Only one thing is known for certain: as we do fall, so too does the world rise to meet us.
In that realm of the cosmos where Jikorra rolls around a binary system (tidally locked with a resonance orbit of 5:4 to a red dwarf, which in turn is locked in an eccentric binary orbit with an F-class star), the elements are…a little messed up. The nebula in which the world turns acts as a shielding device, allowing travel to a small pocket dimension where the laws of physics are fundamentally different. For one thing, the five basic forces in the universe–gravity, electromagnetism, transmutation, multiplanar phasing, and stabilization–are all subatomic particles of their own, creating a system of chemistry unique to the region. Travel too far outside the nebula, and this collapses–but within it, life continues as per normal. Sort of.
Among the elements found in this region of the galaxy are adamantium, the ultimate Earth element; aether, the substance of the localized stars in a complex orbit around the planet; aquium, the source of life; and phlogiston, the accumulation of which causes structures to lose mass. Atomic structure is based on the accumulation of particles representing different forces.
The core planet itself is relatively small, perhaps the size of Mars (although heavier). But it expands. The core of the planet, the taproot (actually a very small white dwarf which has undergone some fairly extensive changes), produces oceanic crust, which slowly but surely loses the accumulated earth and fire particles and picks up phlogiston, oxidizing the crust and making it slowly but surely lift off the surface. It may break apart into islands quite early on, or it might maintain a certain amount of structural integrity, but it will rise nonetheless. At the point where the planet can no longer hold it together, the pieces (now more aether than anything else) will condense into living diamonds, becoming new stars before eventually either falling to Earth or being flung out of the nebula at some speed.
The people of this world live on both levels, the continental and oceanic crust. The Upper People tend to be taller, darker, and more frail, but they have access to more materials. The Lower People–paler, bulkier, covered with downy fur like a seal–are more primitive but tougher. The two basic races can intermarry; it’s been suggested that the ancestors found their way down to the Lower World by accident. Down on the oceanic islands, there is very little phlogiston by comparison to the vast continents already risen from the sea-bed; a person who goes down will eventually lose the ability to go back up, unless aided by others.