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I would like to know, what is the average temperature of Earth as a whole planet, not just its surface temperature. Hope the following scenario will make it more clear what I mean:

Imagine, that we isolated/sealed Earth (up to 30 km above sea level) as it is right now, so that there is no mass or energy being added or escaping and waited several millions of years till the temperature is everywhere the same.

What would that temperature be? When I tried to google it, I only could find references to surface temperature, and not to this.

EDIT: Based on remarks in comment section:

I am not an expert, that is why asked this question, however IMHO of a layman in this area, I presumed that radioactive decay wont play that much of a role (like a +-1%). And the role of a gravity is well beyond me. But the most important thing is, I pretty much wanted to know the average temperature by heat capacity of the Earth at this moment (so basically, all decays and all tidal forces excluded, even if they play significant role).

I would just like to have (a fairly confident) idea about the potential temperature, and no need for highly exact numbers and calculations, +-15% is absolutely fine.

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    $\begingroup$ Rather than a wait of a mere several millions of years, one would have to wait for tens of billions of years, minimum, for a completely isolated Earth to come to thermal equilibrium. While your definition is unambiguous, the answer has zero utility, and this is why you haven't find any references. $\endgroup$ – David Hammen Aug 6 '18 at 14:22
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    $\begingroup$ This is an Earth Science question. It's probably around 2000 degrees, but the "sealing" idea doesn't work, because the interior is powered by nuclear decay. $\endgroup$ – James K Aug 6 '18 at 16:23
  • $\begingroup$ @DavidHammen "the answer has zero utility, and this is why you haven't find any references" -- umm, been on the Internet long? :) $\endgroup$ – barrycarter Aug 6 '18 at 17:29
  • $\begingroup$ @JamesK Eventually it will all decay, and decay again and again until we reach whatever the unknown final state of matter is (hydrogen? quarks? energy?) $\endgroup$ – barrycarter Aug 6 '18 at 17:30
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    $\begingroup$ @barrycarter - I was referring to scientific references rather than the nonscientific woo one finds all over the internet. BTW, the radioactive uranium and thorium eventually decay to lead and helium, while the radioactive potassium decays to calcium or argon. If protons aren't stable, things may decay further, but that's trillions and trillions of years from now. $\endgroup$ – David Hammen Aug 6 '18 at 18:00
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What is Earth's average temperature as a whole?

Sealing the planet or using $E = mc^2$ isn't a practical means to measure the temperature. Here's how the average is usually determined:

Source: Tufts University - Global Warming - Heating by the greenhouse effect

"... the average surface temperature of the planet is 288 degrees kelvin (15 degrees Celsius or 59 degrees Fahrenheit).".

Source: Space.com - What Is Earth's Average Temperature?

"According to GISS, the global mean surface air temperature for that period was estimated to be 57 F (14 C). That would put the planet's average surface temperature in 2017 at 58.62 F (14.9 C).".

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    $\begingroup$ This doesn't really answer the question: the OP explicitly asked about the Earth's entire temperature, not just the surface. $\endgroup$ – barrycarter Aug 6 '18 at 17:28
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    $\begingroup$ Thanks Barry. I did address that point and provide the calculation to derive the total energy. The Windows 2TheUniverse sites says that the core is hotter than the surface of the Sun. Not sure how the information is to be practically used, and someone else has voted to close (after yet another person commented on the practicality of the question). I'll check back for your answer. $\endgroup$ – Rob Aug 6 '18 at 17:35
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Another way of saying this is that you're looking for the average temperature, weighted by heat capacity; you're looking for the total heat of the earth divided by the total heat capacity.

Looking at it weighted by mass, I found this site that gives the mass of the mantle as 67.3% and outer core as 30.8%. The temperature of the mantle is given as ranging from 1000 to 3700 C, while the outer core is given as ranging from 3700 to 4300. This gives a range of 1900 to 3900 C. The calculation would be a bit different if done in terms of heat capacity rather than mass. Almost half of the mantle is oxygen, but it wouldn't be in gaseous form, so it would be difficult to find what the appropriate heat capacity is.

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