Habitable Zone Calculator
The habitable zone is the annular region around a star where a rocky planet with sufficient atmospheric pressure could maintain liquid water on its surface. The boundaries depend primarily on stellar luminosity: more luminous stars have wider, more distant habitable zones. The standard empirical approximation scales each boundary by the square root of the stellar luminosity in solar luminosities, since the stellar flux at distance d scales as L/d squared. Enter the star's luminosity relative to the Sun to find the inner and outer boundaries of its habitable zone in astronomical units.
Habitable zone formula
d_inner (AU) = sqrt(L / 1.1)
d_outer (AU) = sqrt(L / 0.53)
Where L is stellar luminosity in solar luminosities. The flux limits 1.1 (inner) and 0.53 (outer) are in units of Earth's solar flux constant (1,361 W/m squared). 1 AU = 149,597,870 km. These limits correspond approximately to the runaway greenhouse inner boundary and the maximum greenhouse outer boundary from Kasting et al. (1993) as referenced by NASA's Habitable Exoplanets Catalog.
Habitable zones for notable stars
Sun (1 L_sun): 0.95 to 1.37 AU (Earth at 1 AU is inside). Alpha Centauri A (1.52 L_sun): 1.19 to 1.69 AU. Proxima Centauri (0.00155 L_sun): 0.037 to 0.054 AU. TRAPPIST-1 (0.000553 L_sun): 0.022 to 0.032 AU. Sirius A (25 L_sun): 4.77 to 6.87 AU.
Habitable zone: frequently asked questions
What is the habitable zone?
The habitable zone (also called the Goldilocks zone) is the range of orbital distances around a star where liquid water could exist on a rocky planet's surface, given adequate atmospheric pressure. It is not a guarantee of habitability, but a necessary condition based on stellar energy flux.
How is the habitable zone calculated?
The boundaries scale with the square root of stellar luminosity (in solar luminosities). Inner edge: d = sqrt(L / 1.1) AU; Outer edge: d = sqrt(L / 0.53) AU. These flux limits (1.1 and 0.53 solar constants) correspond to the runaway greenhouse inner edge and the maximum greenhouse outer edge from standard climate models.
Where is Earth in its habitable zone?
Earth orbits at 1 AU, near the inner half of the Sun's habitable zone (approximately 0.95 to 1.67 AU using conservative estimates). Venus at 0.72 AU is inside the inner edge, and Mars at 1.52 AU is near the outer edge. Mars's thin atmosphere means it is too cold despite being within the zone.
What are conservative and optimistic habitable zone estimates?
Conservative estimates use flux limits derived from Earth's own climate: the moist greenhouse at 1.015 solar constants (inner) and the maximum greenhouse at 0.36 (outer), giving 0.99 to 1.67 AU for the Sun. Optimistic estimates extend to the recent Venus distance (0.75 AU) and Mars early wet period (1.77 AU).
Does stellar temperature affect the habitable zone width?
Yes. Cool M-dwarf stars have much closer and potentially tidally locked habitable zones (0.1 to 0.4 AU). Hot F stars have wider, farther zones. The spectral type also affects what flux limits are appropriate, as albedo and atmospheric absorption differ for different stellar spectra.
Official sources
- NASA Exoplanet Archive habitable zone definition: exoplanetarchive.ipac.caltech.edu.
- NASA Exoplanet Exploration (habitable zones): exoplanets.nasa.gov.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.