Exoplanet Equilibrium Temperature Calculator
The equilibrium temperature is the baseline temperature a planet reaches when the starlight it absorbs balances the heat it radiates away as a blackbody. It depends on the host star's temperature and size, the orbital distance, and how reflective the planet is (its bond albedo). It deliberately ignores atmospheres, so a real surface can be warmer through the greenhouse effect, but it is the standard first estimate astronomers quote for newly discovered exoplanets. This calculator applies the radiative balance formula and returns the equilibrium temperature in kelvin.
Equilibrium temperature formula
T_eq = T_star * sqrt(R_star / (2 * a)) * (1 - A)^(1/4)
degrees C = T_eq - 273.15
This comes from setting absorbed stellar power (over the planet's cross-section, reduced by 1 minus albedo) equal to emitted blackbody power (over its full surface). The star's radius and orbital distance appear only as a ratio, so any consistent length unit works.
Equilibrium temperature facts
- Earth's equilibrium temperature is about 255 K, well below its 288 K mean surface temperature.
- The difference of about 33 K for Earth is due to the atmospheric greenhouse effect.
- Higher bond albedo lowers the equilibrium temperature by reflecting more starlight.
- The defaults model a Sun-like star at one astronomical unit using solar radius in AU.
- The formula assumes efficient heat redistribution across the whole planet surface.
Equilibrium temperature: frequently asked questions
What is a planet's equilibrium temperature?
The equilibrium temperature is the temperature a planet would have if it absorbed stellar radiation and re-radiated it as a blackbody, with no atmosphere or internal heat. It is a useful baseline for comparing planets, though real surface temperatures can differ greatly due to greenhouse effects and atmospheric circulation.
What is the equilibrium temperature formula?
T_eq = T_star * sqrt(R_star / (2 a)) * (1 - A)^(1/4), where T_star is the star's effective temperature, R_star its radius, a the orbital semi-major axis (in the same length unit as R_star), and A the planet's bond albedo. The factor assumes the planet radiates from its full surface and absorbs over its cross-section.
What is bond albedo?
Bond albedo A is the fraction of all incident stellar energy a planet reflects back to space across all wavelengths and directions. Earth's bond albedo is about 0.31; a perfectly black body has A equal to 0, and a perfect reflector has A equal to 1. Higher albedo means less absorbed energy and a lower equilibrium temperature.
Why does this differ from a planet's real temperature?
Equilibrium temperature ignores atmospheres. Greenhouse gases trap outgoing infrared and raise the surface above the equilibrium value (Earth's surface is about 33 K warmer than its equilibrium temperature). Internal heat, tidal heating, and uneven heat distribution can also shift the real temperature. Treat the result as a radiative baseline.
What units should I use?
Use kelvin for the star's temperature. Use the same length unit for the star's radius and the orbital distance (the formula uses their ratio, so kilometers, meters, or astronomical units all work as long as both share one unit). Bond albedo is a dimensionless fraction between 0 and 1. The output is in kelvin.
Official sources
- NASA: Exoplanet exploration.
- NASA Earth Observatory: Climate and Earth's energy budget.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.