Planetary Equilibrium Temperature Calculator

What Is a Planetary Equilibrium Temperature Calculator?

A planet's equilibrium temperature is a rough estimate of its surface temperature based purely on how much starlight it absorbs and re-radiates as heat, ignoring greenhouse effects. It depends on the star's brightness, the planet's distance, and how much light the planet reflects away (its albedo).

How to Use This Planetary Equilibrium Temperature Calculator

1. Enter the host star's luminosity relative to the Sun (1 for a Sun-like star).
2. Enter the planet's orbital distance in astronomical units.
3. Enter the planet's albedo — the fraction of light it reflects (0.3 is a reasonable Earth-like default).
4. Click Calculate to get the estimated equilibrium temperature.

Formula

• Teq ≈ 278.3 × L^0.25 / √d × (1 − albedo)^0.25 K
• (278.3 K is the approximate equilibrium temperature at 1 AU from a Sun-like star with zero albedo)

Worked Example

A planet like Earth (L=1, d=1 AU, albedo=0.3): Teq = 278.3 × 1 / 1 × (0.7)^0.25 ≈ 255 K (about −18°C) — close to Earth's real airless-blackbody estimate, with the extra warmth of Earth's actual surface coming from its greenhouse effect.