Thanks for the tips. I checked every available path and I'm pretty sure they're all either empty or lead down to the cold zone. (map attached)
I did go back to Cornolia and unlock Prometheus, which I explored for a while. I got to the bottom and killed the boss, then the lava(?) started to rise. I picked up the spark shot on the way up which let me blast through TNT, but the lava kept rising, so I had to keep climbing up. The lava rose so far that it covered up the spaceship, and I couldn't use the spaceship anymore, so it seemed like I was stuck. There was another droplet-shaped upgrade that I could reach above the lava, but it said "Another item is required first" and I couldn't get it. Eventually I died in the lava and respawned, but it loaded the save before I killed the boss/got the spark/raised the lava, so I guess touching the ship while it was submerged in the lava didn't actually save despite showing the icon. I'm pretty sure there was at least one bug involved in all of that but I don't know what exactly. Regardless now I'm stuck on Prometheus too.