I like the idea of buying and managing planets, that's quite a unique take on the theme! I like that there is a variety of different planets, each with different sizes and different structures on them to sell and I appreciate that you have put an indicator on the bottom of the screen to show which tile of the map you are dealing with for placing or selling.
while I do think its a pretty decent loop concept of selling structures on the small planets and then selling those for even bigger planets, it's a little hard to tell what I can and can't buy, as it seemed like i had enough money to buy a building but sometimes I couldn't buy it then other times I could, so I'd suggest just further explaining the rules of that.
also I'm unsure if I purchased a planet or not, I think it would be helpful if you could have a little message pop up just confirming if the payment went through or not, Some sound effects could also help with this.
I like the art style as well it's quite cute and has like an old flash game feel to it haha.
I think some music could really aid the game too, for example having different tracks for each planet to make them feel more distinct.
Good effort! : )