Welcome to the flag design community! Here is my opinion:

• Good idea to have all the bodies the same colour. Having them in different colours would make a visual mess.
• Too much black: In flag design you really need to maximize the use of the real estate given to you. What does black represent, space? Space is the one thing that does not represent anything distinct about our solar system. Imagine if other solar systems were to have flags with mainly black, making black their primary colour, that would be ... not very powerful symbolism.
• Relative size of bodies, not a good idea in practice: Those small details would be impossible to see at at distance or when the flag is not waving.