<p>OK, what happens to this system? This can be answered with some confidence.</p>
<p>First, the 100 moons will start colliding and escaping. To keep big moons from tidally interacting very strongly they need to be fairly far apart (and not have the wrong resonances), but if you place them reasonably far apart the tidal effect on the planet will be minor. This gets even worse if you put your planet close to the star, since now it has a limited Hill sphere. So the likely outcome is a bunch of impacts, escapes and maybe a few remaining big moons.</p>
<p>(The Hill sphere radius is <span class="math-container">$\approx a\sqrt[3]{m_2/3(m_1+m_2)}$</span>, so if you could pack moons so they all stay just outside each other's spheres they would be at <span class="math-container">$a_{n+1}=a_n \sqrt[3]{m_2/3(m_1+m_2)}$</span>, basically a geometric series out to the planetary Hill sphere <span class="math-container">$a_{planet}\sqrt[3]{m_1/3(m_1+m_{star})}$</span>.)</p>
<p>Then the sulphur hexaflouride will start to photodegrade into other compounds. It is pretty inert, but UV can splice off fluorines, producing long-chain sulphur compounds that will likely start condensing out (and fluorine start to escape). A messy iron sulphide-fluoride crust will likely be the result.</p>
<p>The planet will of course still be really hot. But it will basically act as a blackbody (with some greenhouse effect), losing heat as soon as its star runs out.</p>
<p>The Kelvin-Helmholtz timescale for a 20 Earth mass rather dense planet (the radius is not going to be many earth radii due to compression of the iron core) is long, but still not that astronomical. Radioactive decay has timescales of mere billions of years for standard long-lived isotopes; you can of course assume some more unusual very long lived ones to stretch things, but after a few hundred billion years you will be close to background temperatures.</p>
<p>Tidal heating and even some infall of matter is going to keep the planet warmer than the background level for even longer, but we are talking a few Kelvin.</p>
<p>The key problem here is that having heat is not staving off entropy. This is a system that is leaking away its free energy without control. It would be better to encase it in mirrors. That way one can use the Dysonian hibernate-and-wait method.</p>

CC BY-SA 4.0
2024-04-09T07:34:45.927Z
@MiltonTheMeme - The greenhouse effect slows things down, but not by many orders of magnitude. Mirrors can be surprisingly good (99.9999% reflectance) and if you layer foil layers and tune layer distances to avoid near-field effects (basically, they need to be further apart than the longest significant wavelength of the heat radiation) it can get even better. This way you get many orders of magnitude better energy storage.
Anders Sandberg
CC BY-SA 4.0
2024-03-18T22:46:51.773Z
What about adding the mirrors and then having the greenhouse effect on top of that?Would that increase the lifespan of the system energy?
MiltonTheMeme
CC BY-SA 4.0
2024-03-18T16:55:09.263Z
2024-03-18T17:00:30.563Z
2024-03-18T17:00:30.563Z
Anders Sandberg
<p>So what if a 20 Earth mass planet is used, where 10 Earth masses are pure Sulfur Hexafluoride extending to 5 Earth Radii, and the other 10 is iron and nickel which is fully molten with radii of one Earth Radii and also this planet has a 100 moons of 1 moon mass orbiting for tidal heating purposes. So would putting this object at Venus's orbit for 100 million years and then letting the planet soak up all the heat and energy be a way to slow down entropy so much to the point that very small amounts of energy leak away.</p>
<p>Assume that the object has a strong magnetic field so radiation can't do anything and assume that proton decay isn't involved. Would this be a suitable way of slowing entropy down significantly long enough? Also assume that the lifeforms are hibernating till 1 googol year, and then use the energy after using the thermal difference between interior and space. Is this long enough for them to survive till the next big bang where the quantum fluctuation can randomly drop entropy to almost zero but the probability is very low? These lifeforms could in theory also hibernate for 10^10^23 years and also assume they are in orbit around the planet, which is in intergalactic space with no other objects for many millions of light years after the star's death, where it gets moved by lifeforms.</p>
<p>So is this the way to get around entropy, just wait really really long and have a massive energy source to use and try to not use it as much as possible till the quantum fluctuation drops entropy down randomly.</p>

