I love making stuff up. I guess that's why I'm a science fiction writer. It really makes me really happy to think about new ways the world could be, or new worlds that are completely different than the one we live in. I have as yet to write a novel that has no human beings, but perhaps that will happen. But I have written a fair bit that includes alien perspectives, or at least alien characters that get their stories told. As a former biologist, one of my favorite things to do is to make up alien ecosystems and life forms.
But as fun as making this up are, there are some rules that I try my best to follow. I do, for sure, include a little bit of handwaving, because it is impossible to know for sure how things would really turn out, but I do try to limit that to a minimum. I try to basically follow the laws of biology, physics and chemistry. Actually, I think that following some rules is a good thing – it makes specific limits, and limits can actually be good for creativitiy. Instead of a completely blank slate, you get some helpful parameters. If a planet has less gravity, that has consequences, both on humans, and on the evolution of life on a planet (animals can be larger, for instance.) If a planet is significantly hotter, or colder, than Earth, that also has consequences on the evolutionary process. As does the day length, or the length of seasons, if any, or the number of moons (tidal processes get really interesting).That kind of stuff is just a lot of fun to think about.
The Casitian Universe series are set on Earth, and on two other planets, both human populated (with no intelligent alien life.) There is intelligent alien life in the galaxy, and some of those stories are told, but it's not very central to the plot. The first alien species I had to invent in some detail are the Kurool, the aliens that are center-stage of the new series, the Cassiopeia Chronicles (the first book is out, it's called The Right Asteroid). The Kurool were fun to design. They are intelligent herbivores, with six limbs (four legs and two arms). Because they evolved as herbivores, they have very large ears, and eyes, to look out for predators. They use arsenic in place of Phosphorus in their biological processes and genetic material, and they use ammonia for metabolism, along with oxygen.
The other fun part of worldbuilding is thinking up both human futures, as well as ways that human beings could have, or might, end up differently than we are today. I make up different kinds of relationship patterns (the Casitians are a great example.) In a book called “Becoming Queen” that I hope to publish early next year, I have a very, very different marriage style.
I think what's also interesting about worldbuilding is that I know that my perspectives, both political and social, are made pretty clear in the worlds I build. Violent, exploitative species don't fare so well. Peaceful ones do. Social norms are a lot different than the ones we have right now. I like telling stories that explore and expose different ways of living, and ways that humans can live out our most loving, peaceful selves. But of course, that doesn't always happen, and that's also interesting – diving into the wreck of human frailties is neat, too.