question: why is the center of the earth so hot? Is it due to friction? Is every planet this way?
comment: Sort of 2 things. First it was very hot to begin with and hasn't had enough time to cool down. And that rocks decay giving off radiation. When they decay they give off heat. Every rocky planet in our solar system has a little bit of a hot core. Some planets cores are hotter and bigger than others.
passages: Section::::Planets.
  
  Section::::Planets.:Terrestrial planets.
  
  The internal heating within terrestrial planets powers tectonic and volcanic activities. Of the terrestrial planets in the Solar System, Earth has the most internal heating because it is the most massive. Mercury and Mars have no ongoing visible surface effects of internal heating because they are only 5 and 11% the mass of Earth respectively; they are nearly "geologically dead" (however, see Mercury's magnetic field and Geological history of Mars). Earth, being more massive, has a great enough ratio of mass to surface area for its internal heating to drive plate tectonics and volcanism.
  
  Section::::Planets.:Gas giants.
   Earth's internal heat comes from a combination of residual heat from planetary accretion (about 20%) and heat produced through radioactive decay (80%). The major heat-producing isotopes within Earth are potassium-40, uranium-238, and thorium-232. At the center, the temperature may be up to , and the pressure could reach . Because much of the heat is provided by radioactive decay, scientists postulate that early in Earth's history, before isotopes with short half-lives were depleted, Earth's heat production was much higher. At approximately , twice the present-day heat would have been produced, increasing the rates of mantle convection and plate tectonics, and allowing the production of uncommon igneous rocks such as komatiites that are rarely formed today.
   The Earth's internal heat comes from a combination of residual heat from planetary accretion, heat produced through radioactive decay, latent heat from core crystallization, and possibly heat from other sources. The major heat-producing isotopes in the Earth are potassium-40, uranium-238, uranium-235, and thorium-232. At the center of the planet, the temperature may be up to 7,000 K and the pressure could reach 360 GPa (3.6 million atm). Because much of the heat is provided by radioactive decay, scientists believe that early in Earth history, before isotopes with short half-lives had been depleted, Earth's heat production would have been much higher. Heat production was twice that of present-day at approximately 3 billion years ago, resulting in larger temperature gradients within the Earth, larger rates of mantle convection and plate tectonics, allowing the production of igneous rocks such as komatiites that are no longer formed.
   Neutrino flux measurements from the Earth's core (see kamLAND) show the source of about two-thirds of the heat in the inner core is the radioactive decay of K, uranium and thorium. This has allowed plate tectonics on Earth to continue far longer than it would have if it were simply driven by heat left over from Earth's formation; or with heat produced from gravitational potential energy, as a result of physical rearrangement of denser portions of the Earth's interior toward the center of the planet (i.e., a type of prolonged falling and settling).
  
  Section::::See also.
   Section::::Dynamics.:Core Heat Source.
   In "Tales to Astonish" #2 (1959) "I Fell to the Center of the Earth", an archaeologist named Dr. Burke who is on an expedition to Asia travels to the center of the Earth (and also, as he later finds out, backwards in time)--and encounters neanderthals and dinosaurs.
  
  In the "Doctor Who" episode, "The Runaway Bride", a Racnoss warship is found at the center of the planet.
  
  Section::::See also.
  
  BULLET::::- Structure of the Earth, layered structures
  
  BULLET::::- Planetary core, the innermost layer(s) of a planet
  
  BULLET::::- List of science fiction themes
   Section::::Physical characteristics.:Heat.
   Section::::Global internal heat flow.
  
  Estimates of the total heat flow from Earth's interior to surface span a range of 43 to 49 terawatts (TW) (a terawatt is 10 watts). One recent estimate is 47 TW, equivalent to an average heat flux of 91.6 mW/m, and is based on more than 38,000 measurements. The respective mean heat flows of continental and oceanic crust are 70.9 and 105.4 mW/m.
   Section::::Physical differentiation.:Thermal diffusion.
  
  When material is unevenly heated, lighter material migrates toward hotter zones and heavier material migrates towards colder areas, which is known as thermophoresis, thermomigration, or the Soret effect. This process can affect differentiation in magma chambers.
  
