This may seem like a story set in space, and it is. But it is not involving other star systems or a scale so massive. In that way, since each player is a single planet in a single system, it might seem at times more like a NES set on Earth where each person is a nation, but with obvious changes in dynamics given the setting. There isn't really colonization in this game that I am envisioning. A planet, you, can conquer moons and small settlements or bring them to your side through diplomacy. I am not a scientist, astronomer, or physicist, so a lot of what will happen in this game will be for fun. But we will try to get orbits right for the sake of having some kind of visual aid (map). We aren't going to worry about stuff like gravity pulls effecting storm cycles on planets, though. We will say that if a planet is closer to the sun, it is warmer, and if further away, it is cooler. And the visual aid will cover this.
Now to the meat of the story. There is a single star system in space that is so massive, that several habitable planets exist. It is here that civilization as we know it has thrived and existed for some time. Humans are the dominant species, and there is no knowledge of extraterrestrial life. In fact, any semblance of an interstellar journey that has been taken, has come up with lifeless or inhospitable planets. It seems this star system is the garden of humanity. Just as importantly, the material resources that allow humanity to survive in this system, are simply not present in other systems. Humanity has the technology to travel to other stars. This technology is costly and wasteful, so not many members of the planetary confederation have embarked on such journeys, with the exception of wealthy philanthropists looking for an exotic vacation. Extremely expensive science expeditions explored vast amounts of space and found nothing profitable or habitable. The cost is not worth potential failures.
Humanity did not begin on a single planet. Rather, humans developed on several of the planets at around the same time. There were some planets with less advanced or civilized cultures, and others where humans never developed, but were settled early enough by the first interplanetary travelers. Each planet has a different set of flora and fauna, as well as different climates. Some are slightly closer to the sun, others are further away. Some are completely covered in water, others have very little water.
It will be up to the players to create their own planet profile and everything that goes with that planet. I am thinking that we start out as some sort of loose planetary confederation, that always has a bunch of member-to-member fighting and is on the verge of falling apart because the ethics, personalities, and mentalities of each planet are too dramatically different from the next (with some exceptions, which are alliances or at least peace treaties). Trade will also be a factor in some way but I haven't figured that part out yet. I do know that bringing other stellar bodies to your side is going to be a pivotal part of gameplay. The system is so large and moons do exist. Yet for most planets governments are only on the planet, unless they have maintained control over a moon through force or something. Moons are typically settled by those from the host planet for a variety of reasons, and then become independent of their host. I guess some moons were settled by other planets a long time ago as well. Most moons are also quite habitable and might have been settled a long, long time ago when planets were being settled, and so that moon has resisted being occupied by a planetary force for a long time. Depending on the moon,they have a lot of honor and dignity, even "nationalism". Players create the planets and I'll pretty much be responsible for the moons (as the mod). Some moons will not be single governing bodies but rather independent corporations. One goal will be to get moons to join your governing body, whatever it may be. Moons = money, and since many moons are neutral and not part of the planetary confederation, getting control over a moon can mean a lot. Another goal is to gain territory. Moons can be conquered, but so can other planets. Or you can try to get them to join you in some peaceful way (though the military will play a significant role, because it is a hostile system).
This system is old, and so your civilization might very well be thousands and thousands of years old. I haven't really thought about technology for spacefleets or armies.