I've been reading through the Starfinder source book, Pact Worlds. It's a small book, but fits a lot of information into it, so I'm going to post about sections as I finish them.
Bretheda is the eleventh celestial body (counting the Diaspora) from the sun in the Starfinder version of the Golarion system. Like its neighbor Liavara, it's a gas giant with lots of satellite moons. Its governing body, called the Confluence, manages both Bretheda and Liavara, and the native barathu control much of the biotech industry, so this is actually a pretty powerful pocket of space.
Bretheda's native inhabitants are the barathu, which are "a hybrid between a blimp and a jellyfish". They aren't listed in Alien Archive 2 (which is the monster manual I own) so I assume they're detailed in Alien Archive. From their description, though, they sound like less psychically-inclined and more socially-inclined Dreamers (or rather, Dreamers are the psychic versions of barathus).
The barathu aren't the only inhabitants of Bretheda, though. Oma and nuru worms, and other floating creatures, are also sometimes found wandering the gas giant, and the barathu cultivate relationships with many other alien races. They've helped to build floating cities in the skies of Bretheda, and seem to actively encourage collaboration. Part of this is cultural. The barathu dislike direct confrontation and it doesn't seem like they're a warlike race by any means. They do maintain corporate interests in Bretheda's resources, though, so I do wonder their policies are inspired by goodwill or by efficient greed.
There are floating cities and moons all around Bretheda. Kalo-Mahoi is a water world locked beneath a shell of ice, and features underwater cities that are home to a trend-setting fashion scene. Marata is home to the seven-gendered maraquoi. Dykon is a volcanic world and host to some silicon-based life forms. Thyst is a radioactive wasteland.
There are lots more, and the amount of detail varies on each one. I don't think it tends to matter, in the long run. You get the idea that Bretheda, like Liavara, is more like the Diaspora than, say, Castrovel or Triaxus. It's a blank slate, with a little detail here and there, but largely it's meant as a setting for the GM to define. Maybe there's an adventure path out there that locks in some locations, but from what the Pact Worlds provides, I think it's a place you're allowed to explore and define for yourself.
The powers at play are the Confluence, which is the governing body for Bretheda and Liavara, a few different biotech corporations, and a smattering of independent colonies.
Combined, Liavara and Bretheda form a coherent whole that's a dichotomy of a rough-and-ready frontier and prosperous metropolis. There's definitely a feeling that there's both too much to cover in 20 pages and not enough to cover in a paragraph. The authors could probably have benefited from narrowing focus a little, the way Guildmasters of Ravnica did. When the authors of Ravnica were faced with an entire planet, they focused in on just one district, providing the general flavour of the setting. That gave the GM some solid definition for a set number of locations, and allowed the GM to extrapolate characteristics of that small sample over the rest of the world. I admit it would be hard to choose what to focus on in a diverse place like Bretheda or Liavara, but I think the lack of focus ultimately communicates that nothing around Liavara and Bretheda has really been defined yet, so make lots of notes in the margins when you take your players there.
There's a player character theme at the end of the section. The Biotechnician character theme grants +1 INT, the ability to reduce the DC for Life Science checks, and makes Medicine a class skill. Starting at 12th level, you can get one more augmentation than what your race normally allows.
Maybe I'm biased because of my love of Dragonlance and Shadowrun, but Triaxus is [still] my favourite planet in the system. It was exciting in Pathfinder, and it's exciting in Starfinder.
I've decided that the Starfinder worlds need shorthand descriptions, the way one might refer to Magic: The Gathering planes by theme. Innistrad is gothic horror world, Theros is Ancient Greece world, Ravnica is city world, and so on.
So far, this is what I have:
Next in line is Apostae, another gas giant!
Header photo by Seth Kenlon, Creative Commons cc0.