  Section::::Physical differentiation.:Differentiation through collision.
   A planetary core acts as a heat source for the outer layers of a planet. In the Earth, the heat flux over the core mantle boundary is 12 terawatts. This value is calculated from a variety of factors: secular cooling, differentiation of light elements, Coriolis forces, radioactive decay, and latent heat of crystallization. All planetary bodies have a primordial heat value, or the amount of energy from accretion. Cooling from this initial temperature is called secular cooling, and in the Earth the secular cooling of the core transfers heat into an insulating silicate mantle. As the inner core grows, the latent heat of crystallization adds to the heat flux into the mantle.
   On Earth, a significant portion of the heat output from the interior of the planet, estimated at a third to half of the total, is caused by the slow collapse of planetary materials to a smaller size, generating heat.
  
  Section::::History of energy transformation.:Release of energy from radioactive potential.
   The temperature of the inner core can be estimated from the melting temperature of impure iron at the pressure which iron is under at the boundary of the inner core (about 330 GPa). From these considerations, in 2002 D. Alfè and other estimated its temperature as between and . However, in 2013 S. Anzellini and others obtained experimentally a substantially higher temperature for the melting point of iron, 6230 ± 500 K.
  
  Iron can be solid at such high temperatures only because its melting temperature increases dramatically at pressures of that magnitude (see the Clausius–Clapeyron relation).
  
  Section::::Physical properties.:Magnetic field.
   Section::::Dynamics.:Trends in the Solar System.:Outer Gas and Ice Giants.
   Earth's internal heat is thermal energy generated from radioactive decay and continual heat loss from Earth's formation. Temperatures at the core–mantle boundary may reach over 4000 °C (7,200 °F). The high temperature and pressure in Earth's interior cause some rock to melt and solid mantle to behave plastically, resulting in portions of the mantle convecting upward since it is lighter than the surrounding rock. Rock and water is heated in the crust, sometimes up to 370 °C (700 °F).
   The mantle remained hotter than modern day temperatures throughout the Archean. Over time the Earth began to cool as planetary accretion slowed and heat stored within the magma ocean was lost to space through radiation.
   Section::::New Core Paradox.
   The air pressure at Venus' surface is about 92 times that of the Earth. The enormous amount of CO in the atmosphere creates a strong greenhouse effect, raising the surface temperature to around 470 °C, hotter than that of any other planet in the Solar System.
  
  Section::::Planets.:Inner planets.:Mars.
   Small bodies such as comets, some asteroid types, and dust grains, on the other hand, serve as counterexamples. Assumed to have experienced little or no heating, these materials may contain (or be) samples representing the early Solar System, which have since been erased from Earth or any other large body.
  
  Some extrasolar planets are covered entirely in lava oceans, and some are tidally locked planets, whose star-facing hemisphere is entirely lava.
  
  Section::::Geology and geochemistry.:Cratering.
   Earth's internal heat budget
  
  Earth's internal heat budget is fundamental to the thermal history of the Earth. The flow of heat from Earth's interior to the surface is estimated at formula_1 terawatts (TW) and comes from two main sources in roughly equal amounts: the "radiogenic heat" produced by the radioactive decay of isotopes in the mantle and crust, and the "primordial heat" left over from the formation of the Earth.
   In slightly better Latin, with errors amended:
  
  which, when translated into English, reads:
   Earth's internal heat is thermal energy generated from radioactive decay and continual heat loss from Earth's formation. Temperatures at the core-mantle boundary may reach over 4000 °C (7,200 °F). The high temperature and pressure in Earth's interior cause some rock to melt and solid mantle to behave plastically, resulting in portions of mantle convecting upward since it is lighter than the surrounding rock. Rock and water is heated in the crust, sometimes up to 370 °C (700 °F).
   Section::::Observed types.
  
  The following summarizes known information about the planetary cores of given non-stellar bodies.
  
  Section::::Observed types.:Within the Solar System.
  
  Section::::Observed types.:Within the Solar System.:Mercury.
   Section::::Caveats.:Internal energy fluxes.
   One of the ways to estimate the age of the inner core is by modeling the cooling of the Earth, constrained by a minimum value for the heat flux at the core–mantle boundary (CMB). That estimate is based on the prevailing theory that the Earth's magnetic field is primarily triggered by convection currents in the liquid part of the core, and the fact that a minimum heat flux is required to sustain those currents. The heat flux at the CMB at present time can be reliably estimated because it is related to the measured heat flux at Earth's surface and to the measured rate of mantle convection.
   Section::::Heat sources.
  
  Temperature within the Earth increases with depth. Highly viscous or partially molten rock at temperatures between are found at the margins of tectonic plates, increasing the geothermal gradient in the vicinity, but only the outer core is postulated to exist in a molten or fluid state, and the temperature at the Earth's inner core/outer core boundary, around deep, is estimated to be 5650 ± 600 Kelvin. The heat content of the Earth is 10 joules.
  