Upon first playing it is confusing on how to increase your score and hard to understand why the velocity hitting the core of another planet ends up making you die. You should make this apparent (the goal of the game and the rules). You can add a how to play menu and also indicate how to play on your itch.io page.
It is more clearer that it's supposed to arc around the star after playing through a couple times. I would simplify the velocity instructions and have an indication that you will lose if it his the core.
I think this is a creative game that has potential, but because of how the velocity works I think it having a bigger screen size would be more effective so that players can have space to create more planets.
This game would be more enjoyable if it did not have a confusing player experience, highlighted the rules/mechanics of the game and a better on-screen indicator of the entire arc the velocity will go in before moving. Right now it is linear with a slight curve and you can only predict when it will start changing. Instead I think it being a circle would be better so you can decide if it will end up hitting the core of another planet.
Game design suggestions -> I think it would be interesting if there was a speed aspect to how fast it orbits around a planet. Could even play around with the colors of the velocity matching the planet for increasing the score.
Artwork and audio are pretty basic, but okay sfx.