The most obvious thing to look at when wanting to explore the idea of alien planets, for me would probably be Star Wars. The moodboard below shows some images of a few different planets, some from the live action trilogy and some from the current animated series.
Both the live action and animated series draw inspiration from organic life and how weather would affect and change certain environments. Some of the live action environments are obviously more life-like and realistic to fit in with the human actors, yet the others even though we know they're not actually real, they are still believable and the characters do not look out of place. There needs to be a balance between story, environment and characters, they all have to work with one another. In my case it will be making sure the environment fits the story and the lighting and textures marry up to produce a believable game level.
Star Wars: The Clone Wars is the animated series which takes place between Episode II: Attack of the Clones and Episode III: Revenge of the Sith. It expands on The Clone Wars themselves and carries off where Episode II left off. As it is an extension from the live action films, the characters and the environments need to show similarities or else the continuity will be off. It's not been done before so there is the lee-way of creating new stories, and as it is animation, the sets and characters have become quite stylised. Kilian Plunkett, an artist that worked on The Clone Wars said that one of their planets, "Rodia took a long time. It was really hard to figure out what a Rodian city would look like, because all we really had to go on was Greedo (the unfortunate Rodian bounty hunter who met his end in A New Hope). It was hard to base an entire culture on one person." - The Art of Star Wars The Clone Wars
It's interesting to see how even if one character has been to a planet previously, they need to relate it to them in some form. A person who derives from a certain planet will have different qualities and characteristics from someone from elsewhere. This is something I will have to think about, even though I am not creating a character, the environment I build will give some suggestion to what may inhabit that planet. I will also have to come to a decision to whether I will want my level to be realistic or more towards the stylised area. I will have to start drawing up some ideas, trying out both styles to see what looks better and what I think I will be able to achieve. It's one thing to draw something down on paper, it's another thing to model and then texture it in the way you drawn it.