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Comment by kijin | Hacker News Reader

> It is about 2.7 times the Earth's diameter, but weighs almost seven times as much.

2.7^3 = 20 times the volume. But only 7 times the mass. So its density would be only 35% of Earth's density. Earth's density is 5.5g/cm^3. That makes this planet's density 1.9g/cm^3, which looks about right for an object that is partially rock and partially water.

But this ain't a waterworld, it's a steamworld. Imagine the greenhouse effect from all that water vapor!

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Does the density & mass calculation affect what the surface gravity would be? If the planet is more massive than Earth, but less dense, would someone standing on the surface experience a greater weight than we do here?

It in fact does! The surface gravity is 0.91 g[1].

http://en.wikipedia.org/wiki/GJ_1214_b

Gravity depends on m/r^2 where m is the mass of the planet and r its radius. Compared to Earth's, m = 7 m_earth and r = 2.7 r_earth, which gives you a gravity 0.96 times the gravity on Earth. Remarkably similar... But I wonder, if the temperature is 200 degrees, there must be a massive pressure, in order to have liquid water. Where does the pressure come from? Heavy atmosphere?

Pressure is nothing other than gravity pulling down everything that lies above you. So if you go deep enough, there will be enough gas above you to create enough pressure to make water liquid, or even solid.

I wonder whether it will be possible to build a floating city somewhere in the middle of that massive atmosphere. Air conditioning will probably take up the majority of the city's energy consumption, but hey, if you've got water, you can get oxygen, and if you've got boiling water, you can make electricity. Actually, it might be easier to live on this planet than to live on Venus (450C, all CO2).

Boiling water doesn't give you electricity. A temperature gradient gives you electricity. Having a hot, steamy planet won't help you if the air is just as hot as the water.

So maybe this planet is more like "Bespin" than Waterworld!

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