Water Planets ~ There are two kinds of worlds that might be entirely covered with water. One is a terrestrial Earth-like planet that's just covered with a lot more water than our world, but is otherwise still familiar. Or you can imagine a Hot Neptune which is almost totally composed of water that is close enough to its star to not be frozen, but instead have an ocean thousands of kilometers deep and perhaps an atmosphere like a gas giant's, with lots of hydrogen and water vapor.
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