The habitable zone
The habitable zone, sometimes called the “Goldilocks zone”, is the range of distances from a star where a rocky planet with an Earth-like atmosphere could keep liquid water on its surface. Too close, water boils away in a runaway greenhouse; too far, it freezes.
How it is estimated
The boundaries scale with the square root of the star’s luminosity L (in units of the Sun’s):
- inner edge ≈ √(L ÷ 1.1) AU;
- outer edge ≈ √(L ÷ 0.53) AU.
These are conservative limits, close to those of Kasting and Kopparapu’s climate models. More optimistic limits, based on early Venus and early Mars, stretch the zone to roughly 0.75 to 1.8 AU for the Sun.
If you know the star’s radius R and temperature T instead, its luminosity is L = R² × (T ÷ 5,772 K)⁴.
Example: the Sun
For L = 1:
- inner edge ≈ 0.95 AU;
- outer edge ≈ 1.37 AU.
Earth, at 1 AU, sits inside. Venus (0.72 AU) is too close and Mars (1.52 AU) just outside the conservative zone.
Equilibrium temperature
A planet’s equilibrium temperature, the temperature it would have without a greenhouse effect, is:
T_eq ≈ 278.6 K × L^(1/4) ÷ √d × (1 − albedo)^(1/4)
For Earth, with an albedo of 0.3, T_eq ≈ 255 K, about −18 °C (−1 °F). The actual average surface temperature, about 15 °C (59 °F), is higher thanks to the greenhouse effect.
Other stars
| Star type | Luminosity | Habitable zone |
|---|---|---|
| Red dwarf (M) such as Proxima Centauri | 0.0016 | about 0.04 to 0.05 AU |
| Orange dwarf (K) | 0.3 | about 0.5 to 0.75 AU |
| Sun (G2) | 1 | about 0.95 to 1.37 AU |
| Hotter F star | 3 | about 1.6 to 2.4 AU |
Planets in the habitable zones of red dwarfs orbit so close that they are likely tidally locked and exposed to strong flares, which may make them less hospitable than the simple distance suggests.
Beyond the habitable zone
Habitable zone status is only a first filter. A planet’s mass, atmosphere, magnetic field, water content and the star’s activity all matter. Ocean worlds under ice, like Jupiter’s moon Europa, show that liquid water can also exist far outside the classic zone.