It's one of the pieces of fiction that left a deep impression on me. Harsh winter days still remind me of it.
It reads as if it is either between us and the sun, or close enough that you'd see it roughly up to the same distance we are from the sun.
My point? That even this was a much better candidate for a far-fetched event in 2020 than this pandemic ever could be. Scientists, academics, reporters, policy makers and even some more astute or honest politicians have been predicting it for literally years as something that was probably just around the corner.
I had interpreted your "why would it be scary" question as "what kind of bad things would happen if a brown dwarf did get close?", but I guess it was instead a rethorical question implying something about low probability. Hence the confusion.
That said, there is a problematic selection bias in your statement about our solar system. Only stable systems can produce observers, so of course we find ourselves in a stable system. For now ;-)
Like, we just found out the universe is a way, way bigger place--full of planets between the stars. Maybe humans will visit these worlds someday as we planet-hop between the stars. What civilization could we make near these cold, but resource-rich worlds? What if they had tidally-heated moons with liquid oceans... what sort of life would evolve without the energy of a star?
But no, people's first reaction is "OMG is this going to kill me?!" I've even had someone get angry at me for bringing up the topic (that the sun will go nova billions of years from now) and making them depressed. I just don't get it.
> That said, there is a problematic selection bias in your statement about our solar system. Only stable systems can produce observers, so of course we find ourselves in a stable system. For now ;-)
That's why I mentioned other exosolar systems. So far as we can see, basically every stellar object that we look at has stable planetary system. We mostly find planets by periodic transit events, which requires the plane of formation to be aligned with us. You can calculate the statistical chance that this is the case, and sure enough when we look we find roughly that percentage of stable planetary systems. If there were rogue planets or primordial black holes out there likely to encounter us, then those other systems would be just as vulnerable and we would expect to see fewer percentage of visible stable planetary systems than predicted. That is not the case.
We would also expect consistent groups of long-period comets from the interactions of these passing objects with the Kuiper belt and Oort cloud, but there is no consistent grouping.
We would expect a history of random heavy bombardment periods in the crater history of the moon, mars, mercury, and other datable planetary bodies in the solar system. Instead we see just one period, the late heavy bombardment which coincides with the synchronization of the orbits of Jupiter and Saturn in the late stages of solar system formation.
Etc. etc. When I first saw the headline I thought "wait a minute, that cannot be right." And sure enough, it's not. There aren't 100's of rogue planets in our particular stellar neighborhood, but rather scattered around the nearby region.
Very unlikely, of course. But not fictional.