Planetary Systems and Life:
The universe of STARFIST has humanity spread out among more than two hundred worlds. Dan Cragg and I make certain assumptions about the nature of alien worlds and the life they harbor. These assumptions are fairly simplistic, not at all the wildly exotic concepts that one often sees in Science Fiction today. We went with simplistic for two primary reasons: First is that makes it much harder for two guys who aren't scientists to make awful mistakes; Second is, for me at least, it's fairly close to how I interpret current scientific thinking.
The base assumptions are:
Nearly all stars of roughly the same type as ours that aren't part of binary/trinary systems have planetary systems.
Many, possibly a large majority, of these planetary systems have a rocky, iron-core planet within the liquid water range of their primary.
Massive amounts of water, a large satellite, and active geology are necessary for life to evolve on a planet. Water, because that's where the chemicals necessary for life are mixed; active geology to pump the chemicals into the water; and a large satellite to do the mixing.
Once a world has enough dry-land plant life, it develops an oxygen-based atmosphere that humans can usually breathe without ill effects.
Native flora and fauna may be but are not necessarily edible by humans.
Native pathogens infrequently find Earth-evolved lifeforms to be viable hosts. Similarly, our pathogens rarely infect native lifeforms. On those occasions where native pathogens are highly infectious to humans, the planet is quarantined. There's plenty of real estate out there, and pioneering is tough enough by itself -- nobody needs to deal with ravenous pathogens into the bargain.
In short, habitable planets are fairly common, with very little "terraforming" necessary. Life as we know it is also common.
Travel and Communications:
The human occupied or studied worlds are in a roughly ovoid volume more than two hundred light years in radius on its long axis. This volume is called "Human Space."
There are two general kinds of space vessels: "Spaceships," which are used for in-system interplanetary travel, and; "Starships," which are used for interstellar travel.
Starships are equipped with the "Beam Drive," which sends them through "hyperspace," "warpspace," an "alternate dimension," or wherever, at the rate of slightly more than 6 lightyears per day. The Beam Drive cannot be operated, for reasons the authors have never seen a need to elucidate, in a gravity well. Navigation is not precise.
Travel times are such that vacations to other worlds are not taken casually. A straight-line trip from the "upstream" extremity of Human Space to the "downstream" extremity takes about six months.
Communication among worlds is slow for the simple reason that science hasn't come up with any kind of FTL substitute for radio. Interstellar communications is possible only via starship.
Government:
Some two hundred worlds comprise the "Confederation of Human Worlds." This is roughly analogous to the United Nations with an attitude and the teeth to back it up. Member worlds are relatively autonomous with their own governments and laws, but are ultimately answerable to the Confederation. The Confederation, headquartered on Old Earth, has the only military with true interstellar capabilities. The Confederation, as seen in the first and third books of the series, is not reluctant to use its military might to bring member worlds into line -- or even overturn their governments.
There are some human-settled worlds that are not members of the Confederation. They have varying relationships with the Confederation and its members ranging from colony of an individual world, through Confederation protectorate, to totally independent. Worlds that are not members can generally conduct trade only under disadvantageous terms. They also are not afforded any of the protection given member worlds.
Military:
The Confederation Armed Forces belong to it. The Confederation Navy, Army, and Marine Corps have active recruiting programs on just about all human worlds, whether Confederation members or not.
The Navy is the primary means of power projection. It not only has both starships and spaceships, it has atmospheric aircraft.
The Army is the heavy muscle for fighting planetary wars.
The Marine Corps (our guys) is an all-purpose expeditionary force. The Marines may step in between two fighting factions in a civil war to stop hostilities, put down riots, aid in disaster relief, fight a war too small for the Army to bother with, "kick in the door" for an Army invasion, or just about any other situation where the Confederation sees a need to flex some muscle or hit somebody. Marines also provide security for Confederation embassies, Navy installations, and anywhere else the powers that be feel a need for well-armed protection. Marine officers, unlike Navy and Army officers, are all commissioned from the ranks.
A military career is 40 years.
Life:
Most human diseases, including genetic, have been conquered.
Average life expectancy on Confederation member worlds is about 115 years.
Nobody except the idle rich retires before the age of 80, some keep working until past the century mark.
A college education is the societal equivalent of today's high school.