Exoplanet Habitable Zone Calculator
The habitable zone of a star from its luminosity — the inner and outer edges where a planet could hold liquid water — with a planet's equilibrium temperature, stellar flux and year at any orbit.
The habitable zone is where a planet with an atmosphere could keep liquid water on its surface: close enough not to freeze, far enough not to boil off.
How the exoplanet habitable zone calculator works
The habitable zone is where a planet with an atmosphere could keep liquid water on its surface: close enough not to freeze, far enough not to boil off. The edges scale with the square root of luminosity — a star four times as bright pushes them twice as far out. The limits here are the conservative ones from climate models: the runaway-greenhouse inner edge and the maximum-greenhouse outer edge, which for the Sun give 0.95 to 1.68 AU.
Equilibrium temperature is what the planet would be with no greenhouse effect: 255 K for Earth, which the atmosphere lifts to 288.
Formula: inner = √(L / 1.107) AU; outer = √(L / 0.356) AU; T_eq = [L (1 − A) / (16 π σ a²)]^¼
Worked examples
| Inputs | Equilibrium temperature (K) | Note |
|---|---|---|
| Earth around the Sun | 254.59 | 255 K, in the zone |
| A red dwarf | 231.19 | a zone a few weeks across |
| Mars | 209.81 | inside the conservative edge |
FAQFrequently asked questions
What is the habitable zone?
The range of distances where a planet with a suitable atmosphere could keep liquid water on its surface. It is a necessary condition for life as we know it, not a sufficient one.
Why does it scale with the square root of luminosity?
Because flux falls with distance squared. To receive the same flux from a star four times as bright you must be twice as far away.
What are the conservative limits?
The runaway-greenhouse inner edge and the maximum-greenhouse outer edge from Kopparapu's climate models — 0.95 to 1.68 AU for the Sun. Optimistic limits based on early Venus and early Mars stretch to 0.75 to 1.77.
Why is Earth's equilibrium temperature below freezing?
Because it ignores the greenhouse effect. Earth radiates as a 255 K body from high in the atmosphere; the surface is 33 K warmer because of water vapour and CO₂.
What about red dwarfs?
Their zones are tiny and close — a few hundredths of an AU — where planets are tidally locked and exposed to flares. They are the commonest stars, so the question matters.
Where these figures come from
- NIST — CODATA 2018 fundamental physical constants — G and the speed of light
- IAU 2015 Resolution B3 — nominal solar and planetary conversion constants — the astronomical unit, solar mass and planetary radii
- NASA — the US space agency
Last checked: September 2026. Constants are CODATA 2018 (G, c) and IAU 2015 nominal values (solar and planetary parameters).