In order for mankind to have spread out and colonized these planets, they would have to have been terraformed. Artificial gravity and atmospheres are a must. Since not all characters will be in space suits, ie: warlocks only having 20% armor. They would be crushed or float away on planets that were not altered to resemble earth's characteristics.
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That doesn't necessarily mean that every planet's gravity would Earth's exact gravitational constant. I think variety is the spice of life, however slight the variety is. But how would one go about terraforming a gaseous planet? If they have altered gravity, then my vote is for gravity weapons.