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.
This going to sound basic but I find the vacuum part of space to be the most interesting from an engineering POV.
We are all use to having an atmosphere we can convex heat out to but as there is no atmosphere where do you send your excess heat to?
You can only radiate it away, and it’s slooooow
Ah yeah, there’s a radiators modpack for KSP.