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I am working on a super-earth, about 1.5 more massive than Earth. On this planet, there's a large variety of megafauna and flora across the world. The biggest trees over there can grow to be 36 meters tall. Giant long-necked herbivores that live near rivers feed off tall trees and their backs are like living ecosystems for smaller animals. Aggressive insectoid predators, about 9 meters tall, lurk in the forests (when I say insectoid, I mean that they simply look like insects, while they're actually vertebrates). Apart from that the biggest flying creature there has a wingspan of 20 meters, which is about 2 times bigger than the biggest flying animal on Earth, Quatzocoatlus. Now, the question is: How can creatures this large exist on a planet with stronger gravity? One possible solution that I thought of was if the creatures' bones were composed of carbon nanotubes, but I'm not really sure if that's biologically possible. What do you think?

Edit: Just to clarify, the gravity of this world is 1.5g

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    $\begingroup$ "1.5 times bigger and more massive than earth" is slightly ill-defined... a planet weighing 50% more than the earth but with the same density won't have a 50% greater radius. A planet with the same density and 50% greater radius will be much more massive, etc etc. What surface gravity to you want? Carbon nanotube bones won't help with the horrendous energy requirements for flyers on a high-g world. They're likely to be small, if they exist at all... $\endgroup$ – Starfish Prime Nov 26 '19 at 15:08
  • $\begingroup$ James Cameron called, he wants Pandora back ;) Seriously now, this is the staple of Scifi. Go for it. $\endgroup$ – The Square-Cube Law Nov 26 '19 at 15:17
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    $\begingroup$ Do you actually WANT stronger gravity, or do you just want a larger world, and you're assuming that REQUIRES stronger gravity? You can have the 1.5x earth's diameter without having 1.5x earth's gravity, that would be better for your megafauna. $\endgroup$ – Morris The Cat Nov 26 '19 at 15:52
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    $\begingroup$ "The biggest trees over there can grow to be 36 meters tall." The biggest trees in real life are around 100 meters tall, with the tallest known being Hyperion, standing at around 115 meters. So 36 meters tall isn't tall at all for a tree. $\endgroup$ – overlord Nov 26 '19 at 16:04
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    $\begingroup$ Some jim-jam on gravity might be the easiest way to get mega fauna. Also, some changes to the atmosphere, more oxygen and more CO2 at the same time, would juice up both plants and animals. $\endgroup$ – puppetsock Nov 26 '19 at 16:19
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Your wording is slightly unclear but I'll answer on the basis that planet is 1.5x bigger (1.5x the radius) and 1.5x more massive than the Earth.

Now the acceleration due to gravity F = GM/r^2

Compare your world with Earth: Fworld = G(1.5M)/(1.5r)^2

The surface gravity on your world with be 1/1.5 times that of Earth, i.e. only 67% of Earth gravity or about 6.7ms^-2. This is a low gravity world, not a high gravity one.

That should suit your megafauna and megaflora quite well. Note that the tallest tree on Earth is the giant sequoia which can grow to 115m tall, so your suggested size for biggest trees on your world may well be a big underestimate, you might find the big ones reach around 150m. Terran giraffes reach 5.8m and clearly taller creatures than giraffes existed in the past (tallest ever currently believed to be Sauroposeidon at 18m tall), so 9m for a large creature on your world doesn't seem out of the question.

Now this all means that the planet has a lower density than Earth, probably specifically meaning a predominantly silicate rather than iron core. That may suggest it's a rather mineral poor world, though you could easily arrange for a suitable heavy bombardment soon after planetary formation to ensure the planet had been provided with enough heavy elements in the crust and upper mantle to support a rich biosphere.

Edit: on closer inspection, the maths makes a planet with 1.5 x the radius and 1.5 x the mass extremely unlikely - its density would be about 10.5% that of Earth, i.e. half the density of water. However that's not to say a world with that radius but a similar-to-Earth gravity isn't possible: with a density of 0.67 that of Earth (i.e. 3700kg/m^3) it would have the same surface gravity as Earth. That's a reasonable density for rocks, so you just need to state that your planet is primarily rocky rather than with an iron core, which may in turn mean it has a much weaker magnetosphere and you therefore need to place your planet in orbit around a much quieter star to avoid irradiating the surface.

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  • $\begingroup$ A predominantly silicate core has potential issues with generation of a strong planetary magnetic field... $\endgroup$ – Starfish Prime Nov 26 '19 at 15:40

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