Planet Equilibrium Temperature Calculator
The temperature a planet or moon settles at from the light of its star alone, from the star's luminosity, the distance and how much light the planet reflects.
Results
The dashed line marks the value you entered.
What this tool does
The equilibrium temperature balances the starlight a planet absorbs against the heat it radiates back into space, and ignores any atmosphere. For Earth it comes out at about 255 K (−18 °C), roughly 33 °C below the real average of about 288 K — the difference is the natural greenhouse effect, which is why this number is a baseline and not a forecast of the surface. The temperature falls with the square root of the distance, so doubling the distance does not halve it. Bond albedo is the fraction of all incoming light reflected; Earth's is about 0.3.
Formula
T_eq = [ L (1 − A) ÷ (16 π σ d²) ]^¼ · F = L ÷ (4π d²)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
lm | Luminosity of the star (Sun = 1) | — |
d | Distance from the star | AU |
al | Bond albedo (0 absorbs everything, 1 reflects everything) | — |
T | Equilibrium temperature | K |
TC | Equilibrium temperature in °C | °C |
F | Starlight reaching the planet | W/m² |
FE | Starlight compared with what Earth gets | — |
T0 | Temperature if it reflected nothing | K |
Worked example
- Luminosity of the star (Sun = 1)1
- Distance from the star1 AU
- Bond albedo (0 absorbs everything, 1 reflects everything)0.3
- Equilibrium temperature254.6 K
- Equilibrium temperature in °C-18.6 °C
- Starlight reaching the planet1361.2 W/m²
- Starlight compared with what Earth gets1.000
- Temperature if it reflected nothing278.3 K
Limitations
- The formula assumes ideal conditions: no friction losses, no air resistance and no efficiency losses unless you enter them.
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.