Undecim System

Undecim-I
A world with a very thin atmosphere. It gets extremely hot, but during the night it can drop to somewhat liveable temperatures (around -50°C). It's made entirely of rocks.

Undecim-II
Undecim-II is a tidally locked world, quite far from this system's star. Both sides could actually be considered liveable, and the atmosphere is suitable for life. It has rivers, lakes, and oceans, but seemingly no plant life. The rings that orbit it contain common metals.