FactSentinel

The claim is supported by current scientific estimates. The Sun's visible surface, called the photosphere, has a temperature of about 5,500 degrees Celsius (roughly 5,800 kelvin). The boundary between Earth's liquid outer core and solid inner core is estimated to be around 5,000 to 6,000 degrees Celsius, which puts the two in the same range. Because the core's temperature cannot be measured directly, the comparison is approximate, and researchers disagree by several hundred degrees.

The estimates rely on indirect evidence. Seismic waves from earthquakes reveal that the outer core is liquid and the inner core is solid, and that both are mostly iron and nickel. Scientists then use laboratory experiments, such as squeezing iron samples in diamond anvil cells and heating them with lasers, to find the temperature at which iron melts under the extreme pressures found there, over three million times atmospheric pressure. Because the inner core boundary sits at the melting point of the core alloy, that melting temperature gives a reliable estimate of the temperature there. The heat comes from leftover energy from Earth's formation, the gradual freezing of the inner core, and radioactive decay.

A common misconception is that such heat would keep the inner core liquid. Immense pressure raises the melting point of iron, so the inner core stays solid. Another is that the Sun's surface is its hottest region. The Sun's own core reaches about 15 million degrees Celsius, so the comparison only holds for the Sun's surface.