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The coldest place in the universe that we have observed is in the Boomerang Nebula. Similarly to a refrigerator, heat is carried away through a fluid, resulting in a space colder than the surrounding environment. However, in the case of the Boomerang Nebula, the coolant is heated gas being expelled away from the remains of a star - bringing almost all heat energy with it.

This seems like an efficient way of removing heat / cooling a large object - and perhaps it could be applied to a planet rather than a star (or a fridge). For instance, a planet with a runaway greenhouse effect may channel its heat outward relatively quickly, a planet too small to hold an atmosphere may lose gas quickly, or a planet without a protective magnetic field may be stripped by solar winds.

So about how cold can a planet actually be?

  • Any feasible, naturally occuring method of cooling is acceptable; if it is speculative, it must be justified.
  • The accepted answer will be credible and justify a reasonable surface temperature colder than those provided in other answers.
  • Note that this is so rather than "it could be around x degrees because this cold nebula works in the same way" you must provide calculations or citations.
  • The rest is up to you. I have no specifications for this planet other than that it must be colder than the temperatures the cosmic microwave background usually permits; you can choose size, density, composition, location, nearby objects, moons or lack thereof, etc.
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  • $\begingroup$ Do you mean the core? $\endgroup$
    – user6760
    Nov 28, 2016 at 9:12
  • $\begingroup$ @user6760 See the second bullet - I mean the surface temperature $\endgroup$
    – Zxyrra
    Nov 28, 2016 at 12:27

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2.7 Kelvin is the lowest stable temperature

The lowest physically possible temperature is zero Kelvin, equal to -273.15 degrees Celsius or -459.67 degrees Fahrenheit. The microwave background radiation left over from the big bang is at 2.7 Kelvin, so anything just floating about far from any star will at least be warmed to this temperature. Any further cooling, such as gas expansion, evaporative cooling etc, sounds like it would "run out of fuel" fairly quickly on a planetary and geological scale. You may however consider the Chem SE question Chemical reactions with a room-scale cooling effect for endothermic reactions.

The surface of Pluto can get as cold as 33 Kelvin, so we find the really low temperatures even in our cosmic backyard. 10-20 Kelvin is meanwhile a typical low temperature for interstellar gas and dust within our galaxy.

Story-wise in hard scifi, why would we care about the exact temperature? Well, superconductivity may feature in the plot and it turns out that all elements that can become superconducting on their own, become so below 10 Kelvin. Mercury for instance, goes superconducting around 4 Kelvin. Many elements such as Aluminium, even need to go below the cosmic background temperature. However, today we know of many compounds that are superconducting until well above the temperatures of Pluto. (Pluto is reported to have a max temperature of 55 Kelvin)

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  • $\begingroup$ Could you explain why gas expansion wouldn't work for a planet when it's been proven to work for the B. Nebula? $\endgroup$
    – Zxyrra
    Dec 11, 2016 at 5:29
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    $\begingroup$ "Adiabatic cooling" takes place in the B. Nebula. That is: When gas expands, it's cooling down. And it has to keep expanding to stay cool, or the microwave background radiation will eventually heat it back up. A planet with an expanding atmosphere is soon a atmosphere-free planet. The B. Nebula is estimated to be only ca 1500 years old, so this cooling is not permanent or "long term" even on such scales. $\endgroup$
    – Abulafia
    Dec 15, 2016 at 23:12

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