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.
I am going to mention the rogue planets, since no one else has mentioned them here yet. Those unlucky celestial bodies ejected by their home star, destined to fly through the universe alone, dark and cold, forever.
And intergalactic stars, ejected out of the galaxy
Remembers me of voids.