This started as a thought experiment here: http://s1.zetaboards.com/Conceptual_Evolution/topic/4230591/
Please direct all commentary To the Discussion Thread (http://thecbg.org/e107_plugins/forum/forum_viewtopic.php?190031.0)
However, I realized that what I had here would also make a great basis for a steam/diesel/retro-futuristic setting, so I'm going to post the stuff that relates to science/culture/sapient species here, and focus on the biology on the sister thread in the Conceptual Evolution forum. I'm not worrying about game balance here - this campaign is going to be used for a comic, either a webcomic with a friend or (if my editor likes it) a publication for a comic company I recently started working for.
However, the opening post is the same - the concept of this setting is that some of the beliefs we now know are wrong were not, in fact, wrong, and physics takes some new, interesting twists because of that.
So, first, the science:
Phlogiston: Phlogiston was believed to be a substance all combustible materials contained. In this universe, this is actually the case - the sun (and indeed, all stars) are self-sustaining Phlogiston reactors that produce more Phlogiston as they burn, making them able to burn eternally. At the center of the star is the combustible material - Stellium. Burning Stellium has two effects. First, Stellium is unique in that burning it produces more Phlogiston, part of which goes back into reforming the Stellium, making it an eternal energy source. Second, when burned, Stellium releases gasses - oxygen, carbon, hydrogen, nitrogen, methane, and sulfur (along with their compounds) being the primary gases released, which are expelled at high speeds - far enough to reach out to about 1.5 light years. This mixture of gases is called...
Aether: Aether is a mythical elemental proposed by the Greeks to fill the void in space. In this universe, Aether is entirely not a unique element, but rather a combination of gases expelled from the local star - as well as the cast of of burning Phlogiston. Aether has two properties: first of all, it provides a medium to move the gasses within it. Second of all, it is a great conductor for heat, meaning the temperature within a solar system is relatively stable in various regions, rarely dropping below freezing except within the outer borders of the inner solar system and the vast distances between gas giants.
This has one very obvious implication: space, while still lacking gravity, contains a breathable atmosphere - something that carries over into tiny bodies, like asteroids and comets, allowing life to find a way to grow in pretty much any niche.
Gravity and Planets: The interior of a planet is also a Phlogiston engine, but in this case built around Terrium instead of Stellium. The Phlosigton burns the incoming elements strewn into Aether and a form of fission occurs, forming the heavier elements - which rest atop the phlosigton, forming a planet. Burning the Phlogiston in Terrium does not produce any gaseous elements, but instead produces a field that "filters" incoming and outgoing gases between the planet and sun, preventing gravity from accruing too much gas from the Aether or the planet loosing too much of its atmosphere to the Aether. This does lead to a wide variety of atmospheric compositions on various "worlds" - a hard to define word, since most orbital bodies with any degree of ignited Terrium can become full fledged worlds, and almost universally develop life, typically through migration. Gas giants, which have no solid core around their Terrium, glow brighter in the night sky than stars, since only their atmosphere deflects their core's light. In addition, they warm local Aether significantly, meaning the moons of the gas giants are excellent worlds for life.
Once the Aether hits the outer edge of the Ort Cloud, it get's stuck, held in place by the sun's magnetic field - which is a good thing, because it buffers us against the worst of the Lumniferous Aether, Aether produced by the galactic core and containing who-knows-what. (To be defined later - exotic elements and whatnot.)
Evolution follows the normal rules, I've decided - it's going to be more fun to see how evolution works in these strange conditions than also using weird rules for evolution as well.
There are a number of planets and moons now capable of supporting life within our Aether Shell that do not actually exist within reality. Too many to list here - kuplier belt objects abound, not to mention countless trans-Neptunian bodies, asteroids, and all of that is still not even referring to the life on existing planets. Here are some new bodies of particular note:
Vulcan: Sitting between Mercury and Venus, Vulcan is a hot, desert word, where plate tectonics have produced vast mountain ranges. Of special interest in Vulcan, other than it's location, is the presence of large silver seas - believed to be large deposits of elemental mercury, though further study is needed.
Phaeton: Resting between Mars and Jupiter, Phaeton is a smaller planet with a very dense atmosphere - it may Atherrays or other creatures of the Aether evolved here.
Nemesis: A red dwarf (star with low, cool burning Phlogiston Stellium core) that rests within our solar system, about 1 light year out. It only has a few planets to call its own, all of which are small and rocky (sans one giant, Tycho). Life in the Nemesis system evolved independent of life in the Sol system, and produced some creatures that are horrific to our eyes.
Update 1: Some more "science" explained, and the history of Earth.
I've been giving this setting setting some thought (and will respond to comments in the discussion thread ASAP, though might not be able to until tomorrow.)
However, there's some details to the science that need explanation to polish the setting off before I can get into the interesting stuff.