CC BY-SA 4.0
2024-03-16T02:28:37.557Z
I’m voting to close this question because it's not about astronomy.
PM 2Ring
CC BY-SA 4.0
2024-03-16T03:04:22.990Z
At the time spans you are talking about, all your energy will be long gone to thermal radiation.  The only things that are really "well-insulated" at those time scales are supermassive black holes, and that is because Hawking radiation is so very slow.
antlersoft
CC BY-SA 4.0
2024-03-16T14:29:54.340Z
Isn't sulfur hexafluoride a really good greenhouse gas if there are 10 Earth Masses of it, because even with CO2, Venus will remain warm for a long time, but sulfur hexafluoride is 22000 times better at trapping heat than CO2 and that is just for 1 atm, and not 10 Earth masses worth of the material?
MiltonTheMeme
CC BY-SA 4.0
2024-03-17T19:07:53.463Z
@MiltonTheMeme "a long time" is a relative statement. If you took away the Sun, Venus would remain hot for "a long time" on human scales, but not on cosmic scales. I do not know how long it would take to cool down, but certainly less than billions of years, but let's be extra generous and call it 10B years. Replacing its atmosphere with a stronger GHG could extend that to, at best, trillions of years. Let's just call it a quadrillion years. That is an inconsequential fraction of the time span that you invoke when talking about the era where the potential for a new big bang is conceivable.
asgallant
CC BY-SA 4.0
2024-03-18T16:39:58.730Z
OK, what happens to this system? This can actually be answered with some confidence and demonstrates known astrophysics. Working on a proper answer, arguing against closing.
Anders Sandberg
CC BY-SA 4.0
2024-03-18T22:17:38.397Z
Apart from the impossibility of forming such an object, convection would quickly set in and cool the object at the nominal Kelvin-Helmholtz time. Greenhouse effects only work in the radiative part of a gaseous object.
AtmosphericPrisonEscape
CC BY-SA 4.0
2024-03-18T22:43:31.800Z
Even with convection all the heat only disperses within the atmosphere and the heat still has to escape the planet so the Greenhouse effect prevents it from escaping past the radiative part so the heat will still be trapped in the atmosphere and a lot of the heat will be near the surface.
MiltonTheMeme
<thermodynamics><entropy>
Would having a 20 Earth mass planet with lots of greenhouse gasses be the way to survive till the end of the universe for lifeforms?
MiltonTheMeme
# Would having a 20 Earth mass planet with lots of greenhouse gasses be the way to survive till the end of the universe for lifeforms?

(Asked by: username_0 on 2024-03-15)

So what if a 20 Earth mass planet is used, where 10 Earth masses are pure Sulfur Hexafluoride extending to 5 Earth Radii, and the other 10 is iron and nickel which is fully molten with radii of one Earth Radii and also this planet has a 100 moons of 1 moon mass orbiting for tidal heating purposes. So would putting this object at Venus's orbit for 100 million years and then letting the planet soak up all the heat and energy be a way to slow down entropy so much to the point that very small amounts of energy leak away.

Assume that the object has a strong magnetic field so radiation can't do anything and assume that proton decay isn't involved. Would this be a suitable way of slowing entropy down significantly long enough? Also assume that the lifeforms are hibernating till 1 googol year, and then use the energy after using the thermal difference between interior and space. Is this long enough for them to survive till the next big bang where the quantum fluctuation can randomly drop entropy to almost zero but the probability is very low? These lifeforms could in theory also hibernate for 10^10^23 years and also assume they are in orbit around the planet, which is in intergalactic space with no other objects for many millions of light years after the star's death, where it gets moved by lifeforms.

So is this the way to get around entropy, just wait really really long and have a massive energy source to use and try to not use it as much as possible till the quantum fluctuation drops entropy down randomly.

