Advanced devices made with no knowledge

[Hornbein]

You could always have the superscientific Bad Guys incoming on a rogue planet with intent to replace the Earth or Jupiter or whatever. They could cleverly mold their trajectory to do so. It wouldn't even take that much energy, because if you calculate it all a million years in advance your steering has great leverage.

In "real life" the presence of a Sun would be extremely disruptive to an ecosystem unadapted to it, but your readers will never figure that out. They will believe that orbit around a Sun is highly coveted.

Hmm, you could have two rogue planets. One harbors the aliens, the other is just a station to transmit power to the place. The station gets a close orbit around our Sun, while the inhabited planet hangs out near Pluto. Hey, I'm starting to like it...

If you want to break away from the dominant Good Guys/Bad Guys Shooting It Out paradigm, you could have the story about getting used to the idea of sharing the Solar System with a far superior civilization.

 

[newjerseyrunner]

I had a thought pop into my head this morning and remembered this thread. You don't have to make it so that your species has no knowledge, just no living members of your species. In the 600s, Europeans had technology that none of them understood and couldn't replicate: no one understood how a water wheel worked, they just knew because the Romans figured it out. The collapse of a highly organized civilization to an agrarian one would suffice. Imagine with our own species, an economic problem on a global scale, all powerful nations' infrastructures are collapsing, as that happens, they're become more volatile and eventually end up fighting. A brutal war would destroy almost all of the infrastructure, and the last desperate attack of a dying superpower unleashes a biological weapon that decimates the population of the entire planet. With the combined knowledge of the awakening of that civilization having been stored digitally, all advanced knowledge is lost in an instant. Only scattered bits and pieces remain: a modern burning of Alexandria. With no winner and a devastated planet, most of us would hunt or farm, yet generations later we'd have solar panels to run our equipment, but not a sole would understand even on a rudimentary level how it works.