Earth's inner core is solid even though it is hotter than the surface of the Sun.

The Earth's inner core is a dense sphere primarily composed of an iron-nickel alloy with a radius of approximately 1,220 kilometers. While the surface of the Sun burns at about 5,500°C, the inner core is estimated to be between 5,200°C and 6,000°C. Under normal atmospheric conditions, iron melts at 1,538°C, but the conditions at the center of the planet are far from normal.Seismological data collected by researchers like Inge Lehmann in 1936 first suggested the existence of a solid inner core within the liquid outer core. The pressure at this depth is calculated to be roughly 330 to 360 gigapascals, which is over 3 million times the pressure at Earth's surface. This immense gravitational weight from the rest of the planet forces the iron atoms into a solid crystalline lattice despite the vibrating energy of the heat.This phenomenon is explained by the Clausius-Clapeyron relation, which shows that the melting point of a substance increases as pressure increases. In 2013, scientists at the French research agency CEA used X-ray diffraction to determine that the melting point of iron at these pressures is approximately 6,230°C. Because the actual temperature of the core is slightly below this pressure-induced melting point, the inner core remains a solid mass.The inner core is actually growing at a rate of about 1 millimeter per year as the Earth slowly cools. As the liquid iron in the outer core loses heat, it freezes onto the surface of the inner core. This process releases latent heat, which helps drive the convection currents in the liquid outer core that generate Earth's magnetic field.