I never thought of writing fiction as discovering a new world, but that is exactly what it is. You are creating a new world that no one has heard about. Each detail is brand new. Each character is newly born and alive....Until you write their death, of course. But even through imagination and writing, you can find a creative way of bringing them back to life in the story.
The world built in the story is different than the "real world" and you can make it your own, whenever and however you want.
You could have your creatures breathe water, hydrogen, barium, or whatever.
They could get energy from sunlight.
They could be driven by the moon.
They could fly, or swim, or float.
They could be solid, liquid, or gas....or all three.
Anything is possible because it is your world and no one can stop you from creating it.
Your writing is your baby and you give it life via your imagination.
Keep writing and you will ever expand the possibilities of your story.