This is going to be tricky and goofy, but we'll see how it goes.
Cosmology is kind of a big deal in DnD. There are all kinds of planes and weird places to go, with bat-shit crazy mechanics and demons and devils and angels and so on. Planescape is a wondrous setting with majestic maps of gloriousness. Unfortunately, I have sort of stepped away from the traditional (by traditional, I mean the best, i.e. 2nd Edition) cosmology because I am obsessed with doing my own thing. It would be so ridiculously easy for me to just say Hekinoe is a prime material plane and be done with it. I would willingly sacrifice my "uniqueness" to be able to say "Let's go to Sigil!" I restrain myself though, because I think concepts like aliens and extraterrestrial entities are far more interesting than demons and gods and MAGIC!!1! Now, the cosmology (I'm trying to use that word as much as possible) of Hekinoe is basically its solar system. The planes are planets or planetoids or large asteroids or whatever you want to call them. The demons, devils, ysgardians, and so on dwelling on them, are alien civilizations that the common man of Hekinoe has no knowledge of.
The reason this 101 is tricky is because I literally know nothing about astronomy or space (other than the fact that its wondrous, unguessable variety and majesty terrify me down to my very bones). Also, like I said, no one on Hekinoe has any knowledge that there are other civilizations and races on these worlds. I mean, there are telescopes and astronomers and such, but shit, Hekinoe is a long way off from space travel and moon rovers. Maybe once they get a handle on revolver technology we can send a fleet of giant bullets out of a cannon to the moon in true steampunk tradition.
I think we'd have to use Gamma World for that scenario though. Heh.
For the sake of simplicity, I have modelled this solar system after our own. I'm not recreating the wheel here, just trying to give you all a better picture of this place.
Errolauk, the Sun of Hekinoe: Errolauk is the sun, it is hot and sun-like. I'm not going to get into its solar radiation output or this or that. It is a sun, and it does its job keeping the planets of the system rotating around it and not freezing over in the empty void of space. In it dwell the Saevoi of the system, and a few other civilizations of energy based, heat and radiation devouring, lifeforms whose whims and motives are likely completely alien to The Robust Five. Encircling Errolauk are rings of fire and energy, and it is in these swirling bands of energy that the Saevoi dwell.
DnD Analog: The Elemental Plane of Fire, the Saevoi are the Saevoi, the energy-based lifeforms work quite well as Fire Elementals and Efreeti.
Erromenle, 1st Planet From The Sun: I'm not going to play the "this planet is xAU from the sun and this one is this many" and so on game. The first planet is far enough from the sun that it didn't burn to a cinder. Erromenle is a small planet, maybe a quarter of the size of Hekinoe (which I have decided is maybe slightly larger than Earth). It is a barren thing with only the thinnest of atmospheres, and nothing that is breathable on the surface. The planet is a shattered dead thing, full of craters and the dead remains of volcanic channels and reservoirs. If there is water on it, it cannot be seen. The atmosphere is thin, but full of dirty grey windstorms buffeted by solar radiation that scour the surface of the planet and are forced down all the volcanic channels of the place, howling as they whirl through the innards of the planet. Within the very core of the planet is a corona of ever shifting nebulous energy that resembles the bands of fire and energy that ring the sun of the system. The only place that living things (as we recognize them) can dwell safely is within the screaming windstorms near this orb of pulsating chaos at the core of the planet. The light of the core generates everything from oxygen to plant life, to massive chunks of minerals, and just as quickly devours it back into its mass to reconstitute something else. Suffice it to say that if something lives there, it is alien to you an I and should be shunned.
DnD Analog: The Windswept Depths of Pandemonium with a dash of The Ever-Changing Chaos of Limbo, sans the giant frog demons (Slaadi) and planar monks (Githzerai).
Hoplemel, 2nd Planet From The Sun: Hoplemel is a Hekinoe sized planet, glowing with a crayon yellow color with a hint of greens and whites in some spots. Nothing is known about the surface of the world because its atmosphere is thick with yellow and green clouds of gases. Unlike Erromenle, this thick atmosphere of cloudy, toxic gases doesn't whirl around in storms, it just kind of hangs there occasionally creeping across the surface. There are breathable aspects of the air there, but the majority of it would be toxic to us and the denizens of Hekinoe. Floating through the atmosphere of this yellow planet are gaseous creatures that are formless and silent, but live nonetheless, as they float through the deadly gases of their homeworld.
