Space and radiation
Planetary Equilibrium Temperature Calculator
Estimate the temperature of an idealized planet whose absorbed stellar radiation is balanced by emitted thermal radiation. The result is an effective radiative temperature, not a prediction of surface weather or climate.
Estimate equilibrium temperature
Why the factor of four appears
A planet intercepts stellar radiation over a circular cross-section with area πR², while its total spherical surface has area 4πR². Under the uniform-redistribution assumption, the globally averaged incoming flux is therefore one quarter of the incident flux before albedo is applied.
Albedo changes absorbed energy
A Bond albedo of 0 would absorb all incident stellar radiation in this simple model. A higher albedo reflects a larger fraction back to space, leaving less energy to be balanced by outgoing thermal radiation and lowering the calculated equilibrium temperature.
Equilibrium temperature is not surface temperature
An atmosphere can make the surface temperature differ substantially from this effective radiative estimate. NASA research discussing planetary equilibrium temperatures specifically warns that using equilibrium temperature as a habitability indicator has important limitations because albedo varies and atmospheric greenhouse effects can raise surface temperature above the equilibrium value.
Sources and further reading
- NASA GISS: Albedos, equilibrium temperatures, and surface temperatures of habitable planets - context on equilibrium temperature, albedo and greenhouse limitations.
- NIST CODATA fundamental physical constants - reference source for the Stefan-Boltzmann constant.
Related exploration
Calculate a blackbody peak wavelength, read about heat in space, or review thermal radiation.