Floating planet

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Floating planet

Postby gladiolas » Sat Mar 15, 2014 2:38 am UTC

How come nobody's done this before?

A planet less dense than water will float.

So we construct a Dyson sphere whose thickness is way more than the radius of a gas giant.

We give it at least one sufficiently deep ocean.

We may or may not enclose the floating planet in Plexiglass or something to keep it from dissolving.

But, how fast would it dissolve?

Let's give it floating moons and a ring system too.

I had trouble coming up with something with a reasonable surface gravity. Feel free to adjust the numbers.

Here's what I came up with for the Dyson sphere.

Inner radius 9e9 meters
Outer radius 9.177027840e9 meters (110000 mile thckness)
Density 5000 kg/m3
Mass 1.2965587e29 kg
Gravity .7 of Earth.

We could set this on an Alderson disk instead if you don't think this whole idea is a waste of time.

Thanks for any suggestions.

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Re: Floating planet

Postby Tass » Tue Mar 18, 2014 1:44 pm UTC

Planets are held together by gravity. Putting it in something with way more gravity has a few problems to say the least.

For one the water of such an ocean would break under its own weight. If the Dyson sphere itself is made of indestructible unobtanium, and you try to put a million km deep ocean on it, it would stratify, giving you some exotic form of of high pressure solid oxygen at the bottom followed by metallic hydrogen and finally a gaseous hydrogen atmosphere.

If you enclose a planet in a likewise indestructible shell and drop it in this "ocean", then it will sink until the dept where the densities match and stay there, but the inside will not look like a planet any more. Rather the former core will be at the bottom and the top will be near vacuum.

All of this ignores massive amount of gravitational energy released as these things settle. The whole thing would be burning hot for hundreds of million of years. Depending on the initial dept of the ocean it might even ignite fusion, so what you would have would be a shell shaped star on top of your, really quite prodigious, unobtanium.

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Re: Floating planet

Postby snowyowl » Wed Mar 19, 2014 4:34 pm UTC

The difficulty here is in trying to reconcile a planet held together by its own gravity with an external gravitational field. The ocean is denser than the planet, so each volume of ocean will have a stronger gravity than the same volume of planet. If the planet were heavy enough to hold together, it would be too heavy to float.

Anyway, the sheer size of the thing breaks chemistry - any ocean big enough to drop a gas giant into it will probably ignite and become a star.

Basically the planet has to either be made of rigid lightweight unobtainium, or small enough that it can behave as a rigid object with normal materials. Drifting way off-topic here, not many rigid things other than icebergs float in water, but if the parent was made of mercury or molten lead (>327°C) then a 20-kilometer chunk of rock would float just fine. A bit underwhelming for our purposes, but it's a starting point.
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Re: Floating planet

Postby lorb » Tue Apr 08, 2014 4:41 pm UTC

So size is what breaks it? What if I make a "planet" from wood, or ice or whatever with a 10meter diameter and throw it into the pacific. Than use some magic to scale the whole solar system up until my "planet" is actually planet sized. At what point would it break? and why?
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Re: Floating planet

Postby mathmannix » Wed Apr 09, 2014 1:50 pm UTC

lorb wrote:So size is what breaks it? What if I make a "planet" from wood, or ice or whatever with a 10meter diameter and throw it into the pacific. Than use some magic to scale the whole solar system up until my "planet" is actually planet sized. At what point would it break? and why?

Wood holds together (is rigid) for small "planets" like your 10m object. Wood does not hold together for larger objects. Think of how there is a limit on tree size (maybe a little over what the biggest sequoias and redwoods actually are) based on structural integrity. Sure, a giant ball of wood - whose diameter is five times the height of the tallest redwood - might hold together in space, but not under earth gravity. I think it's like that. It would "break" at different sizes for different materials, but no material (except for the elusive "unobtainium") can hold together at moon or planet sizes.

A realistic planet scaled down to your 10m wouldn't be anything like a solid rock or ball of wood. It would be much closer to a ball of sand. Not held together by anything, not fused into glass, just a 10-meter sphere of sand. Or sugar. Or wet powdered sugar. How well would any of these survive being tossed into the Pacific Ocean? (Hint: they would not.)

Oh, and that's for rocky (earthlike) planets; the sand would sink. Since we were originally talking about gas giants, picture a 10-meter ball of methane gas with nothing holding it together, which would be lighter than the water (float) but heavier than the air. That wouldn't stay together, period.
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Re: Floating planet

Postby gladiolas » Fri Apr 18, 2014 9:38 pm UTC

Too bad, this doesn't sound promising. But I persevere.

We don't attach retro-rockets and land it on the planet like a rocket ship. That would take ridiculous amounts of energy.

I did mention having the planet inside a hard, transparent ball of something like plexiglass. That would hold it together, whether it's a gas giant or a rocky planet.

We build the hollow sphere on a flat lowlands plain, pour in the ingredients to build the planet on the surface of the Dyson sphere, then flood the lowlands with water to form the ocean, and the planet begins to float...?

Or maybe not. Would decreasing the surface gravity of the Dyson sphere help any? Using a different liquid than water?

I have a better idea. Leave the planet sitting on dry ground. It's still inside its Plexiglass ball.

Sell tickets to tourists wanting to visit the most enormous structure on the Dyson sphere. Give guided tours to the interior. (There'd be an airlock and stuff.)

Maybe once in a while they do flood the lowlands just to watch the planet spin around in the flood waters.

Does that seem any more ridiculous than the original? :) :roll:

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Re: Floating planet

Postby Xanthir » Sat Apr 19, 2014 1:27 am UTC

I'd say that the gravity couldn't possibly work out, but if you're already in a Dyson sphere, you have gravity generators. (You can't get centripetal gravity from spinning a Dyson sphere, as it would produce a gravitational gradient from full power at the equator to zero at the poles.)

So hey, since you're already using godtech, anything's possible.
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