While Thalassa is fictional, the science of the Procyon system in SOT is based on real research and our current understanding of stellar evolution and binary star systems.
Gas & Dust
The planet began two billion years ago as frozen clouds of dust and molecules dispersed across vast areas of space. Somewhere in the galaxy, a star went supernova and the shock wave disrupted the clouds. Although the movement was imperceptible, over millions of years, weak gravitational forces caused molecules to gather and rotate in space. Due to variations in density two planetary discs emerged. Most of the mass, more than 99.9%, collapsed into two stars in a binary system. The tiny amount of remaining material coalesced to form planets and debris in the emerging solar system.
The first protostar to emerge was massive. As it collapsed from gravitational forces it formed a star two and a half times that of the sun — 2.5 solar masses. The second protostar formed a smaller star, 1.5 solar masses. Over millions of years, as the molecules coalesced into the cores of the two protostars, temperature and pressure increased until gases, mostly hydrogen, began to fuse into deuterium and then helium. The resulting nuclear fusion balanced the collapsing gravitational forces. As the stars ignited they radiated light out into the far reaches of the universe. The Procyon system was born.
The larger star was blue-white and burned brighter and hotter than its smaller yellow-white companion as they orbited each other over a 25-year period. Because the star system had formed from the supernovae remnants of previous giant stars, which had created heavier elements through fusion, the Procyon stars, and their eventual planets were rich in heavy metals, especially iron, nickel, and traces of heavier elements including radioactive isotopes.
The remaining debris was scattered throughout the binary star system as primordial rock, dust, water ice, frozen gases, and organic compounds which formed asteroids and comets that rained down on the planet’s surfaces, slowly enriching them with water and other compounds. Early in its history one particularly large mass, the size of a small planet slammed obliquely into Thalassa and shattered it into pieces that were thrown into orbit to coalesce into a small moon orbiting the young planet. It would later be named Lono.
As the stars formed and blinked on, tiny remnants of the molecular clouds that orbited them began to form into small planetesimals, meters to kilometers in size, which eventually coalesced from countless collisions into protoplanets and eventually planets. Around the larger star, this process resulted in the formation of two gas giants, both larger than Jupiter and orbiting close to the star. Due to gravitational interactions between the stars and the gas giants only one planet was formed around the smaller star: a small rocky planet about the size of Mars. Billions of years later astronomers on earth would name the planet Thalassa, after the primordial god of the sea.
As Thalassa cooled it formed a core, mantle, and outer crust. Because of its high concentration of heavier elements, the core created intense heat which powered convection within the mantle, creating a strong magnetic field. For hundreds of millions of years, volcanoes drove crustal plate movement and belched gases and ash onto the planet, which began to fill with water in low-lying basins as it cooled. These volcanic processes also pushed heavier elements from the core to the surface, creating rich deposits of heavier metals, including gold and platinum which settled on the deep-sea floor. Tremendous pressure in the volcanic plumbing deep in the mantle slowly turned some carbon into diamonds, which were pushed to the surface.
After 600 million years, as the small rocky planet continued to develop, the larger star, because it was burning faster and hotter than the smaller one, began to run out of hydrogen in its core. As fusion shifted outside of the growing helium core to a hydrogen shell, intense heating and pressure caused a collapse of the core and an expansion of the star’s outer atmosphere as it transitioned into a red giant. Rather quickly, as the red giant grew, it consumed its inner gas planets. When the red giant ran out of fissionable material it violently ejected its atmosphere into space and then collapsed into a small, hot, and extremely dense, earth-sized carbon core: a white dwarf. In the process, its nebula further enriched the Procyon system with heavier elements, including carbon, iron and other heavy elements that dispersed into space amongst the primordial debris. Although the white dwarf was still hot, it had lost most of its luminosity and it was now orbiting the smaller star once every 40 years.
The death-throes of the larger star caused pronounced changes in the gravitational forces around the two stars and disturbed the orbits of long-period comets in the system’s oort cloud, sending a renewed flood of debris raining down on Thalassa. For over a billion years, as the planet belched volcanic gases and steam, and pushed heavier elements to the surface, the oceans grew, and the planet cooled. Eventually, reduced mantle convection slowed crustal plate movement, and magma from the remaining heat formed a volcanic bulge on the surface. Repeated eruptions on the bulge resulted in a massive surface volcano. But as volcanism subsided, and the planet’s heat dissipated under its deep oceans, the bulge began sinking on the thin crust and rising sea levels eroded the volcano to an underwater plateau.
As the planet cooled it would have taken the path of many before it: as the molten core cooled and plate tectonics ceased, Thalassa would have lost its magnetic field and the solar winds would have stripped off its atmosphere and its oceans would have dissipated into space.
But 10 million years ago, an event happened that had a profound effect on the future of Thalassa. An object, perhaps a rogue planet ejected from a distant solar system, moved through the Procyon system and obliquely collided with the small ocean planet, and was captured as Thalassa’s second moon. The new satellite began a highly elliptical orbit around the planet that cast it far out in space and then swung it back perilously close over a two-year period, causing considerable havoc on the planet, including renewal of volcanism. the moon would be called Hina.