How cold is space? Why does the planet not freeze? originally appeared on Quora: the place to gain and share knowledge, empowering people to learn from others and better understand the world.
Empty space has no temperature.
However, if you were to leave an object somewhere in deep space, far from any stars, planets or other bodies, it will eventually come into thermal equilibrium with the cosmic microwave background (CMB), which is thermal radiation with a temperature of 2.7ºK. (That’s -454.81ºF or –270.45ºC.)
Stars continuously produce heat, which they radiate into empty space. They also receive a little radiation from the CMB. However, the power of thermal radiation is proportional to the fourth (!) power of temperature. So a star like our Sun, with a surface temperature of nearly 6,000º K, will emit trillions more times the heat that it receives from the CMB.
As for planets, they receive a lot of heat from the Sun. They also emit heat into deep space. And given that a planet like the Earth has an average surface temperature of nearly 300º K, it, too emits millions of times more heat than it receives from the CMB. No problem; it receives plenty of heat from the Sun. Ultimately, a planet like the Earth receives the same amount of heat, on average, from the Sun that it emits into deep space, so its temperature remains approximately constant.
In fact, if space were not this cold, we’d all be in deep trouble. The Earth could not shed the tremendous amount of heat it receives from the Sun. Eventually, its oceans would boil. So we really need the cold of deep space as a sink for our waste heat. As does the Sun… if it could not radiate its heat into deep space, it would blow up soon as its temperature would continue to increase due to the ongoing nuclear fusion in its interior.
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