What is a habitable zone?
The habitable zone is the range of distances from a star where a rocky planet could have liquid water on its surface. Too close, and a runaway greenhouse boils the oceans away. Too far, and even a thick carbon dioxide atmosphere cannot stop them from freezing.
Being inside the zone is not a guarantee of habitability — the planet also needs enough mass to hold an atmosphere. See Step 2: Choosing the orbit.
How this calculator works
The amount of starlight a planet receives falls off with the square of its distance. So the edges of the habitable zone can be found from the star's luminosity L and the amount of starlight (relative to what Earth receives) that marks each edge:
- Distance (AU) = √(L ÷ S), where S is the starlight at the edge relative to Earth's.
- Conservative zone: inner edge S ≈ 1.1 (runaway greenhouse), outer edge S ≈ 0.35 (maximum greenhouse). For the Sun this gives about 0.95–1.7 AU.
- Optimistic zone: inner edge S ≈ 1.78, based on the idea that Venus may have had surface water until about a billion years ago; outer edge S ≈ 0.32, based on evidence that early Mars had liquid water.
- Orbital period uses Kepler's third law: P (years) = √(a³ ÷ M), with a in AU and M in solar masses.
This is a simplified model. Research models also adjust the edges for the star's temperature — cooler stars give off more infrared light, which shifts their zones slightly outward — and for the planet's mass. Treat the numbers as a good first estimate.
Example stars
- Proxima Centauri is so dim that its habitable zone lies roughly 15 to 25 times closer to it than Earth is to the Sun. Its planet Proxima b orbits there with an 11-day year.
- TRAPPIST-1 is even dimmer, and several of its seven Earth-sized planets fall inside its tiny habitable zone. Read more about planets around red dwarfs.
- Sirius A is about 25 times brighter than the Sun, pushing its habitable zone out to roughly where Jupiter and Saturn orbit in our solar system.
Learn how star types differ in star types explained.