I admit that I fell into this trap myself when I started to seriously work on a fantasy world six years ago. So let’s call it a typical beginners mistake. I was young and stupid, just like probably everyone else. I frequently look around in RPG forums for discussions about advice on worldbuilding and I’m always more than happy to share my experiences and discoveries and present various options that I think might be of interest and useful for the people who asked.
But there is this one thing that regularly comes up, almost always by people who are just starting out, that makes me somewhat… let’s say “frustrated”. It’s people trying to make a fantasy world by attempting to model a physically, chemically, biologically, and even astronomically realistic planet. I think the initial consideration is valid. You don’t want to end up people pointing out that your rivers are running upstream or in circles or that you can’t have an ecosystem in which all the animals are carnivores. A basic understanding about geography and ecology is a good thing to have to avoid the most embarassing blunders.
But then there’s always people who start worrying about the size of their world’s sun, it’s radiation output, and it’s distance from the planet. Or how spiders can only get so big because of the way their respiratory system work unless you increase the amount of oxygen in the atmosphere considerably. It’s a typical mistake and I made it as well when I started, but such exercises are ultimately pointless.
The first thing is that it’s still fantasy. If you’re writing really hard science-fiction, then I can understand the desire to show off your real-world science knowledge and make the alien environments physically sound. But why bother worrying about the oxygen circulation in (real world) spiders or the maximum weight supported by bones (real world) mammals and then add 50 meter fire-breathing flying reptiles and ghosts? As soon as you start adding fantastic elements, you already establish that real world physics do not apply to this world.
The other thing that makes such efforts futile is the question how such information would ever become known to players or readers? Is anyone ever going to dissect a giant spider and ask for a description of its whole internal anatomy? Is any character ever going to make an atmospheric analysis or calculate the energy output of the star based on its chemical composition and mass? If a detail can never plausibly come up in the fiction, then it’s useless.
The amount of scientific knowledge you need to make a pretty plausible fantasy world is really very basic stuff: Planets are colder at the poles and warmer at the equator. Climate varies little between summer and winter near the sea and varies greatly further inland. You need to have larger numbers of herbivores than carnivores. Water flows downhil; rivers only meet but don’t split up (except for coastal swampy river deltas). As a simple rule of thumb: As soon as you start adding measurements and make calculations, you went off doing exploring real-world scientific phenomena (which is a good thing), but it’s irrelevant for your worldbuilding.
I also want to make an appeal to dream big. An Earth-like planet with medieval culture is unlikely to really inspire people and amaze them. When you set out to build a new fantasy world, then you really should consider to build one that is different and something people haven’t seen many times before. Of course there’s some personal stylisitic preference talking here. I’m unashamedly a huge fan of Morrowind, Planescape, and Star Wars and 70s and 80s fantasy art. There seems to be very little fantasy of this kind to be around these days. But it’s fantasy, you’re allowed to be fantastical.