I picked up Distant Worlds via PDF download the other day, and most of what I read, I liked a lot. The Paizo guys packed a lot of content into just eleven planets, which quite impressed me. I had been developing a PF campaign setting more or less based on the real-world solar system. A few of my ideas paralleled what Distant Worlds did--though instead of placing a fifth intra-Belt planet beyond Mars (or Akiton), I had included a Io-like world after the manner of Le Verrier's hypothetical torch-orbit planet. Otherwise, I had Venus (or Castrovel) as a lush jungle planet, Akiton/Mars as a cold desert planet (with a breathable if harsh atmosphere), and the dwarf planets Ceres, Pluto, and Eris as interesting (if inhospitable) places to visit. Given that the gas giants are so far from the sun, I would have had all the gas giants serve as net heat emitters and at a much greater efficiency than Jupiter (unscientifically exaggerating the effects of gravititional contraction), allowing some of the larger satellites such as Europa, Titan, and Triton to support a wider variety of climates. Jupiter would by far have the greatest heat output, its habitable moons being more like Venus or Mercury, Saturn's moons more closely resembling the temperate regions of Earth, and Uranus' and Neptune's moons mimicking everestine Mars or the global tundra of Ceres.
Before DW was released, I liked having the Elves on Venus as a steamy world of swamps and jungles (a meme known in sci-fi since the early 20th century) while putting the Dwarves on Mars as the steadfast subterranean race that they are: lush beards to ward against the fierce cold of Martian winters and the grit of frequent dust storms, beings obsessed with massive excavation projects and inured to hard labor and heavy burdens, and a race continually at war with and defending against frequent incursions from the enemy races on the surface (giants, orcs, and goblinoids). In other words, each race's innate traits suggest their original homeworld before they lived among the humans on Earth.
Thus, one of the few things I thought DW came up short was a certain planetary origin for all the other core races (not including humans). Probably the author intended only that the elves should be non-native to Golarian, leaving the dwarves, halflings, gnomes, and humans to have arisen on that temperate planet with the line from page 17 "Elves are relatively familiar to Akitonians, thanks to the planet’s connection to Castrovel and Sovyrian, but smaller races such as half lings, dwarves, and gnomes tend to be regarded with unguarded fascination or derision." Though I would like to think, as anthropentric as it might be, that only humans are the true natives of Golarian, and all the other "core" races come from somewhere else.
Except for the passage just quoted, it wouldn't be too great a leap to put the Dwarves' origin on Akiton, with the Shobhad-neh as a primeval racial enemy. An explanation for their presence on Golarian is needed; perhaps some of their kind had unearthed a subterranean portal to the underdark of Golarian? or maybe they were transplanted by a long-forgotten master race which had bred some of them for manual labor, and gave them the ability to remain effective on worlds of stronger gravity than Akiton's?
Given the Halflings' fear resistance, halfling luck, heightened senses, and comparatively greater longevity compared to humans, they would fit well on Triaxus. Their fear resistance (and superior dexterity and charisma) could have arisen as defenses against the tyrranical reptillian rulers of the Drakelands. Their saving throw bonuses may have given them the ability to survive as a race continually active through Triaxus' slow change of seasons, and this same great length of year may have contributed to the halfling's super-human longevity. A halfling's heightened senses (while free from suffering any light sensitivity) might be an adaptation from the necessity to hunt scarce game during the eighty-year winter season.
In most D&D-esque incarnations, Gnomes are posited as distant relatives of the Dwarves. Pathfinder Gnomes are described in the core rulebook as fey rather than dwarf-kin, which lets us be more flexible. We mght say that gnomes are not native to anyplace in Golarian's star system at all, but are rather distant ancestors of extraplanar fey who "went native" a very long time ago. Their journey and subsequent settlement need not be limited to Golarion, as Castrovel, Triaxus, or Verces would all serve as pleasant homes for their kind.
As a natural extension of a Dwarven origin on Akiton, a logical next step might be to place the orcs and all goblinoid races on AKiton too, to explain the martial tactics evolved by the Dwarves against these foes. The light sensitivity trait common to these races may be inferred as a result of being used to the fainter light of the Akitonian sun. I would not expect this rivalry to have originated from an ancient inter-planetary conflict, without also explaining how the technologically backward orc- and goblin-kin could be involved in space travel or portal use, if not as mercenaries or conscripts serving a more advanced and disciplined military power. The ability of orcs to crossbreed with humans (and perhaps other humanoids), may also have been inculcated into them as a generational biological/genocidal weapon by their spacefaring overlords, and could explain their presence on Golarion and other worlds besides Akiton.
Any other playable race could receive similar treatment, retroactively identifying their origin by using their innate racial traits as clues.
The problem you run into there is that Mars, as a light-graviy planet half the diameter of Earth, would tend to produce elf-shaped natives, not dwarf-shaped ones. You could give Mars a super-dense core, but that runs into all kinds of interesting questions like "how did this material get created as a part of stellar evolution producing elements only up to iron?"
Gnomes are dimensional travelers, not inter-planetary ones.
