Heidelberg is New Europa V, one of the Twelve Worlds.
Physical Planetography
Heidelberg is an ocean planet, without an accessible solid surface (unless one counts huge floating ice caps similar to the north polar ice on Earth.) It has an atmosphere composed mostly of water vapor, nitrogen, carbon dioxide, oxygen (from photodissociation of water in the upper atmosphere, from which the hydrogen subsequently escapes), oxides of nitrogen, and argon. The oceans of liquid water give way to a core of exotic hot ice forms under extreme pressure. Oceans are relatively acidic from carbonic and nitrous acids; other major ions in the ocean are alkali and alkali earth metals and halides. The greenhouse effect is quite strong.
Gravity is somewhat lower than Earth. The day is faster than that of earth, and the planet has a pronounced equatorial bulge.
The planet is almost devoid of metals.
Oxygen supplementation is required for both humans and Biran. (note need to check; maybe there would be enough O2? What would be holding O2 in equilibrium? Would all the nitrogen be oxides of nitrogen by now?)
The average surface temperature is 14C. There is negligible seasonal variation. The climate is relatively benign.
Culture and History
Heidelberg was colonized by humans and Biran, who live in relative proximity on floating cities (with an artificial reef below and a dome above.) The water chemistry is marginally suitable for Biran, requiring some adaptation to the nitrate concentrations.
The various floating cities are politically independent under the United Cities of Heidelberg, a loose confederation which represents them in the Allied Worlds but which holds little power over the Cities.
Economy
The economy of Heidelberg is founded on organic chemistry using deuterium-deuterium fusion for process heat and power. Advanced polymer materials are used extensively. Metals are imported mostly from the Good Moons, with some metal products being imported from outside the system.
Fusion power plants are extensively exported; the New Europa system has a complex interplanetary shipbuilding industry.
Luxury goods and agricultural products are exported to the Good Moons.