Hey great game for a first entry, welcome aboard and hope you have a lot more fun and success with LD! Thanks also for your review of my group's project. My ideas on your game follow:
Graphics: Really liked the graphics on your game. The background was nice but not distracting and the planets were varied and interesting. They added a great touch of polish which is a real differentiator for LD games. Some suggestions I would have would be to match the resolutions of the game objects, and settle the colour palette down on certain levels. I found the screen looked a little busy at times. Also, I think that making the player ship a slightly more complicated shape (e.g. a triangle like from Asteroids) might help the player guess the trajectory of the ship when they let go of the beam.
Audio: The sound effects in the game sounded really professional and the BGM is a great thing to have. The little touches of polish always go a long way to set your game apart. One comment I would have here is to match the gain a little more closely - some sound effects were much louder than others (and the BGM), which prompted me to turn the whole thing down.
Gameplay: I liked the difficulty of the game, but there were a couple of things that would make gameplay a little more intuitive and fair. Like many of the above, I think that the tethering system could be a little more intuitive. Many suggested using the center of the planet, although I like the idea of the surface, since your velocity gain can sometimes be more the closer you let yourself come to the planet, adding a risk/reward element. This clashed a little with the limited beam time mechanic however, and frequently I found I ran out of gas because the spin was slower than I expected. In keeping with the gas use theme, I think the beam could recharge quickly when not in use, incentivizing minimum use but not punishing you for the sometimes unintuitive physics. Also, I think that the radius at which you can attach to the planet could be made more clear, as did many previous commentors. Some polish elements you could also add: (1) let the player choose when to launch the ship. Since it launches automatically I was caught off guard the first few times and since I didn't recognize the ship's shape right away I was confused at first what was going on. (2) add a 'restart' button. The countdown timer is too long on most levels and you can become stuck. It's no fun to refresh the page when that happens, and waiting is kind of a pain too. They're easy changes to make a big difference to the UX!
Thanks a bunch for your submission, it's really polished and a great idea for your first entry. Looking through your To-Dos, I really liked the level editor idea and score system. I agree with you on those. A ghost racing system could be fun too. Hope you had fun making the game and enjoy the reviewing! Hope to see you back at LD again :D