It's a game mode!

You can enable it by setting your world name/seed to skyblock when creating a new world, as suggested by a random game title/tip.

I recommend playing it after you've beaten the game on a normal world.

Upon your arrival I suggest to cut down and re-plant the planet's two trees and make a shelter close to the two obelisks so that you don't get eaten by local fauna on the first nights.

This game allows you to hit enemies through blocks and closed doors so a little "foyer" can be a good way to fight creatures without taking damage.

If you have figured out the seed yourself, you might be wondering where on earth are the praffins and how are you supposed to progress.

Perhaps you have already tried to expand the planet a little and haven't had much success.

Here's the thing: although the planet is smaller, it still uses "normal" rules for creature spawns, so you have to build a platform where the surface usually is.

That's just about 240 blocks above the surface, 2.5 stacks of ladders for ease of measurement.
Probably build on the "home" side of planet (marked with ↓ on the compass) so that you don't run into hot/cold biomes.

Once you're up there, you can make a platform from any solid block, like wood or stone.
Here I made a platform that spans 99 blocks in both directions.
Then you walk back and forth until one of the fellas spawns.
Note: if cobbles are spawning on the platform, it's too low.

Once you have access to wax, you can start building a (proper) house and generally adding more layers to the planet.

The hopping "cobbles" drop stone and ores. I think good first uses for ores are:

• Copper: a copper sword will make creatures slightly easier to deal with, and a copper pickaxe will make it easier to work with blocks and trees. You can craft a grill afterwards so that you can cook all those eggs.
  
• Gold: Shop Computer will give you access to a variety of blocks and items that are impractical or impossible to obtain in this tiny world, including more ores!
  Leftover gold can be used for crafting a set of armor and/or tool upgrades.
  
• Iron: an iron spear 1-hits all creatures that spawn on the planet's surface and will allow you to walk around more freely.

(...anything else?)

A look at the game's source code reveals that there's another, undocumented world type / special seed, lattice.

It looks like this:

This one seems to be structured more normally apart of the gaps.

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Thanks for reading!