There could very well be millions of small brown dwarfs zooming around the galaxy. Mostly undetectable. Imagine one showing up and passing near the solar system. Planetary orbits could be changed at little or a lot by it’s passing. Life is so fragile. We are lucky to be in the sparsely populated region of the galaxy... lucky so far.
Might explain “the Grand Tack”....https://en.wikipedia.org/wiki/Grand_tack_hypothesis