**Tags:** thermodynamics, entropy

> **Comments:**
> **username_1**: I’m voting to close this question because it's not about astronomy.  
> **username_2**: At the time spans you are talking about, all your energy will be long gone to thermal radiation.  The only things that are really "well-insulated" at those time scales are supermassive black holes, and that is because Hawking radiation is so very slow.  
> **username_0**: Isn't sulfur hexafluoride a really good greenhouse gas if there are 10 Earth Masses of it, because even with CO2, Venus will remain warm for a long time, but sulfur hexafluoride is 22000 times better at trapping heat than CO2 and that is just for 1 atm, and not 10 Earth masses worth of the material?  
> **username_3**: @username_0 "a long time" is a relative statement. If you took away the Sun, Venus would remain hot for "a long time" on human scales, but not on cosmic scales. I do not know how long it would take to cool down, but certainly less than billions of years, but let's be extra generous and call it 10B years. Replacing its atmosphere with a stronger GHG could extend that to, at best, trillions of years. Let's just call it a quadrillion years. That is an inconsequential fraction of the time span that you invoke when talking about the era where the potential for a new big bang is conceivable.  
> **username_4**: OK, what happens to this system? This can actually be answered with some confidence and demonstrates known astrophysics. Working on a proper answer, arguing against closing.  
> **username_5**: Apart from the impossibility of forming such an object, convection would quickly set in and cool the object at the nominal Kelvin-Helmholtz time. Greenhouse effects only work in the radiative part of a gaseous object.  
> **username_0**: Even with convection all the heat only disperses within the atmosphere and the heat still has to escape the planet so the Greenhouse effect prevents it from escaping past the radiative part so the heat will still be trapped in the atmosphere and a lot of the heat will be near the surface.  

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## 1 answers

### Answer by username_4 on 2024-03-18. Score: 1

OK, what happens to this system? This can be answered with some confidence.

First, the 100 moons will start colliding and escaping. To keep big moons from tidally interacting very strongly they need to be fairly far apart (and not have the wrong resonances), but if you place them reasonably far apart the tidal effect on the planet will be minor. This gets even worse if you put your planet close to the star, since now it has a limited Hill sphere. So the likely outcome is a bunch of impacts, escapes and maybe a few remaining big moons.

(The Hill sphere radius is $\approx a\sqrt[3]{m\_2/3(m\_1+m\_2)}$, so if you could pack moons so they all stay just outside each other's spheres they would be at $a\_{n+1}=a\_n \sqrt[3]{m\_2/3(m\_1+m\_2)}$, basically a geometric series out to the planetary Hill sphere $a\_{planet}\sqrt[3]{m\_1/3(m\_1+m\_{star})}$.)

Then the sulphur hexaflouride will start to photodegrade into other compounds. It is pretty inert, but UV can splice off fluorines, producing long-chain sulphur compounds that will likely start condensing out (and fluorine start to escape). A messy iron sulphide-fluoride crust will likely be the result.

The planet will of course still be really hot. But it will basically act as a blackbody (with some greenhouse effect), losing heat as soon as its star runs out.

The Kelvin-Helmholtz timescale for a 20 Earth mass rather dense planet (the radius is not going to be many earth radii due to compression of the iron core) is long, but still not that astronomical. Radioactive decay has timescales of mere billions of years for standard long-lived isotopes; you can of course assume some more unusual very long lived ones to stretch things, but after a few hundred billion years you will be close to background temperatures.

Tidal heating and even some infall of matter is going to keep the planet warmer than the background level for even longer, but we are talking a few Kelvin.

The key problem here is that having heat is not staving off entropy. This is a system that is leaking away its free energy without control. It would be better to encase it in mirrors. That way one can use the Dysonian hibernate-and-wait method.

> **Comments:**
> **username_0**: What about adding the mirrors and then having the greenhouse effect on top of that?Would that increase the lifespan of the system energy?  
> **username_4**: @username_0 - The greenhouse effect slows things down, but not by many orders of magnitude. Mirrors can be surprisingly good (99.9999% reflectance) and if you layer foil layers and tune layer distances to avoid near-field effects (basically, they need to be further apart than the longest significant wavelength of the heat radiation) it can get even better. This way you get many orders of magnitude better energy storage.  

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