Donaldson R F Exoplanet Equilibrium Temperature Calculator
The theoretical blackbody temperature of a planet, from its star and orbit — no atmosphere included.
How this is worked out
A planet's equilibrium temperature balances the stellar energy it absorbs against the energy it radiates away as a blackbody: Teq = Tstar × √(Rstar / 2a) × (1−A)⁴ₛ&sup4;, where a is the orbital distance and A is the Bond albedo — the fraction of incoming light the planet reflects rather than absorbs (0 = perfectly black, absorbs everything; close to 1 = reflects almost everything, like fresh snow or thick cloud). This is a theoretical radiative-balance figure, not a forecast of surface conditions: it ignores the greenhouse effect, internal heat, rotation and atmospheric circulation entirely, so a real surface or cloud-top temperature can differ substantially — Earth's equilibrium temperature works out to about 255 K (−18°C), well below its actual ~288 K (15°C) average surface temperature, with the roughly 33°C gap being the greenhouse effect.
Frequently asked questions
What albedo should I use if I don't know it?
0 is a reasonable, cautious default (treats the planet as absorbing all incoming light, giving the warmest plausible equilibrium estimate); Earth's actual Bond albedo is about 0.306, mostly from clouds.
Why does the formula use the star's radius rather than its luminosity?
They're equivalent by the Stefan-Boltzmann law (luminosity depends on radius and temperature together) — this form just avoids converting back and forth, since stellar radius and temperature are usually what's quoted for an exoplanet host star.
Related tools: Habitable Zone Calculator, Star Luminosity, Radius & Temperature Calculator, Kepler's Third Law Calculator.
Source / Last verified: standard radiative-balance physics (built directly on the Stefan-Boltzmann relation also used by this site's Star Luminosity, Radius & Temperature Calculator); read/checked 1 October 2026 against two commonly-quoted reference figures for Earth: with its actual Bond albedo of 0.306, this tool gives 254 K (matching the commonly-cited ~255 K); with albedo 0 (a perfect absorber), it gives 278 K (matching the commonly-cited ~279 K reference value for that case).