section_path: Internal structure/Internal heat
section_name: Internal heat
paragraph_text: Internal heat
  
  Uranus's internal heat appears markedly lower than that of the other giant planets; in astronomical terms, it has a low thermal flux. Why Uranus's internal temperature is so low is still not understood. Neptune, which is Uranus's near twin in size and composition, radiates 2.61 times as much energy into space as it receives from the Sun, but Uranus radiates hardly any excess heat at all. The total power radiated by Uranus in the far infrared (i.e. heat) part of the spectrum is 1.06 ± 0.08 times the solar energy absorbed in its atmosphere. Uranus's heat flux is only 0.042 ± 0.047 W / m, which is lower than the internal heat flux of Earth of about 0.075 W / m. The lowest temperature recorded in Uranus's tropopause is 49 K (−224.2 °C; −371.5 °F), making Uranus the coldest planet in the Solar System.
  
  One of the hypotheses for this discrepancy suggests the Earth-sized impactor theorised to be behind Uranus' axial tilt left the planet with a depleted core temperature, as the impact caused Uranus to expel most of its primordial heat. Another hypothesis is that some form of barrier exists in Uranus's upper layers that prevents the core's heat from reaching the surface. For example, convection may take place in a set of compositionally different layers, which may inhibit upward heat transport; perhaps double diffusive convection is a limiting factor.
  
  In a 2021 study, the ice giants' interior conditions were mimicked by compressing water that contained minerals such as olivine and ferropericlase, thus showing that large amounts of magnesium could be dissolved in the liquid interiors of Uranus and Neptune. If Uranus has more of this magnesium than Neptune, it could form a thermal insulation layer, thus potentially explaining the planet's low temperature.