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Imagine a world in which you have all the creatures from the game franchise "Monster Hunter"...

enter image description here

Screenshot from the upcoming Monster Hunter World game

...and when I say all the creatures I mean enough creatures of each species that they are all genetically viable.

Knowing there is HUGE monsters (like Jhen Mohran), they live in HUGE natural areas (maybe not in one place for all the being of one specie).

What could be the size of such planet also taking account there is some hunters (obviously) killing them down ?

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    $\begingroup$ Does it need to be a spherical planetoid? Someone else, would you be so decent as to suggest the Ringworld? $\endgroup$ – can-ned_food Oct 12 '17 at 9:56
  • $\begingroup$ Looking at those hunters lifting their enormous weapons and swing them in the air with their thin arms... they should be on a planet with less mass than Earth! Proof? Try hunt a rajang when it goes super saiyan and lose it... jumps up and turn itself into a kamahamaha wave or spirit bomb! $\endgroup$ – user6760 Oct 12 '17 at 10:05
  • $\begingroup$ Yes it has to be round. $\endgroup$ – Tagadac Oct 12 '17 at 10:10
  • $\begingroup$ Since not everybody knows the game, could you give some more details on how many different species there are and how big they are? Body weight is probably more important then length or height, since it is a better measure for how much space they need and how much they eat. $\endgroup$ – Till Oct 12 '17 at 12:11
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    $\begingroup$ You might want to reword the question to "What is the plausible smallest size a planet with numerous huge creatures?" Your current form has no end toward the bigger size of the planet (why not a planet with a size of the sun?) $\endgroup$ – Vylix Oct 12 '17 at 18:26
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"Naïve" answer: our tiny planet has hosted species of all sizes, ranging from amoeba to Apatosaurus

Apatosaurus had an average length of 21–22.8 m (69–75 ft), and an average mass of 16.4–22.4 t (16.1–22.0 long tons; 18.1–24.7 short tons). A few specimens indicate a maximum length of 11–30% greater than average and a mass of 32.7–72.6 t (32.2–71.5 long tons; 36.0–80.0 short tons).

More than the size of the planet, it's the subtended biomass that limits the size of the animals. Again, when the dinosaurs ruled, the available biomass was higher, and thus the size of the animals could be bigger.

Today we cannot get anything bigger than an elephant of a blue whale, and the planet size has not changed.

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  • $\begingroup$ Yes there was big animals on Earth. But one of the biggest monster is the Dalamadur (monsterhunter.wikia.com/wiki/Shah_Dalamadur) which is ~440m. Could Earth be capable of having such creatures with all the others BIG species ? $\endgroup$ – Tagadac Oct 12 '17 at 10:26
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    $\begingroup$ One big factor affecting dinosaur size was the concentration of oxygen in the atmosphere. More oxygen = larger animals. $\endgroup$ – BrettFromLA Oct 12 '17 at 23:10
  • $\begingroup$ I suggest to have this answer especially highlighting that planet size can be any big. Since its worldbuilding, the hunting itself depends on the hunter species, so what they prefer, that will qualify. A triple size of Jupiter can be just as good as a Pluto. Tiny dinosaurs can prove to be feasible when the hunter species are similar ratio smaller. I think it is very well scalable. $\endgroup$ – Sonic Oct 13 '17 at 13:42
  • $\begingroup$ @Sonic, I find hard to imagine a dinosaur bone working on Jupiterish planet $\endgroup$ – L.Dutch Oct 13 '17 at 15:36
  • $\begingroup$ @L.Dutch, it depends on the conditions you set. If you set the very same conditions for Jupiter and dinosaurs, as our current science knows about it, then yes. A huge planet with dust-filled atmosphere will not support carbon based green vegetation and connecting fauna. Though Jupiter is 11x times bigger than Earth, it has only 2.5x bigger gravity. Not all Earthian things will work on Jupiter, but human thigh bone can endure 1 tonne pressure. Please let me know the aspect of your doubt. $\endgroup$ – Sonic Oct 30 '17 at 8:19
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On Earth, the thing that prevents creatures getting any larger than they do is gravity. If you want bigger animals, you need a lower surface gravity.

It turns out that the maths for this isn't as simple as smaller planet -> lower gravity - the actual equation is Surface Gravity = Mass / Radius^2

Of course, Mass = Density * Radius^3 (for a sphere), so assuming that you wish to have a similar composition (and so density) to Earth, surface gravity is directly proportional to the radius of the planet, and so you will want a smaller world.

Of course, if you make it too small then the atmosphere might escape, but that's a whole other set of problems.

