Phantom, and its sole moon Apparition.
Stealthily eluding confirmation for a year after its moon was discovered, Phantom is a Carbon planet, so dark it is almost invisible to the naked eye, except for its faintly glowing atmosphere. It orbits the Grey Dwarf Ghoul, just outside of the Creep Nebula. With its wide, circular orbit around Ghoul, it regularly passes through the Creep Nebula, at which point it picks up some of the nebula's gases (though it leaks gas almost as quickly). Its thick atmosphere is not breathable to humans and there is no surface water, but the planet is otherwise habitable with relation to its gravity and other critical features.
There is one important catch, however. Landing on Phantom is practically impossible to most ships. Why?
Phantom is covered in jewel- and industrial-grade diamonds. Any unprepared ship landing here is likely to be shredded to bits if not otherwise severely damaged in the process. Where there appear to be clear patches of "soil", there are actually patches of pure carbon in the form of powder or graphite. Using rocket powered ships on Phantom is a death wish.
As for its moon Apparition, the story is rather different. Similar to our own moon in appearance, Apparition is covered in silica, water ice, nitrogen ice, and a mixture of minerals and other forms of rock. Its relatively thick atmosphere is breathable during warmer months (when Phantom is closer to Ghoul and outside of the Creep Nebula). It occasionally serves as a layover station for travellers passing through the region.