Penumbra

Summary
Penumbra planets have absolutely no sunlight, and this is their defining characteristic. The surface is often covered in a bit of a humid mist, with a strong plethora of plant and animal life. Trees comprised of energy wood can also be found on the surface and will yield Penumbrite when put into an Extractor. The main means of obtaining Penumbrite however is to dig underground, the same depth as most vanilla ores, and look for faint blue glowing ore.

Appearance
These worlds are very dark, but often lit up with bioluminescent plant life. However, bringing a flashlight or torches is recommended. The surface is often comprised of void dirt, a dark and easily-mined substance that proves useful when extracted from. It's also common to find black tar here, both on the surface and underground. What you will not find much of is light. It exists, but in small quantity. These planets are almost oppressively dark, and can often be identified on the star map just by looking for planets using black and other dark colors.

They have been seen to commonly orbit dark stars but they can spawn anywhere, orbiting any star.

Dangers
There are relatively few dangers on Penumbra worlds, despite how intimidating they may appear. The atmosphere is warm, but not dangerously hot. You will need any suit protections to explore one, but mid-tier weapons and armor are recommended. Penumbra worlds are similar in threat levels to Proto Worlds in terms of the hostile wildlife, but there is a lack of deadly weather here. The surface is usually relatively safe, provided you can fend off the less hospitable residents. In terms of hazards, the most noteworthy are the Black Slime blocks (which will poison you when stepped upon) and radioactive stone, which will give you radiation poisoning. Using poison protection can prevent this, and dumping healing water on them can turn them into blue slime that heals you instead. The lack of light is the most dangerous thing you'll encounter here, so be absolutely sure to bring some lights if you plan to dig, lest you fall to your death.

Codex description
Pitch black for most of their existence, these rare worlds have some sort of environmental barrier to the sun. A natural sunglasses-effect in the ozone, or something. Whatever the case...dark and spooky.

Notable Features
The occasional underground deposit of Moonstone, Prisilite and Penumbrite makes visiting a penumbra world even more profitable. Additionally, as of an unspecified update, these worlds can now contain small pools of liquid Aether, which can further your profits if extracted.