For my alternity game I have created a univers based on the "colonisation" of nearby star who may have habitable or life bearing planets. Then one of my friend got into the "job" of building a civilisation for a star battle board game, his planete as a few probleme namely that it is not in the habitable zone. Little parenthese here for those of you less science fiction (or science periode) inclinded. The Habitable zone is the area around a star where liquid water can be found. For a G stars kinda like ours this zone is about a few time the diameter of earth roughtly centered around the earth orbit. This is the area of a star where life is the most likely to evolve (at least life as we know it). End of the parenthese. So in discussion with this friend (we'll call him bob1 (in case I have other friend called bob ;-)) we had the problem that life must have had evolved on this planet or there would not be a civilisation for him to build (talk about geeky.). However recent measurement of Europa (one of Jupiter's moon) show that liquid water may exist under a few kilometers of ice. Europa is way outside our star habitable zone it shouldn't have water. It didn't take much more to get geeky people (Bob1 is an engineer... it hard to get geekier then that.) to consider what might make a inhabitable planet habitable. First the single most important thing for a planet to be habitable is that it as oxygen and water which means it must be at least above zero deg C. This can be achieve in many ways first you can have the simple logical way: a star heat the planet. Then you could have a planet orbiting a giant planet which tidal disturbance might produce enought friction to heat some of the planet. (like would be the case for Europa). Also, you could (maybe this one is far fetch) have a venus like planet (heavy greenhouse gaz atmosphere) just outside the habitable zone it might trap enough heat to keep water above freezing. All those way lead to some deduction (confirmed in a few scientifc publication) that what a planet require for life is free energy (as in not caught up in something not that cost nothing). Thinking about that made me realise there is another source of free energy me and bob1 had not considered... radiation. You see right now radiation is heating the core of the earth a similar phenomenon could happen in the crust of a planet (radiation might might it unhabitable but right now I'm just looking for a source of heat). Zone bob1 planet could be in the situation where it is outside the habitable zone of the stars (it orbiting a brown dwarf another kind of stars but too light to produce any meaningfull amongts of light) and throught the tidal interaction and maybe a little help from overabundant radiation provide condition for liquid water and life to emerge. It probably would be a life very different from our own with a mutation rate (and maybe even a reproduction rate) way greater then ours but it might just work.. now I just have to mail the guy and tell him that ;-)
I just had to clear up my idea here first and to deploy another WME (weapon of mass eduction of course).