Userscripts.org

OK, this is an idea my friend had for a site:

You register, and you get your own solar system. You can add a long description to it. You can also create your own planets, and add a description and a picture to it. It would just be for fun, to practice your creative writing skills and have a laugh.

I'm thinking of making it - but how would the planets interact with each other?

EDIT:

1. A currency for each planet, with conversion rates compared to £GBP (or $USD, if you're culturally inept)
2. The ability to declare war on other planets - of course, the outcome would be decided manually. A list on the planet's description page of the battles fought, won, and lost.