Water

What would happen if you made a planet out of water and nothing else?  What if you had a giant water droplet, as big as the Earth, suspended in space with only its own gravity to hold it together?  What would such a world be like?

The obvious answer: it would never happen.  Real planets are mixtures of many elements and compounds.  A 100% water planet is as unlikely as a planet of 100% carbon or oxygen.

But I don’t care.  I’m not examining this idea to include in a story.  It’s a hypothetical extreme case that I hope will help me understand how planets are formed.  You might even call it a thought experiment.

Based on my limited knowledge of geology, I know the Earth’s core is a giant, solid mass.  Despite the fact that the core is thousands of degrees Celsius, it doesn’t melt.  The enormous pressure exerted by all the layers of rock, magma, etc on top hold it together.  So what would happen at the core of a planet made only of water?

Based on my research, I believe it would solidify at an extremely high temperature, just like the core of the Earth.  Water, when at high temperatures and under enormous pressure, will form a special kind of hot ice.  Researchers have created this ice in the laboratory, and they call it ice VII (using different temperatures and pressures, scientists have created sixteen varieties of ice).

Assuming the water planet is close enough to its parent star to allow liquid water to exist on the surface, I expect this world’s one ocean would be subject to the same Coriolis forces as Earth’s bodies of water, but without the interference of continents.  Near the poles, we’d probably find ice (ice Ih, the kind you put in your drinks), and there might be an atmosphere of water vapor.  Of course since there’s no carbon, there’s no life.

This is an ongoing project, and I still need to do more research.  I’m particularly curious to know if hot ices, like ice VII and similar forms, glow as they give off heat; and if they do, would that glow be visible from the surface of my water planet?  I should also mention that while 100% water planets are extremely unlikely, scientists have discovered at least one planet outside our Solar System that may have abnormally large amounts of water.  It’s called GJ 1214 b.

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