DnD Analog: Elemental Plane of Air, and Air Elementals and similar creatures (although, they should probably generate clouds of cloudkill if they are brought to Hekinoe and perhaps do acid damage).
Hekinoe, 3rd Planet From The Sun: Hekinoe is Hekinoe, perhaps you've heard of it. It is Earth-like and has civilizations in great abundance and variety. It is a swell place.
DnD Analog: The Prime Material Plane.
Hekinoe's Moon: I'm being silly here. Hehe. The moon of Hekinoe is like a moon, it hangs in the sky illuminating the night with the reflected light of the sun. Hekinoe's moon is actually a massive concave disk of wolf-iron encrusted on its Hekinoe facing side with space debris. On the other side of it, filling the concave portion of the disk is a roiling sea of that energy that rings the sun and lies within the heart of Erremenle. Riding the waves of that Cosmic Sea is a silver city of strange metals and archaic architecture built upon a broken block of stone from a larger world. I've spoken of this place before. Trailing through space as the moon orbits Hekinoe, dangerously close to that planet, is a tail of asteroids and debris that often impact Hekinoe. These asteroids and debris are usually fragments of the moon's wolf-iron, but sometimes they are bits and pieces of the other side's city of stone and silver that occasionally break off due to unrest or disaster.
DnD Analog: The Prime Material Plane, most prime materials have planets with moons.
Moreenke, 4th Planet From the Sun: Moreenke is slightly smaller than Hekinoe, and is a planet of burning craters and volcanic destruction. The planet is dark, with most of its surface composed of scorched rock, and where it is not blackened by fire, it is usually a pit or river of superheated lava. The planet is a broken thing that has been impacted for thousands of years by debris from the nearby asteroid belt, and many of the volcanic craters and lava filled pits are the result of those repeated impacts. The atmosphere of the planet is mostly sulfurous smog that would choke the life out of anything living on Hekinoe. There are three civilizations on this world, and they are hardy creatures that live and prosper on this death world. The first is an ancient race called the Molengel, the original inhabitants of Moreenke. They once ruled the surface of the world and dwelled in huge monolithic cities built of Morrenke's equivalent of obsidian, a nearly indestructible substance that kept the worst of the lava flows of the planet from burning down the empire of the Molengel. At some time long ago, the Molengel were attacked by two other civilizations, the Mon-ed and Vil-ed. Both of these two civilizations were resistant to the harsh atmosphere and heat of Moreenke and this allowed them to freely wage war on the Molengel. When they had razed enough of the Molengel's fortresses, they quickly made camp in the ruins of the planet and decided to wage war on each other. Massive armies wage war between the two races in the lava-lit craters of the planet. Meanwhile, far beneath the surface of the planet lurk the Molengel in dark pit-like cities of obsidian, waiting for the chance to bring death to the two other races. The Mon-ed are a crude, war-like race of a tremendous population, possessing a society that is little more than might makes right, combined with an industrious ability to create weapons from any material. The Vil-ed are just as violent, but they are far more concerned about longevity, and the ruins they occupy often show evidence of repair and fortification so that the Vil-ed have a large network of interdependent of fortresses scattered across the planet's surface. Often times, these fortresses are connected by lava tubes that the Vil-ed have walled off or widened to accommodate the movement of their armies. The Molengel population is nearly extinct, and they are forced to remain underground biding their time, only ever striking against their oppressors when the odds favor them tremendously.
DnD Analog: The Blood War, Baatezu, Tanar'ri, and Yugoloths, sort of.
Dikorpseken, 5th Planet From The Sun: Dikorpseken is only slightly further from the sun than Moreenke. This planet has one feature, it no longer rotates as it orbits the sun. This leaves one side in perpetual night, and the other in never ending scorching daylight. On the dark side of this planet, there is only the cold dark, and on the light, there is only scorching desert and humid jungle. There are native civilizations on the light side, ranging from primitive hunters up to those that dwell in silver cities of glass that harness the unrelenting energy of the sun. The shadowed side of the planet is cold and dead, a mirror of the light side. The civilizations on the dark side are pale, colorless, mirror of the other as well. Primitive hunter societies are brutish cannibals that eat anything within reach, and the silver cities of glass are ruined and cobwebbed, covered in the hardy root-like plants that are all that can grow beyond the light of the sun.
