Planet Equilibrium Temperature Calculator

About this calculator

A planet's equilibrium temperature is the theoretical temperature at which the energy it absorbs from its star equals the energy it radiates back to space, ignoring greenhouse effects or internal heat. It's calculated from the star's luminosity (L), the planet's orbital distance (d), and its Bond albedo (A) — the fraction of incoming light it reflects — using T_eq = [L(1 − A) ÷ (16πσd²)]^(1/4), where σ is the Stefan-Boltzmann constant.

Astronomers and astrobiologists use this formula as a first-pass estimate when characterizing newly discovered exoplanets, checking whether a planet falls within a star's "habitable zone" before more detailed climate modeling. It's a simplification — Earth's actual surface temperature, for instance, is warmer than its equilibrium temperature because of the greenhouse effect — but it remains the standard starting point in exoplanet research and astronomy education.