want the surface to be Earth-like, that will mean your planet has an atmosphere about the same height as Earth's atmosphere, which will be much thinner compared to the size of the planet than Earth's is. This means that the global wind circulation patterns will be very different. Earth's atmosphere looks like this, being composed of six circulating 'cells' of air:

enter image description here CC BY-SA image by Kaidor, from Wikipedia

The reason there are six cells has to do with the thickness of the atmosphere compared to the planet (among other things), so on your planet it will look different. I imagine you would either have a lot more cells, or it won't be broken up into cells in the same way and could be a lot more chaotic. This might work in your favour though, because it might mean you can have giant deserts (even bigger than Earth's), or whole continents where it's really windy, and other such interesting things for your world's travellers to find.

  • On a related note, if your characters live in a temperate region they will have to travel a really long way to find anywhere with a tropical or polar climate, and so on.

  • Evolution might be weird. A larger planet gives more opportunities for species to get isolated and take their own evolutionary path. So (for example) instead of just mammals and marsupials there might be many different types of mammal-like animals. Or there might be somewhere where dinosaur-like animals still survive. There might even be other intelligent species if you travel far enough, as there would be more opportunity for two intelligent species to evolve without meeting each other.

  • Reaching orbit or escape velocity will be for all intents and purposes impossible. The speeds needed to orbit or escape your planet will be vastly higher than the speed needed to orbit the Earth, and it's most likely that it would never be technologically possible to reach those speeds starting from a sub-orbital flight. If your civilisation reaches space age technology the best they can hope for is to send rockets on huge parabolic trajectories, only to fall back down again. (So there will never be any satellites, but ICBMs will still work, and you might also see suborbital flights used for point-to-point travel.)

  • Meteorite impacts might be much more extreme. This one was wrong sorry. Orbital speeds are generally slower in your world, but your planet also has a very large gravity well, which will speed meteorites up before they impact. I'm just going to guess it all cancels out and meteorite impacts are about the same as they are on Earth.

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    $\begingroup$ If this planet has "the same surface gravity as Earth," won't its escape velocity be similar, too? $\endgroup$
    – Tom
    Oct 5 at 4:20
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    $\begingroup$ @Tom no, because escape velocity depends on the size of the gravitational well you're in, not just on the surface gravity. Think of it this way: suppose someone built a solid sphere around the Sun with a radius of 4 million km. Standing on the surface of that sphere the gravity would be a bit less than 1g. But orbiting around that sphere is just the same as orbiting around the sun, and its escape velocity would be the same as the sun's escape velocity - much higher than Earth's. $\endgroup$
    – N. Virgo
    Oct 5 at 6:54


Leave gravity alone. Instead, some extremely advanced race built a Earth-orbit-sized shell of material 4000 miles thick and the inside is somehow handwaved to be totally empty. This gives an apparently Earth-normal world except it's the size of Earth's orbit.

Not only do you get great areas to explore but while flight can develop normally there can be no space program without major breakthroughs (to get into orbit you need antimatter rockets) so it never gets pre-empted by camera satellites.

You'll want a very thick atmosphere to protect you from grains of dust, that will make flight easy but slow.

  • $\begingroup$ The planet is an dyson sphere. A long time ago some aliens built an dyson sphere that was later abandoned for unknown reasons. $\endgroup$ Oct 6 at 8:36

Definitely not

The first question you have to ask yourself is : what is the biggest terrestrial planet size we could find in the universe. Lucky you, someone already asked this question, on this sub : See here. The answer provided is : ~2Re (with Re: Earth radius).

Now to give you a bit of a solution, in the case we are taking this 2Re Planet as your planet. The surface area would be 4 times the Earth's one (A=4.pi.(2Re)^2). And four times the earth surface is really huge, considering your planet is heavier (and to add to this, let's suppose it has a bigger gravity) a ship would be slower to run across the globe.

The first recorded circumnavigation took 3 years without the exploration part, the second one took 11 years. Considering your ship does not have plans to rush the circumnavigation, is slower to perform the trip, has at least 4 times the perimeter (if going straight because earth perimeter is pi.Re.2) to cross (but you could say it's for discovery purposes so not straight at all, with big pauses to explore etc..). Having a 100-years trip could be reasoneabale with this configuration (without considering the logistic and the fact humans does not live 100years).

EDIT: I only answered considering we are in the actual world (with the same physical constants). Messing with those parameters is almost impossible without affecting the entire universe and one could believe messing that much with the gravitational constant would make life impossible (if not the whole universe) as another user said.

  • $\begingroup$ More mass doesn't necessarily mean greater gravity. Planetary density plays a huge role. Just look at Saturn: 95 Earth masses at little over 1g. $\endgroup$
    – BMF
    Oct 3 at 14:15
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    $\begingroup$ @BMF Indeed, but we are speaking about a terrestrial planet in this case so in most of the known case it is what's happening. Furthermore it was an idea to justify the vessels being slower on the sea in the OP's world. But you are right, shoud edit this to be more precise as it is definitely poorly worded, thanks. $\endgroup$
    – ohpif
    Oct 3 at 14:32

Actually the question barely has any meaning.

Changing gravity won't change any object's mass. Gravity applies to mass, but doesn't modify it. The same way, you will not say : "Can I get away with reducing my magnet to the point where there is barely any iron remaining in the coin?".

