In terms of planetary evolution, these worlds are actually very common. However, in most circumstances the Pelagic Type development stage is very brief, and so these worlds are quite uncommon.
Best described as a moist greenhouse world, Pelagic planets form when a planet accrues enough greenhouse gases to initiate planetary warming on a massive global scale. This causes the evaporation of oceanic environments and the breakup of water into hydrogen, which then is lost to space, and oxygen which forms compounds with other elements and is incorporated into the crust. The oceans, however, will remain intact for a few hundred million years, largely because of the presence of large amounts of carbon dioxide in the atmosphere. However, these oceans remain exceedingly hot, on average around 200 to 300 degrees Fahrenheit, and are kept liquid by the immense atmospheric pressure.
A breaking point is eventually reached, though, when enough hydrogen is lost to space and the atmospheric pressure falls below the point which is required to maintain the oceans. The result is a swift and cascading loss of the oceans to evaporation. These worlds are both young and old in age, and are typically formed when a water-rich world is transformed by increasing stellar output, either by a maturing sun or by a sun that has begun to move off of the main sequence and evolve into a subgiant.
A Pelagic world at its height will have an almost imperceptible transition between the atmosphere and the ocean. Life may be present in both environments, but they tend to be microbial forms. Older worlds may have microbial multicellular biomes, and in the depth, sheltered from the heat, there may still exist macroforms, holdovers from when the world was a mature Gaian Type planet. But all life is doomed, for the planet is destined to lose its entire atmosphere as the sun continues to heat, its stellar winds slowly eroding away the thickened, expanded atmosphere.
The full cloud cover creates a planet that, from space, is blindingly white.
Enhanced for clarity, we see the hot but wet surface of a Pelagic planet's world ocean. Temperatures do not begin to reach comfortable levels until depths of several hundred meters.
Uses for Pelagics: Surface colonization of these worlds is extremely difficult. However, suboceanic habitats can be constructed, and often natural resources are very abundant.
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