This game is getting more and more fun. I've decided to ignore the tiers and the bosses and all that, and just explore using what I have available from the start. Turns out, you can get many hours of enjoyment out of the game without ever fighting a boss.
The leather armor helps, once I figured out how to make it. And a better pick makes getting coal faster, so you can explore without too much worry about fuel. I'm currently on a level 2 arid planet. The sound you make when you walk across the bones... Oh my god it's perfect.
At night, I box myself in with dirt so nothing can get me, and head down. This time, I went really far down. At one point, I saw some silver embedded in an enormous wall of gravel. I decided to go for it, and from a distance, used the matter manipulator to remove a single tile of gravel. What I didn't realize was how far up the gravel (and above that, fine sand) went. The cave in lasted for several minutes, but I managed to stay safe. I didn't get my silver there, but it opened up a passageway down to some sort of ruins. There were beasties down there (nothing that could hurt me though), and loads of tiny little houses and streetlights, like some tiny gnome city had been built underground. There was also a great deal of silver and even some gold in the soft brick walls, so I mined and mined and mined.
I have no idea how long I was down there. I was underground for so long, I wasn't even sure I remembered what planet I was on. At last, I reached the bottom of the tiny town, and decided to head back up. It was a long way, but fortunately I was able to follow the trail of torches and blue dirt (from my starting planet) back to the top. My crops had finished growing and the sun was just coming up. I'm not sure if I was under for just the night, or a whole day. But man, that was fun. Time to move on....