Thank you everyone for all the positive feedback! It's great to hear so many enjoyed playing the game. I had great fun developing it; it was my first time using Godot and I found it very intuitive.
I'd also like to respond to some of your feedback, as there are a few themes running through the comments.
Firstly, the flight controls. For a game like this it is common to have completely frictionless physics where every thrust and turn must be compensated for to change direction. Personally I dislike that feeling, so I put in some dampening physics (i.e. the ship's movement and turning slow down if you let go of the controls). I played around with the variables for a bit until it felt right to me, but @jackaljk and an anonymous reviewer found the controls awkward. Others, like @senover-games, enjoyed the flying. @james-joubert suggested using the mouse to navigate, and if I had implemented something like that it might also have worked for touch controls (I tried the game on my phone's browser only to find that the ship would launch and then explode).
Secondly, the trading economy. I've always liked the idea of flying around and trying to find the most profitable deals, but in practice I think that is difficult to do well in a game. As @mossieur-patate points out there are some obvious dominant strategies, but maybe the alternative would have been too boring and arbitrary. I couldn't spend much time balancing this because the entire trading and economy system was added in the last few hours of the compo. As @douwe-ravers points out the trading, upgrading and exploration are all linked, so I'm glad I was able to get one upgrade into the game, but as you might guess from the very limited UI, I didn't have time to add more upgrades as I would have liked. @deniz-aras, @zubspace and @jackaljk all mentioned a "buy/sell all" button, which would have been good in hindsight!
Thirdly, the planets. There are in fact 7 planets to visit, but they are spaced out so that you need the upgrade to get to two of them. @hneumann, @senover-games and @f1krazy found it difficult to navigate to (or even find) the planets. @3ncryp73d and @f1krazy suggested a minimap. This was something I wrestled with while planning out the solar system; 10 seconds of fuel can actually get you quite far, in terms of the number of "screen widths" you can travel in that time, so to present the challenge I had to make the planets even further apart than that with fuel collectables in between. One option is to make the ship travel more slowly and the planets closer together, but I didn't think it would feel as exciting to fly. This is the compromise I came up with: the first planet launches you "upwards" and if you keep flying in that direction, you have to deviate slightly to collect the fuel but you will eventually come across the "second" planet. On that planet the "Chat" option tells you about pressing the M key, which I implemented as a really simple camera zoom out. This enables you to at least see the planets from further away, but still get the sensation of speed when zoomed back in again (however the parallax breaks while zoomed out). When you take off from a planet, the surrounding fuel collectables are generally in directions such that, if you collect them and keep travelling in the same direction away from the planet, you'll eventually come across another collectable or planet. Towards the end of the compo I wanted to add little arrows around the edges of the screen pointing to nearby points of interest, but I ran out of time to implement them.