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    $\begingroup$ It's not as simple as a smaller planet. Two planets can have the same mass, but different surface gravities if they are of different sizes. The planet with the bigger radius will have a lower surface gravity. Depending on their mass & density, smaller planets can surprisingly have higher surface gravity. See this webpage for more information: psychologytoday.com/blog/world-wide-mind/201211/… $\endgroup$ – a4android Oct 12 '17 at 11:59
  • $\begingroup$ I've never heard of that before; guess I should add some caveats to my answer. $\endgroup$ – walrus Oct 12 '17 at 12:47
  • $\begingroup$ Thank you for your response, I learned a lot of things ! So maybe the solution is not about the size nor the surface gravity but maybe in the composition of the atmosphere of the planet ? See @DannyBoy response. $\endgroup$ – Tagadac Oct 12 '17 at 16:08
  • $\begingroup$ Oxygen levels aren't really the limiting factor for non-insects (I've commented on their response) - if that were the case then we wouldn't see mammals larger than the largest insects. Some more SE questions that may be relevant: worldbuilding.stackexchange.com/questions/316/… worldbuilding.stackexchange.com/questions/57348/… worldbuilding.stackexchange.com/questions/40640/… $\endgroup$ – walrus Oct 12 '17 at 16:41
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Dinosaurs

The reason the dinosaurs were so big was mainly due to the higher levels of oxygen available in the atmosphere. According to this, this was also the reason why all arthropods were also much bigger than today. This because they "absorb" oxygen through their carapace. This method of breathing is limited and thus the higher oxygen level is what enabled them to achieve the size they did.

This also applies to all other animals that breathe or need oxygen. A higher oxygen level while all other parameters are maintained, enable an increase in size.

Your world

In your case, the only real thing your planet would need is not an increase in size but an increase in oxygen levels. This can easily be accomplished with greater flora or more "efficient" plant life.

I read an article that I cannot find now that stated that one of the factors for greater oxygen levels in the prehistoric era of the dinosaurs and even before them, was in part due to the fibers and cells in trees of that time. Bacteria and smaller insects didn't know how to process these substances and later evolved to do so. But when they could not, the dead trees simply lay there and didn't decay as they should. This lead to less carbon dioxide emissions as well, also being a cause for bigger arthropods and the like.

Summary

To be able to produce a planet that could sustain massive animals such as monster hunter animals, increase oxygen levels. Their size, as mentioned being ~400meters, should be irrelevant since a planet the size of earth is still absolutely massive. If you want to, just increase the size to your liking and change the composition of the planet to have a mass that equates to 1G, but is bigger in volume. In short, greater volume, lower density.

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  • $\begingroup$ Thank you for your response. Didn't see this "problem" with this angle of view. I think I'm not aware of how big is my planet ? $\endgroup$ – Tagadac Oct 12 '17 at 16:09
  • $\begingroup$ This response seems to conflate the limiting effect that oxygen has on insects with all animals. Insects breathe (to dramatically over simply) through their skins, which restricts their size because the amount of skin (and therefore oxygen they can take in) increases as the square of their size, while their volume (and so amount of oxygen they need) increases as the cube. Mammals etc solve this problem with lungs, and the main limit on their size is gravity (the square cube law again). The Monster Hunter animals don't look like insects to me... $\endgroup$ – walrus Oct 12 '17 at 16:37
  • $\begingroup$ Yes, insects mainly benefitted from the higher oxygen levels but so did vertebrates. Perhaps not equally as much, but still sufficient to allow creatures such as Diplodocus Hallorum to exist. I do agree with you that gravity does limit the MAXIMUM possible size of animals. I thought your answer was good as it was so adding your information would just be repetitive. The information in my answer was another aspect on creature size. $\endgroup$ – DannyBoy Oct 13 '17 at 6:56
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The monster hunters planet can be up to roughly twice the radius of Earth

Rather than try to think of how big an area is required to support all the different species, a different approach is to simply ask how big could a planet be given what we already know?

Judging by the picture the monster hunter world is an earth similar planet with exotic creatures that appear to be adapted to around 1g. If the surface gravity is 1g and if the planet has the same composition as Earth then the size of the planet must be the same as the Earth. The planet could be bigger if it were less dense than Earth. But how much less dense might it reasonably be? Assuming that it’s a rocky planet like the Earth the obvious way of making it lighter is by removing the iron core.

The density of the Earth is 5.5g/cc, but the density of the crustal rock is only around 2.7g/cc. So if the iron core were removed and replaced by crustal rock the density would be roughly halved. Gravitational force = 4*3.142*Gdr/3
Where
G = the gravitational constant
d = density
r= radius

So halving the density allows a doubling of the radius for the same gravitational force. Therefore the monster hunters plant can’t be that much more than twice the radius of the Earth.

There might be a bit of wriggle room for the following reasons but each of these is limited:

The rocks might be lighter than we have on earth
but not that much before we run into unrealistic rocks.
There might be a lot more water present
but not that much or we would end up with a water world.
The surface gravity might be greater than on Earth
but not that much before we would expect to see very different animal adaptions.

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