Light: When I'm not on my iPad I'll edit the original post to reflect this, but decided that light looks like it does in reality as opposed to the reversal I had before - for the setting to work right in my mind, the glowing incandescent Aethershell is just too much.Â
Gravity: In reality, all that gas that's being constantly expelled should coalesce into solid bodies. I feel the need to explain why that doesn't happen here, and that's simple - Aether works as a sort of anti-gravity, keeping them from being pulled together like that (also why the gasses don't just fall back into the star.) The reason planetary bodies can form is because Terrium actually repels Aether, even when not burning, and large enough masses of Terrium allow the formation of physical objects in space. Also, Aether is weaker than gravity, so large objects are pretty much unmoved by it.
Atmosphere - There are two types of atmospheres, the solar one and planetary ones. The solar atmosphere is detailed in the previous post. Planetary atmospheres take one of two forms. For bodies without a burning Terrium core, they just use to solar atmosphere, the gasses from that providing more than enough to breath. For ones (like Earth) that have a burning Terrium Core, the atmosphere is considered "locked" - when the Terrium started burning, it created a shell that kept the current solar atmosphere in but allows for only minimal gas exchange between solar and terrestrial atmosphere. Terrestrial atmospheres can vary wildly from the solar norm thanks to volcanism, chemical reactions, and biological influences - for example, the abundance of plant life on Venus means it is much more oxygen rich than the solar atmosphere, while Io's high volcanic activity keeps the planet's atmosphere high in carbon dioxide.Â
In addition, this limited gas exchange means solar temperatures have little impact on planetary temperatures - instead, that works like it does in reality, so while the solar atmosphere outside of Earth is typically between 80-90 degrees F year round (with only minor variations), the terrestrial atmosphere of Earth varies as much as it does in reality.
As a final note, there are solar winds, because Aether is weak compared to gravity - gasses aren't pushed away from the Sun in an outward blast, but rather in a spiral pattern as gravity tries to pull it back in. This can create solar windstorms that engulf huge areas with intense wind - hardly enough to effect an Earth-like body or even the moon, but enough to perturb the orbit of comets and asteroids. Also, since Terrium does not create an atmosphere lock unless burning, those without a burning core are subject to intense winds, creating some fascinating erosion patterns.
Now, on to the cultures of Earth.
Most of Earth's development happened as it did in reality until we get to the 1500's. While the age of exploration and colonization and all that was going on, there were some major differences in the New World. First of all, the arrival of an extra-terrestrial animal life form known as "Drach" by the natives in Meso-America was a huge alteration - the Aztecs, Incans, and a few other cultures were able to domesticate Drach, [note=Drach]The origins of the Dratch remain a mystery - it's believed those that landed in South and Central America were the last of their kind, perhaps fleeing an extinction event on their homeworld. And even greater mystery is how they survived planetary entry - with no flight, they couldn't have simply fallen out of the sky. Regardless, a Dratch is a predatory hexapod that bares some resemblance to the hexapods of Venus (though nothing conclusively points to that as their homeworld yet) roughly the size of a horse, it's primary weapons being venomous mandibles. They are pack hunters and likely would have been devastating to the local ecosystem had they not been so quickly and easily domesticated - they took to humanity quite well, and humans were fast to domesticate the few they had. No Dratch exists in the wild.[/note] which served at beasts of war: Meso-America was much more unified when the Spanish arrived, a factor combined with the fact that North American had already been exposed to smallpox so it was much less devastating meant that spain was forced to found colonies elsewhere - Texas and California had a much heavier Spanish presence. This same problem led the French to put much more effort into Quebec, making that a much more populated region.Â
Those factors don't become relevant until well after America gains independence from Great Britain - the early 1800's saw a slew of rebellions in the New World, with Quebec breaking from france, Texas and California from Spain, and of course, the American Civil War - which the Confederacy was able to win. The Wild West was gobbled up by the USA, CSA, ROT (Republic of Texas) and ICR (Idependent Calfornia Republic), though the Aztecs managed to hold onto their land all the way up into what is now New Mexico and Arizona.Â
This meant that North America, instead of being dominated by a supergiant from sea to sea, was a collection of broken, fighting factions in the beginning of the 1900's, when two important things happened.
The first was the obvious one - World War 1 broke out, much as it did in reality, only the ROT, ICR, and CSA were on the same side as Britain, France, and Russia, while Quebec, Canada, and the USA sided with Germany, Austria, and Japan. (Mesoamerica was busy with it's own conflicts during this, and largely stayed out of the first World War.) In this scenario, the latter group emerged victorious - and set the stage for World War 2, this time with a Confederate dictator being the Hitler of that war.
The second is major for a different reason - in the early 1900's, humanity first took to the sky, the first steps towards leaving Earth's atmosphere. During World War 1 deposits of Terrium began to be found across the globe (presumably broken off the Earth's core and floating to the top), and it's properties greatly enhanced our understanding of the Universe we live in.
So that's where I'm stopping for now - humanity on the brink of finally leaving our little corner of the solar system, and also on the brink  of once again going into total war - this time with machines powered by fragments of Terrium instead of fossil fuels, machines of unimaginable destruction. [note]I plan on taking this setting all the way to present day, and eventually providing an actual timeline. This is just where I'm stopping for the moment[/note]
Added more information to last post, reserving this space for the eventual timeline.