[johndallman]

There are quite a lot of campaign possibilities, but they're mostly for pretty science-ey campaigns. Read Arthur C Clarke's novel Rendezvous with Rama if you haven't already: it's rather relevant.

The first thing to decide about this planet is its trajectory. If it seems to be something that's fallen in from the Oort cloud, it has huge potential for illuminating the history of the solar system, as well as trying to figure out what perturbed its orbit.

If it's on a trajectory that implies it didn't originate in our solar system, it's really interesting, but it will be moving fast enough that it won't be around for long.

If it's in orbit around the sun, its period is a very important question, and I'd advise you to make it at least 5,000 years so there's no question of it showing up in historical records. That implies a very eccentric orbit, like that of a long-period comet, which is the only way it could have escaped notice until 2100.

If it's going to come close enough to another planet to perturb it significantly, or even collide, the solar system has a big problem: the technology to divert or demolish a large body isn't available.

Landing on it depends on its mass. Even with THS technology, a lander that can go down a Mars-sized gravity well and get its upper stage back out to orbit is not a trivial job. It may well be easier to send a one-way robot lander with SAIs who get transmitted back off the planet. One thing that will be of extreme interest is its isotope ratios: those can say a lot about its formation and its relationship to our system.

If it has life on it - that's a big deal, especially if it's not part of our solar system.