I get that this is an idle game, but I would still appreciate a main menu to access show instructions of some sort explaining how this game works. Other than that, I think it's really interesting

The game lacks UX. Would be nice to have a different sound for buttons when they have an effect or don't. Also, you can grey out  inaccessible buttons

The game loop is broken for such an idle genre. You can heat the sun enough to a super nova and reset the system; you don't need planets at all for this. Matter needed less and accumulated faster than heat. 

I would recommend making the progress loop more focused on planet buildings because it has some potential for visually interesting growth of your system.

Also consider adding some randomness to colors, planet sizes, and so on, so the system can look different each time you reset it.

Nice visuals and the calm music works well with the space theme. It should be noted that the button "Outer Ring 2" does not fit within the button.  The same applies for the buttons to add a moon, when the cost reaches 100.