Did the Norse know about Uranus, Neptune, and Pluto?
Those planets are considered "modern" discoveries, but it seems to me that the Norse probably knew of their existence, because the World Tree has nine worlds and our solar system has nine planets.
Just to take it a step further, I will attempt to match the nine worlds to the nine planets:
Mercury = Asgard. Mercury is the fastest of the planets, and traditionally associated with Odin.
Venus = Alfheim. Venus is the brightest planet from our perspective, and strongly associated with beauty, making it an ideal candidate for the realm of the Light Elves.
Earth = Midgard, duh
Mars = Muspell, because of its fiery color.
Jupiter = Jotunheim, because it's the largest of the planets.
Saturn = Vanaheim, because idk where else Vanaheim might fit
Uranus = Niflheim, because of its icy blue color.
Neptune = Svartalfheim, perhaps because its deep blue hue speaks of ancient wisdom and fabulous treasure.
Pluto = Helheim, because it's frigid and dark and the most "netherworldly" of the planets.
It's interesting how you can match the worlds almost 1:1 to the planets based on their physical appearance, and they end up in pretty much the same order (with Asgard being the highest and Helheim being the lowest, etc.). The only one that doesn't really seem to fit is Vanaheim.
The Norse cosmology says Midgard is surrounded by a wall separating it from the giants of Jotunheim; the solar system has an asteroid belt that separates Earth from the planets known as "giants." There's a lot of parallels here, but I hope I'm not just overthinking it?