Overall a nicely put together concept statement for a unique sounding tower defense game. As someone who struggles to get in to tower defence, the hook of having to defend an entire planet and the environment topology of that (e.g. a round surface) makes it sound interesting at least (albeit likely a source of considerable difficulty).
Tower defence games in this unit often struggle from scope creep, and the list of features in your game treatment point to that potentially being a problem here too. Without even considering the list of stretch goals, there is a bunch of work to do be done in terms of the different enemy types, implementing upgrade systems, and balancing the game. A big time-sink you can expect as well is developing a UI for managing towers, and generally this will be difficult to then write controller support for (I haven't played a tower defense with a controller, but they're fairly rare given that unit selection is clunky with a controller).
Given that scope could be an issue here, I encourage you to prioritise things that are core to your game being fun (I think particularly lean in on your unique planet/space-based ideas and the aspects where gravity is involved). Consider going through the game modelling process taught in the lectures to work out what features are more feasible than others.
Scope issues aside, this sounds like a fun tower defence game, looking forward to seeing how it goes!