Rustum

Twenty light years away, Rustum, a planet of e Eridani, hotter and more oceanic than Earth and with one and a quarter terrestrial gravities, has a semi-permanent cloud layer separating two life zones (eg, lower spearfowl are larger) and containing nebulo-plankton browsed on by occasionally glimpsed cigar-shaped, jet-propelled air porpoises.

Theory: the dense air carries fine particles scoured by wind from surface rocks into the clouds where water drops dissolve out minerals which are consumed by microscopic organisms preyed on by larger life forms like the porpoises which rise by filling a bladder with biologically generated hydrogen, eat by sucking in air and plankton and move by blowing air out.

Vegetation is blue-tinged green, brown or yellow. Sea level air pressure is too high for most human beings who therefore colonized an above-the-clouds plateau, High America, where the Swift and Smoky Rivers from the western Centaur Mountains join to form the Emperor River. Hercules Mountains are to the south.

Rustum has two moons:

the outer Raksh changes apparent size as seen from Rustum, sometimes appearing twice the size of Luna as seen from Earth, and raises tides in a lowland lake;

Sohrab moves fast enough for its motion to be seen, like the hurtling moons of Mars in Edgar Rice Burroughs' John Carter series.

Three thousand colonists, transported from Earth in suspended animation, increase their numbers by both natural and exogenetic births. Little native life is edible so they grow terrestrial crops. Because the ecology supporting the crops is not yet firmly established, harvests are poor and most colonists must farm immense holdings with children transported to the Anchor village school in a publicly owned airbus. More scientifically oriented colonists become, e.g., iron miners or lowland explorers, competing with farmers for machinery and differing culturally, being more pragmatic and hedonistic.

The day-night cycle is longer than Earth's so Rustumites must be active at night although Dan Coffin wonders why they could not adapt to forty hours of activity and twenty of sleep. 

Public policy is settled by televisual discussion without formal government. Military defense is unnecessary and public services are voluntary but one elected official, the mayor, administers laws, presides in debates, judges disputes, oversees medicine and education and collects taxes.

Most sea level explorers must wear helmets but a minority who can live there comfortably settle and begin the colonization of the entire surface of Rustum.