Merkuro

 This is a follow-up to this post.

Merkuro is the closest planet to the sun and tidally locked, so that it has a searing Dayside where temperatures climb high enough to melt lead, and a frigid Nightside that's cold enough to chill oxygen to a liquid. Only the narrow band of the Twilight Belt is close to habitable, and it is a badland riven by canyons and caves, wracked by storms. 

Human explorers would have been surprised to find life here, had the Vrusk of Marva not tipped them off. Liquid gases flow underground from the Nightside and melt to provide breathable, if rarified, air for cave complexes and even deep canyons in the Twilight Belt. Here peculiar invertebrate life developed, amorphous like amebae, but multicellular. One species (if such a term has any meaning with these lifeforms) developed sentience. These unlikely, elastic creatures, the plasmoids, would become one of humanity's best allies in the System.

Plasmoids are somewhat mysterious in that while they did not have particularly advanced technology at the time of human first contact, they possessed the theoretical framework to understand advanced concepts, and took to modern technology easily.  Some have proposed that plasmoid knowledge, perhaps even plasmoid sentience, may have come from consuming earlier interplanetary explorers (perhaps members of the Precursor race that inhabited New Terra before humanity's arrival) and absorbing their knowledge. It is a credit to plasmoid broadmindedness (and perhaps their renowned sense of humor) that they do not find such speculation offensive.

There is a human-operated spaceport in the Twilight Belt, called Solar City. This is a mining and broadcast power planet for New Terra. Most of it's inhabitants are roughnecks, miners, and technicians who do their hitch and then return to New Terra with a big paycheck. There are naturally ramshackle saloons, gambling houses, and other places of entertainment that tend to grow up around such camps to separate the workers from their money.