Personally, I find Brown Dwarfs to be absolutely fascinating. An object that isn’t quite a planet and isn’t quite a star, but something in between.
What would one even look like? Would it look like a gas giant that’s glowing red, along with swirls of gas in its atmosphere like Jupiter? Or would it resemble a star and have a fiery surface like the sun? I prefer to imagine them as glowing gas giants but I don’t know how realistic that is.
Gas giants in general are fascinating to me as well, I really hope we send a probe into one of the gas giants with a camera before I die. I’d absolutely love to see what it looks like inside a gas giants atmosphere before the probe gets crushed by the increasing pressure as it descends.
The idea behind the heliosphere and all its related parts are really interesting. Something about a giant space bubble/teardrop flying through the galaxy that blows my mind. Learning about termination shock and the sink basin example really gave off this weird sense of cosmic grandeur that can be somewhat replicated on earth. As above so below kinda thing.