Exoplanet Habitable Zone Calculator
Estimate the inner and outer boundaries of a star's habitable zone based on its luminosity, spectral type, and a planet's mass. Use it when assessing whether a known exoplanet could support liquid water on its surface.
Last updated: September 2026
Formula below · 2 sources (nasa.gov, Wikipedia) · Updated Sep 2026
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About this calculator
The habitable zone (HZ) is the range of orbital distances where liquid water could persist on a rocky planet's surface. This calculator uses the Kopparapu et al. (2014) model: each edge sits where the stellar flux equals an effective flux S_eff that depends on the star's temperature, and the distance is d = √(L / S_eff) AU, with L in solar units. The inner (runaway-greenhouse) edge also depends on planet mass — heavier planets can sit slightly closer in — and is interpolated between the published 0.1, 1 and 5 Earth-mass fits (masses outside that range are clamped). The outer (maximum-greenhouse) edge does not depend on mass. Spectral type sets the star's temperature (F 6,500 K, G 5,780 K, K 4,500 K, M 3,300 K); cooler stars emit more infrared, which planets absorb more efficiently, so their HZ sits at lower flux — slightly farther out for the same luminosity. The result is the midpoint of the inner and outer edges. For the Sun and an Earth-mass planet the conservative HZ runs from about 0.95 to 1.68 AU, centred near 1.31 AU.
How to use
Suppose you are assessing a K-type star with a luminosity of 0.4 solar luminosities hosting a 2 Earth-mass planet. Step 1 — for a 4,500 K star the runaway-greenhouse flux for 2 Earth masses is about S_in ≈ 1.01 and the maximum-greenhouse flux S_out ≈ 0.285. Step 2 — inner edge √(0.4 / 1.01) ≈ 0.63 AU; outer edge √(0.4 / 0.285) ≈ 1.18 AU. Step 3 — the calculator returns their midpoint, about 0.91 AU.
Frequently asked questions
What is the habitable zone of a star and why does it matter for finding life?
The habitable zone is the range of orbital distances where a planet receives enough stellar energy to maintain liquid water on its surface — a prerequisite for life as we know it. Too close, and water evaporates; too far, and it freezes permanently. Identifying a planet within its host star's HZ is one of the key criteria scientists use when evaluating exoplanet candidates for potential habitability. It does not guarantee life, but it narrows the search significantly.
How does stellar spectral type affect the location of the habitable zone?
Cooler M-dwarf stars emit far less energy, so their habitable zones sit very close in — often within 0.1–0.4 AU. G-type stars like our Sun place the HZ around 0.95–1.37 AU. Hotter F-type stars push it out beyond 1.5 AU. This matters practically because M-dwarf planets in the HZ may be tidally locked, facing unique climate challenges. Beyond luminosity, spectral type shifts the edges slightly: cooler stars put out more near-infrared light that water and CO₂ absorb well, so their HZ sits at lower stellar flux (Kopparapu et al. 2014). This calculator uses those temperature-dependent fluxes.
Why does planet mass influence where the habitable zone boundary falls?
More massive rocky planets have stronger surface gravity, which compresses the atmosphere into a thinner, drier column and delays the runaway greenhouse, so their inner HZ edge moves slightly inward (Kopparapu et al. 2014 found the runaway-greenhouse flux rises from about 0.99 to 1.19 solar fluxes between 0.1 and 5 Earth masses for a Sun-like star). The outer edge barely changes. The effect is a few percent in distance, small next to the uncertainty from clouds and atmospheric composition.