DnD Analog: Plane of Shadow, Plane of Radiance, Negative Material Plane, and Positive Material Plane.
Asteroid Belt: Beyond Moreenke and Dikorpseken is a large, circular field of rocks that orbit the five inner planets and the sun. Most of this belt is just dead rocks and space debris. Among the rocks though, are huge square or rectangular blocks of wolf-iron. Much like Hekinoe's "moon", they are encrusted with dust and craters and space debris, so much though that some have lost their blocky shape and resemble something more mundane. Some of these massive cubes sport the insane machinery of some ancient interstellar war on their surface, others are hollow. Many of these cubes are so massive that they possess gravity of their own and retain an atmosphere, and across their surface, various races of the system wage war with scavenged weapons in the hopes of looting their enemies and escaping their own block of wolf-iron for one that is larger and and bears more spoils to be looted.
DnD Analog: The Infernal Battlefield of Acheron.
Rekpornen, 6th Planet From The Sun: Rekpornen is a ball of blue half the size of Hekinoe. It is believed that the planet was once a small planetoid with an extremely dense core that magnified its gravitational pull and allowed it to draw other elements to it. Surrounding this dense core of metals is a layer of thick ice, but above the ice is the deepest ocean within the system. In the ocean is an abundance of aquatic life, ranging from kelp to sea serpents the size of, well, big sea serpents.
DnD Analog: Elemental Plane of Water, Water Elementals, and other aquatic lifeforms.
Cardinep, 7th Planet From The Sun: Cardinep is a large planet, bigger than Hekinoe. The inner portions of the planet are rife with volcanic activity, the planet's core is actually a molten ball of iron. Most of the planet is solid rock with caverns and volcanic channels spread out below the surface. The planet has no atmosphere to speak of, but there is no surface life, so that is irrelevant. Within the earth and caverns of the planet are hordes of mineral based lifeforms, along with a robust assortment of stone, minerals, and ores that would shame Kusseth and Whurent.
DnD Analog: Elemental Plane of Earth, Earth Elementals, some dragons, various oozes, Stone Giants, etc.
Gorelon, 8th Planet From The Sun: Gorelon is a semi-radiant planet that looks like a large star when seen from Hekinoe. The bright atmosphere of the planet completely shields the surface of it from watchers (although the sights beneath the atmosphere can be viewed by those using sorcery to aid their endeavors), not via yellow clouds or fire, but from a golden radiance that suffuses the atmosphere. Beneath this radiance, is a seething chaos of madness. Gorelon is not a true planet, it is actually the remnants of three wrecked spacecraft that have fused together and had the shielding surrounding their Cosmic Engines crack open. The unmanaged energies of the Engines have resulted in a planetoid that is awash with the unpredictable fires of creation. Beyond the golden radiance is a limitlessly huge planet that exists in between the laws of physics where creatures of flesh exist in horrific variety. Seething maws of venom and fangs the size of dragons seethe and howl, while smaller creatures skitter around on their tongues. Cities of flesh sprout walls of living teeth and fingernails and sit atop giant, pale grey, brain matter. Gorelon is a twisted place and many an astronomer has gone mad pondering its mysteries.
DnD Analog: The Far Realm and everything that plane entails.
Jhrehylem, 9th Planet From The Sun: Jhrehylem is a grey planet, warmed by the insane energies of Gorelon and shielded from the chaotic effects of those energies by the thick, grey, atmosphere of the planet. Everything within the planet is cold and grey. The planet is warmed by the radiance of Gorelon, and this keeps if from becoming a place of frozen water and gases, but it is by no means a warm place. The atmosphere of the planet is composed of huge clouds of ash and dust. The volcanic core of the planet has almost gone out and the thermal channels belch only ash now. The gravity of the planet is immense, pulling all manner of space debris to the planet's surface, filling it with huge craters. There is no true light in Jhrehylem, only the dusky twilight that is a byproduct of Gorelon and the feeble (relatively speaking) rays of the sun. Jhrehylem is a grey place, there is no color here and the inhabitants of this planet must constantly struggle against a despair and ennui that are seemingly encoded into their very DNA.
DnD Analog: Gray Wastes of Hades.
So there is a bit about the solar system. It isn't complete, and there are plenty of random celestial bodies that I will flesh out as needed. I'm sure none of this will ever come into play in our current campaign or later ones centered around Hekinoe.