It's the law

When we're world-building for SF we're not quite as free as it might first appear.  Our world has to be built according to laws of some kind, and then we have to stick to them.  As I'm writing SF rather than fantasy, my worlds have to conform to the known laws of science.  That doesn't mean that it isn't possible to have exotic aliens, it just means I have to work much harder to justify their existence. 

In the interests of story, we generally want our human characters to be able to move about freely when they're on-planet, and that usually means putting them on a planet that's not dissimilar to earth.  But if you think that limits you too much, think Avatar.  In many respects that world was familiar.  It had jungles, an atmosphere, oceans.  But within those parameters is scope to be exotic.  The bioluminescent forest at night was an extension of the bioluminescence some plants on Earth have, but scaled up and with every plant glowing it made for an alien world.  There were horses and birds on that world too, and flying dinosaurs.  Every creature has a similar counterpart on Earth, and yet they were truly unique.  

Pandora has trees too, but while they're recognizable as such, some are much more than mere trees. In the Tree of Souls I see the form of Earth's weeping willow in its arching branches, but everything else is different.  The tree has its own form of sentience, a role as memory-keeper, something no Earth tree can do.  The Na'vi too are recogniseable.  They're a standard humanoid body form, but they're ten feet tall and much thinner in the body than humans,

The rules we set up for building our world will influence the society and culture of the people who live there.  The Na'vi live in the forest, and respect and worship the natural world.  Species who live on the ocean where there are huge creatures would most likely build up legends, and probably some kind of religion, around the creatures.

If there are several different species populating your world, each will have their own culture and moral code.  If they are organised societies they will have some form of law.  What is forbidden for them?  Are they free to choose their friends?  Or is there a strict caste system that prevents free thought?  And what happens when an outsider comes in and shows that their rules don't make sense?

Consistent biology, creature design, and laws and cultures arising out of the environment and what its people do there, allow us to tell a convincing but alien story,

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.