Gravity will change the intensity with which any mass is attracted to another, though. Maybe that's what you meant. This is a couple of consequences I can imagine.

  1. Mass Decreasing gravity does not decrease molecules' density (mass for a given volume). So if you want the gravity of your bigger Earth to feel the same for humans living on its surface (and you want it, otherwise the atmosphere is not retained and escapes to space, oceans evaporate, and there is basically no life, like on any object with a very low gravity like asteroids or the Moon), keeping the idea of weak gravity force intensity, you will actually need much, much more mass. That bigger Earth with the same feeling of gravity at its surface would thus require much more of the same material that makes earth to compensate the weakness of the gravity constant, leading to actually having a much bigger mass.

  2. Planet cohesion Any neighbouring object (the central star of the planetary system, a natural satellite) applies a "tidal force" to its neighbour. To put it simply, when the moon is exactly above, say, Rio de Janeiro, then it applies a stronger gravitational attraction to Rio than to Taiwan, because Taiwan is on the opposite part of the Earth and is therefore 13,000 km farther. Oceans make it obvious with tides because they are liquid, but the Earth's crust is also slightly distorted. If the difference of force's intensity between Rio and Taiwan were too high (either because a neighbouring object would be too heavy, or too close), the crust might first crackle, leading to so many earthquakes and volcanos, then the next step would be the Earth breaking in crumbles. That's what happened to the Shoemaker-Levy comet, coming too close to Jupiter in 1992. And you guessed it: the more your planet has a big diameter, the more it's easy for an object, even quite remote or quite light like for example the central star of the system, to just break it by tidal force. Actually, that planet would never appear. That's why you don't see huge telluric planets as @ohpif points, and why huge planets are gas planets, like Jupiter or Saturn.

  3. Star birth If the gravity is much lower, then it would take a lot more gas and a lot more time for stars to appear, and probably for galaxies too, if they can appear at all. With the figures you said, it would probably take billions of times more time. And stars would be billions of times bigger too (in volume) in order to have enough mass so that the small gravity is able to however produce the huge central pressure that is enough to "light up" the star (actually: start the thermo-nuclear reactions producing light). Black holes and everything would also be much bigger.

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    $\begingroup$ You're correct that changing the gravitational constant will not change the mass of a particular planet or star, but it absolutely will change what constitutes a "stellar mass" versus a "planetary mass". If G were lower, the weight of the sun's mass would not produce sufficient pressure to keep a nuclear chain reaction going, so it would not be a star. The question isn't asking how to lower G to reduce the mass of a star (which can't be done), it's asking how to lower G so that an object with a particular large mass becomes a planet and not a star. $\endgroup$ Oct 4 at 17:06
  • $\begingroup$ "Can I get away with reducing my magnet to the point where there is barely any iron remaining in the coin?" ... do you mean strengthening the magnet? I would say that. What is wrong about saying that? Consider a hypothetical coin with hypothetical iron content that is just barely high enough to be magnetically moved by said magnet. Seems like a valid question. This is worldbuilding, not a physics lab. $\endgroup$
    – cowlinator
    Oct 5 at 1:23
  • $\begingroup$ @NuclearHoagie You're perfectly right. The question starting with "Can I get away with...", I was just pointing a couple of side-effects to address to "get away" with that, the main one being the structural cohesion (or even the possibility of appearance) of such an amount of matter that would be barely tied together with a very weak gravity. $\endgroup$
    – Freedim
    Oct 5 at 11:15
  • $\begingroup$ @cowlinator No, I did not mean strengthening the magnet, I meant reducing it, like cutting it and taking a half, a quarter etc. Do what you want to the magnet, it will not magically add or remove iron inside the coin. But I think I misunderstood the OP anyway, so my example with the magnet does not actually apply. However, the side-effects to be addressed remain, if you want to "get away with it". But as others pointed, one doesn't need to address any physics consequence of an idea, as long as your narrative doesn't bump into those consequences. $\endgroup$
    – Freedim
    Oct 5 at 11:22

From a physics perspective I think this becomes more problematic than just having a really large planet and ignoring the physics (if this is fantasy rather than SF even moreso).

If you particularly want a huge world that obeys physics there are a couple of ideas you might consider:

  1. A planet with a significantly different construction.
  2. In SF, constructs like a Ring or Sphere take the mass of a planet and transform it artificially to give far greater surface area. You might employ such a device (which could include a 'planet' which is basically hollow) and decide if the inhabitants know or not. It might take centuries or millennia for them to realise.

If you want your setting to be science-based, stay away from quantum/relativistic stuff where the things will get really, really messy.

On the other hand, from the classical physics standpoint, you have to deal with:

  • The internal heat exchange of the planet in square-cube law context. Our Earth still (4.5bn years after its formation) has an internal ocean of molten rock and iron. Your planet will have to cool WAY longer in order to have a solid surface at all. Expect strong volcanism, tectonics and geothermal activity as well.

  • Either much deeper gravity well (if you want comparable to our own surface gravity) with much harder or impossible space travel - or - weaker surface gravity, thicker and very turbulent atmosphere with storms featuring a great deal of dust, sand, snow and water. And, splashy water bodies. Or something in between.


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