To be realistic, planets must conform to certain laws of physics, astrophysics and geology. I wrote a number of blogs on this topic in 2010, and thought I'd put links to them here, so they can be read in order.
1. Planet Building and Astrophysics (8/15/10)
2. Planet Building and Astrophysics (8/22/10)
3. Planet Building and Astrophysics (8/29/20)
4. Suitable Suns (9/5/10)
5. The Greenhouse Effect (9/12/10)
6. Water (9/19/10)
7. Tropical Cyclones (9/26/10)
8. Weather (10/3/10)
9. What Would an Alien Sun Look Like? (10/10/10)
10. Sky Color (10/17/10)
11. Suns and Moons all Over the Place (10/24/10)