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I’m trying to do some world building for a game I’ve been working on, and so I’ve been designing a planet that would fit large fauna and flora. What I would like to know is if the mass of a planet has any affect on the size of organisms on said planet.

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  • $\begingroup$ Wouldn't the basic research Worldbuilding SE asked of you have shown that the simple act of moving on larger planets with greater gravity required more muscle, basically meaning bigger bodies? Either way, how is this not off-topic as a real-world Question? $\endgroup$ – Robbie Goodwin Oct 17 '20 at 21:54
  • $\begingroup$ The gravity of the planet might, but I can't see how the mass would. $\endgroup$ – NomadMaker Oct 18 '20 at 18:03
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Basically, yes: the less Mass, the easier it is to get big. So there is an inverse relation.

Big things need to eat either lots of small things or other (not quite so) big things, unless they are parasitic or predators in which they can live off bigger things.

A major driving force for the sheer quantity of biomass required for super large animals is always going to be plants. Given that plants thrive on CO2, there’s a bounded correlation between CO2 and rampant vegetation. Dinosaurs lived on an earth that had CO2 levels which are toxic to humans.

So big fauna are more likely on large (more surface area = more sun), low density (light gravity) planets with high CO2 - all of which can provide rampant vegetation that then feeds the intermediate species allowing for gigafauna.

Oh and stability. Evolutionary complexity requires lots of time - so you need some form of protection against solar particles - such as a magnetic field driven by an iron core.

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Necessary premise: we are talking about a sample with size 1, since at the time I am writing it we know only one planet hosting life.

The main effects the size of a planet might have on the maximum size of its life form is directly due to gravity, and indirectly due to the biomes supporting the large life forms.

  • Gravity: obviously, the stronger the gravity the larger the load on the anatomic structure for the same mass. A healthy human's legs would be crushed by the weight of the same human in Jupiter gravity.
  • Biomes: to sustain a large animal you need a large food chain. This has happened in the past, when for some reasons being large was quite normal (see dinosaurs).
  • Biomes 2: living in water or on land makes a big difference. The blue whale can sustain its 150000 kg thanks to the water it swims in.
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It's reasonable to say that climate is what limits the size of current organisms on Earth, because we know that in past climates, many kinds of organism grew much larger: 70cm dragonflies, 15m sharks, 2ton rats, etc. If the size of the planet is not what limits the size of organisms today, there's no reason to assume it was the limit in the Cretaceous period either; in other words, dinosaurs on Earth might have been even bigger with a different atmosphere.

The good news is that gives you a lot of leeway regardless of how big a planet you propose, because we have no data on what limits that imposes. The bad news is we have no data; if you want to propose mile-high trees, it's guesswork what kind of planet you'd need.

We can do a lot of napkin calculations, though, mainly using the line-square-cube ratio: when you double an object's length, you increase its surface area by 22=4, and its volume by 23=8.

If you double the Earth's width, its volume (and therefore its mass and gravity) is eight times greater. So the atmosphere is eight times as dense, and a cat's lungs will be able to absorb eight times as much oxygen. But the cat weighs eight times as much, so it will need eight times as much energy to stand up, to pump blood to its head, and so on. But on the other hand, its mass is the same so it doesn't need any more energy to move horizontally. This (very loosely) implies that it could grow somewhat bigger, but not twice as big.

You can keep adding details to this, but my instinct is that there is no one term that will dominate as you vary the planet's size between zero and infinity – I would guess that, roughly speaking, cat length is proportional to planet size, but with diminishing returns at either extreme (a sigmoid type of curve).

Another angle of attack would be to consider that size is a function of time: it takes longer to grow bigger. Since an organism with a lifespan of a day evolves much faster than one that lives for a thousand years, the former will always be better adapted to changing conditions. To grow really big you need an environment that doesn't change (though this doesn't relate to planet size in any simple way).

Short version:

Within the relatively narrow range of sizes possible for solid planets (let alone Earth-like planets), there is no obvious relation to the size of organism those planets could sustain.

