Re: Very strange stuff
There's also a grey zone between a wandering gas giant planet and a brown dwarf star. Jupiter emits more energy than it receives from Sol, because something (probably a very small amount of hydrogen fusion) is going on in its core.
"Torn loose by gravitational interactions" implies some sort of catastrophic interaction such as another star passing close to a solar system. That's not necessary. Any solar system with more than two bodies is stable only in a statistical sense. A 3,4, ... N body gravitationally bound system is chaotic, and it is always possible that what appears to be a stable orbit will in fact end up with one of the planets ceasing to be gravitationally bound to its sun.
You'll be unsurprised to know that the future of the solar system has been carefully modelled. Earth is safe for the next 200M years or so. Beyond that, we can't say. The observations aren't good enough to distinguish longer-term stability from its opposite. Such is the nature of a chaotic system. An unmeasurably small difference today may be the difference between earth remaining in orbit or not, 300M years hence. "Past performance may not be a reliable guide to future performance"!
Back to mini brown dwarfs or large wandering planets, it's possible that these might be the last habitable places after all the universe's stars have burned themselves out. Has anyone ever written a far-future SF story set on or within one?