Panthalassic worlds are truly massive terrestrial planets, on average massing three to five times that of Earth. During planetary formation, these worlds accrete beyond the "snow line", that point in a solar system where abundant ices are found, and which lend themselves to the rapid creation of Jovian worlds. however, momentum loss through the interaction with the accretion disk causes inward planetary migration. While in many cases fully formed Jovians end up in the inner solar system, some of these worlds have not yet had time to accumulate the sheer mass of a Jovian, and instead cease their growth within this 3 to 5 Earth mass range.
As these worlds settle into the warmer regions of their solar system, a massive layer of water forms, approximately equal to 400 times the amount of water that the Earth possesses. Such massive oceans can easily reach depths of over 100 kilometers. The atmospheres of these worlds remain rich in primordial hydrogen during their youth, but after only a few million years of being exposed to the greater stellar intensity, this hydrogen will have been eroded away, leaving behind a massive atmosphere of water, nitrogen, and carbon dioxide. Curiously, because of the absence of a rocky crust or the attendant tectonic activity, oxygen that forms from the photodessication of water in the atmosphere remains abundant. On most worlds, this presence of oxygen would be an indicator of life. On Panthalassic worlds, it is an indicator of planetary formation.
These planets have a distinct geological makeup. The core of the world is comprised of approximately one Earth-mass of heavy metals, with another two or so Earth-masses of lighter rocky materials composing the mantel. The crust of the planet, buried beneath the stupendous oceans, is made up of another two earth-masses of ices.
A Panthalassic Type world from space (image used courtesy Don Edwards).
The world ocean of a Panthalassic planet.
Panthalassics of Note
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