Hi All

Sky & Telescope quote a suggestion that the mysterious Sedna might be the result of a near-star passage by a low mass red-dwarf or brown-dwarf, and might in fact be an exoplanet stolen by the Sun...

Sedna's Origin

...which is a wild concept in my mind. What if it holds alien ruins from aeons ago?

Far-out suggestions aside it does indicate that the Extended Disk of the outer system holds a lot of mass. I would bet money that would be the most likely place for visiting ETIs to set up base, venturing in Sunwards to explore, but parking their starships around cometoids full of handy volatiles. Reaction mass and/or fusion fuel. And well out of sight.

Adam