Maybe you've heard of this one before, maybe you haven't! Either way, let's take a closer look at the facts and see if this holds up at all...
Firstly, if you've not got a decent understanding of the structure of the Earth, then I'd recommend reading this article before continuing with this one. After having gained a fundamental understanding, you can then return to this one without asking yourself a million and one questions along the way!
Otherwise, let's get into it.
I've set the question up so that it's pretty much identical to what you might hear in colloquial conversation, but in reality, it doesn't really make much sense!
I mean, the temperature at the core of the sun is about 15 million degrees Celsius (27 million degrees Fahrenheit), where as temperatures at the surface of the sun are on average around 6,000 degrees Celsius (11,000 degrees Fahrenheit). Therefore, it largely depends on where in/on the sun we are talking about...
Highlighting "on average" here, since the surface of the sun is a very violent, temperate environment where sunspots regularly appear which are regions of much cooler activity.
15 million degrees Celsius is definitely out of the equation, as the temperatures at the core of the Earth simply cannot be the same as that at the sun's core, given the fact that the atoms undergo nuclear fusion at the core of the sun and have 2 trillion trillion million kgs of matter bearing down on them!
Image credit: NASA
6,000 degrees Celsius, on the other hand, is far more realistic.
And, in fact, that does appear to be the case!
The Earth's core is thought to be around 5500 degrees Celsius, which is near enough as hot as the surface of the sun. But how could this be the case?
I gave a slight hint earlier when I referred to the pressure of matter bearing down on the sun's core, as the same can be said for the core of the Earth, just on a much smaller scale.
The relationship between pressure and temperature isn't a simple one, but it can be briefly described as this:
In order to increase the pressure of a substance, some form of work has to be done - the pressure can't randomly change without anything being applied to the substance. It's this work that transfers energy to the atoms that make up the substance, in the form of kinetic energy. Since temperature is essentially a measurement of the total energy of a substance, we can say that the temperature will increase as the kinetic energy of the atoms increase, which itself can be due to a increase in pressure. Hence, they are indirectly connected.
As for what pressure actually is.... Well, that's a bit more complicated and I've discussed it in this article, which you're of course more than welcome to give a read!
The reason why this is important is because we know that the pressure at the core must be astronomically high, as its gravitational pull accelerates the entire weight of the Earth towards it.
In this situation, it is the acceleration due to gravity that is doing the work to the core that is causing temperatures to exceed 5000 degrees Celsius, in the same way as described above - understanding that it's the work done that increases the kinetic energy of the substance at question, which we interpret as a higher temperature, is absolute crucial if you are to go on and understand more complicated applications of this.
So that's why the Earth's core is so hot, but if you've read the other article that I mentioned previously, then you'll know that the Earth's core is in fact a solid.
That's a pretty strange observation, as Iron and Nickel, which together predominantly make up the Earth's core, having melting points far below 5500 degrees.
Again, this has to do with the pressure at the core.
The melting point of Iron is around 1500 degrees Celsius.... At room pressure - that's a very important part to remember.
Just like temperature, pressure plays a role in a substance's state of matter. The higher the temperature, the more energetic the particles are and hence the more likely they are to overcome their attraction to one another, and move around freely. When this happens, we refer to these substances as being either liquid or gas, with gases being more energetic than liquids.
When you heat Iron up to over 1500 degrees Celsius, the particles begin to separate from each other due to their increased kinetic energies, which is what transforms the solid into a liquid. Continue to heat the now molten Iron to temperatures of over 2800 degrees Celsius, and the particles will separate further to form Iron gas! So how on Earth could the Iron that's heated to over 5500 degrees Celsius at the core remain as a solid?!
Well the answer, as I've alluded to, has to do with pressure.
Pressure works in the opposite way to temperature when it comes to manipulating a substances state of matter - an increase in pressure will force atoms closer together, and hence make them more "solid like".
Therefore, even though the temperature is high enough for solid Iron to melt into molten Iron and even boil into Iron gas, the pressure is high enough to stop the iron particles from separating, hence they remain a solid regardless of their kinetic energy.
This is exactly why the freezing points, melting points, and boiling points of substances are given at room pressure which is what we define as 1 atmosphere (roughly equal to 101,000 Pascals).
In summary, the temperature at the core of the Earth is pretty much as hot as the surface of the sun, although despite this temperature being above the melting and even boiling points of the elements that make up Earth's core (Iron and Nickel) at room pressure, the pressure at the core is far from 1 atmosphere, and is responsible for preventing the particles at the core from separating into liquids. Therefore, the core remains a solid. In fact, the pressure at the core is thought to be around 3.6 million atm (atmospheres)!
So there you have it....
Earth's core is as hot at the surface of the sun!
One thing that's worth commenting on now though, is that the more massive a planet is, the higher the temperature at the core (generally speaking), since the stronger gravitational field is allowing more mass to pile on against the core, hence more work is being done, and the particles at the core have more energy, which we measure as temperature.
To name an example, Saturn is 95 times as massive as Earth, and it's core temperature is thought to be around 8500 degrees Celsius! You can read all about Saturn and the other 6 planets here if you're interested :)
Thanks for reading!