Kaywell/Gememma Formation

The binary Gememma system formed around 7 or 8 billion years ago in a distant part of the galaxy, drifting around in deep space. It was formed abnormally metal-rich for its era due to being formed out of the gas of not one but two supernovae. After a troublesome and flare-heavy childhood which lasted 4 to 5 billion years, it finally settled down and became a calmer star. By that point it had severely depleted the original thick atmospheres of its planets, all of which had had very thick atmospheres initially. Lowel and Gannovar's orbits, heavily inclined and eccentric respectively, are the result of a long gone earth-mass planet which once occupied the system in an unstable orbit, or perhaps a flyby of a rogue interstellar planet. Gememma's companion star, which orbited several hundred au away from Gememma, is totally lost, and is thought to be similar in mass to Gememma.

Kaywell forms from a proplyd around two billion years ago. The disk is more metal rich than Sol's, and more massive as well. The cores of the giant planets form at around the same time while the disk is still full of gas. Tyepolbynar and Reander form in the outer system while Shol forms down at around 0.2 au from a huge amount of rock and iron, and easily sweeps up the gas in the nebular as it migrates down to its current position. The leftovers from Shol's formation would form a quite heavy gas giant on its own eventually. Earth-mass planetary cores are already forming, and there's many dozens of them. They're disrupted by the downward migration of Tyepolbynar as it sucks up the vast amounts of gas in the disk and falls inward. Reander migrates down, following Tyepolbynar part of the way down to around 3 au, before a smaller companion forms behind it which pulls Reander back out to more or less its current orbit, clearing the space between what is now Egad and Reander. The companion either collides with Reander later or is ejected at some point. By the time Tyepolbynar reached its modern orbit its moons had already finished forming, and were icy in composition.

The inner system began to form. The remaining planetary cores, of which there were still many Earth masses, coalesced into two planets, Mesbin and Valyr. Mesbin was hit by a super-earth-mass planet and spent some time as a synestia which resulted in the formation of Derbin and a rapid rate of rotation. It accreted what was left of the hydrogen in the disk to form a modest gas-giant-like atmosphere, and its tidal drag pushed Derbin further and further outwards. Another planet, nicknamed "Clement," orbited in Mesbin's leading trojan for a long time. It would have had a comparable mass to Derbin and Valyr. A few other earth-sized cores were roaming around and there were still plenty of moon and mars sized planetary embryos, which would occasionally strike the big planets. Once the gas disk had completely dissipated, Clement's orbit became unstable and it collided with Mesbin. This turned Mesbin into a synestia once again.

The hydrogen, now heated up so by the collision, was rapidly being bled off into space. The rock vapor torus surrounding the rapidly rotating mantle of Mesbin began to accrete three moons: Kerbmun, then Graymun, then "Notmun", each one two orders of magnitude less massive than the last. As one was finished, tidal drag forced it out of the synestia, and the next could form from a drastically reduced stock of material.

The result was three moons (four counting Derbin, five counting Derbin's own captured Derminmus), a rapidly spinning planet Mesbin, and a relatively thin hydrogen atmosphere which hugged the oblate Mesbin and formed an equatorial disk leeching out into space.

Notmun spent some time in a nice stable orbit where Graymun is now, but orbital instabilities between Kerbmun, Derbin, and Graymun ended up forcing Notmun dangerously close to Mesbin, to the point where the small moon was torn apart into a vast ring system embedded within the atmosphere disk. By this time, Mesbin life, including organisms similar to CavernGoo™, may have evolved. It's possible that there could have been life living in the rocky atmospheric rings of Mesbin.

The atmosphere was slowly being leeched off to space. But tidal drag was slowly slowing Mesbin down. Eventually the atmosphere disk would be detached from Mesbin's atmosphere, and Mesbin's atmosphere would stop being lost to space. However, this took so long that the pressure on Mesbin's atmosphere at the surface was truly pitiful, and surface conditions no longer supported liquid water. When the atmosphere disk dissipated, the rings were no longer held stable, and began to be torn apart by tides from Graymun and Kerbmun. The ring material would either fly out and hit Graymun or, much more likely, fly down and hit Mesbin. All that remained was eventually a diffuse moonlet system and the core of Notmun, which is now known as Statmun. Much, much later (only a few million years ago) Mesbin would be hit by a massive asteroid right on the North Pole. The resulting crater basin was deep enough that the entire atmosphere settled down in the crater, crushed to a pressure of 1 atm.

Statmun is brought into a Mesbin-stationary orbit by being locked into a weak resonance with Graymun, which at the same time coincidentally has the same weak resonance with Mesbin. Even after the resonance is broken, Mesbin's lumpiness from continents keep Statmun locked in a stable 1:1 orbit.

(Troymin was captured somewhat recently by Kerbmun from interplanetary space, by the way)

1 billion years ago, something shook the Kaywell system. That something was pale orange in color, the size of the planet Shol, and an awful lot heavier. Gememma was crossing through the domain of space near Kaywell. Its companion went one way, Gememma went the other, and suddenly Kaywell was now Gememma's companion and Gememma's former companion was now a lone star. The resulting near-circular orbit plaid havoc with the system. A planet orbiting beyond Reander was lost to deep space, and the comets were totally disrupted. A late-late-heavy bombardment struck the system.