Consistent worlds

 One of the joys of being an SF writer is having the freedom to create my very own worlds for my stories.  But this can be a big challenge too.  If I'm creating a world where humans are to move about freely, it limits me in how wild that world can be.

The atmosphere must be the same as ours to be easily breathable, and the gravity needs to be in a range we can deal with.  We know that being in zero-g for any length of time results in muscle wastage.  Being on a low-g planet for years would make it very difficult for humans to go back to Earth's heavier gravity later.  If your world is smaller than Earth is it at risk of losing its atmosphere like Mars did? And where is the water on that world?  If humans are to live there easily in any numbers the world must have an abundant supply.

And then there's the question of what we would eat on other worlds.  Some native foods might not be good for our digestion.  And we all know the havoc introducing non-native plants has caused in different countries on Earth, it could be even more devastating on a planet with poorly-understood ecosystems. Japanese knotweed, anyone?

Our created worlds have to be consistent.  Evolution shapes plants and animals to deal with the challenges of their particular ecosystems.  It would work the same way on other planets.  If you invent a fabulous predator, you have to work out what it preys on.  And where do those prey species feed?

And I haven't even started thinking yet about the nature of the societies that live on that invented world.  But that's another story for another day.