Really neat game! The concept of a game that does not have any particular conflict--only a series of tasks to do--is always interesting, and I don't think I've seen a combination of a construction theme and a space theme like this before.
Initially I thought that building planets to match all the percents was going to be a huge pain, but I noticed the "weight" parameter pretty quickly and discovered it was quite straightforward to build planets simply by grabbing a particular weight of each component. I actually really liked that the requirements were stated in percentages rather than weights, as that allowed me to make a couple gigantic planets just for fun.
(Although, at the end, I decided to just be efficient... multiples of 5% will always divide evenly into 20 parts, so 20 was the total weight of most of my last few planets)
On the margin-of-error problem that some people noted below: one potential way to make it better, I think, would be to just reduce the units visible to the player. For example, always display the orbit position as round(orbit / 10) (or round(orbit / 10) * 10 if you want to keep the same units), and the percentages as round(percent / 5) * 5, so that it's clear I can only change the orbit in large increments, and that I can only change the percentage in steps of 5%.
Overall I didn't experience any real roadblocks playing the game. The one suggestion I would make is to try to make the transition from the hub world to the level much smoother. I think the hub-world concept becomes less fun when the transition between hub and levels is not very smooth.
I mean, I imagine it's not possible to actually remove the loading time, but if you had an animation fading to and from black (or maybe some sort of animation involving the truck and the warp gate?), it could be a lot smoother.
One other thing I thought would be cool was more variety in planet models.. especially based on maybe planet scale and maaaaybe planet composition, although that one might be tricky.
Overall, I had a pretty chill time with this game. The core concept of building planets as a mundane job is, in my opinion, quite novel, and is executed pretty well.