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  • $\begingroup$ Important to note that insect size peaked before amniotes really diversed, in combination with a high atmospheric O2 concentration. Whilst the oxygen would have helped insects reach larger sizes, competition with vertebrates is what seems to be what limits their size. Climate itself was not the sole factor in dinosaur size. Their anatomy was simply much better than mammals at producing large body plans. High sea levels and the structure of the continents also likely played a role. As for megalodon, the emmergence of toothed whales may have also driven them extinct in combination with climate. $\endgroup$ – Zac Walton Oct 18 '20 at 17:04
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Yes... But no.

As mentioned already, direct factors include gravity and available biomass. It is important to remember that the rules are different for marine, terrestrial and aerial ecosystems. Marine ecosystems enable larger organisms because the bouyancy force of water negates most of an animals weight. This allows huge amounts of plankton and algae to inhabit the upper ocean which sustain must more complicated food webs than their terrestrial counterparts. On land, ecosystems are predominantly confined to two dimentions. Light does not penetrate rock like it does water, meaning that your producers are limited to the surface available. Most plants grow tall to increase their surface area, whilst roots harvest minerals from underground. Aquatic producers do not typically grow large as they can absorb nutrients straight out of their surroundings.

Terrestrial environments

Terrestrial fauna sizes are dependant on available biomass. On a lower gravity world, bones and tissue may be less dense, meaning an elephant-sized alien would have less mass than an elephant. Or more importantly, an alien with the mass of an elephant would have larger dimensions (more volume) than an elephant. This scaling would be effected by the square-cube law. This only works if the food source remains at the same density. Since leaves aren't load-bearing, leaves on a low gravity world might be just as nutricious, even though the wood of the tree is less dense. Things like grasses however could potentially be less dense (or not, there are no known alien biospheres so it's at your disgression).

Increasing the surface area of a planet would increase the amount of space for plants to grow, meaning there would be more biomass available, however, this also comes at the cost of increasing gravity unless you compromise on density (which effects your planets ability to retain an atmosphere).

The arangements of continents is probably the most important factor in determining how productive your environment is. Having all your continents bound together in a supercontinent would give your creatures a huge habitat range, but would reduce your biodiversity as the best adapted animals would simply dominate. The cretaceous is a good template as it had a warm climate with high sea levels which is hugely productive for marine or coastal ecosystems. Large continents are good but supercontinents can lead to huge desert areas (Pangea, Eurasia). Natural boundaries such as seas and oceans are essential if you want to have different types of large fauna - Compare the diversity of megafauna in North and South America before and after the Great American Interchange. Earth's surface is 75% water already so there's plenty of room to increase the land mass without increasing the size of the planet. Just don't increase it too much as oceans are neccessary for climate regulation.

Aerial Environments

In the air, the density of nutrients is only a fraction of in water because the air is 1000 times less dense. An extremely thick atmosphere can increase the productivity of aerial environments but comes at a cost. The greenhouse effect is dependant on how much greenhouse gas (CO2 being the most abundant) is in the air. Increasing the atmospheric thickness should be done with inert gasses where possible, lest you end up with venus. Oxygen is also important as respiration depends on partial pressure whilst flammability depends on concentration - at high densities, there is a trade-off between fire becoming impossible and oxygen reaching dangerous levels (oxygen is a great electron donor).

Marine environments

Marine environments wouldn't be effected much by the mass of the planet. A larger planet has a larger surface area, which increases the available energy that can be harvested from the sun. Increasing the radius of the planet however, comes at the cost of increasing gravity, which impacts terrestrial ecosystems. Blue whales are pretty damn huge but their feeding stategy becomes inneficient beyond their maximum recorded size (~33m). However, the primary limit on ocean productivity on earth is the availability of iron in the ocean. Increasing the amount of iron or iron oxide in the crust could allow iron-rich dust to saturate the oceans. If the iron input was constant, this would increase the density of plankton in the oceans, making filter feeding more economical. The maximum size and/or population of blue whales could increase significantly, potentially where predation of blue whales may be sustainable for large predators. So marine environments don't need huge planets to create huge creatures. Water also doesn't vary much in density (salinity is a bigger factor than gravity on water density) so weight would not change much with gravity.

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