Exoplanet Habitability Calculator
Estimate an exoplanet's effective surface temperature and habitability based on its host star and orbital parameters. Use this when evaluating whether a discovered or hypothetical planet could support liquid water.
Last updated: September 2026
Formula below · 2 sources (nasa.gov, Wikipedia) · Updated Sep 2026
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About this calculator
The calculator estimates a planet's temperature from the stellar flux it receives. The equilibrium temperature of a planet with Earth's albedo (0.3) is T_eq = 255 K × L^(1/4) / √d, where L is the star's luminosity in solar units and d is the orbital distance in AU; this follows from balancing absorbed starlight (∝ L/d²) with thermal emission (∝ T⁴). The star's temperature does not enter once luminosity is known (it is shown for reference only). The result is then multiplied by a rough atmosphere factor f_atm (1.5 thick, 1.0 Earth-like or none, 0.7 thin) to mimic greenhouse warming or its absence; this factor is a crude illustration, not a climate model (Earth's real greenhouse lifts 255 K to 288 K, Venus's lifts about 230 K to 737 K). A result between about 273 K and 373 K suggests liquid water could exist.
How to use
Suppose a planet orbits a star with luminosity 0.5 L☉ at 0.6 AU and has a thick atmosphere. Step 1 — L^(1/4) = 0.5^0.25 ≈ 0.841. Step 2 — √0.6 ≈ 0.775. Step 3 — T_eq = 255 × 0.841 / 0.775 ≈ 277 K. Step 4 — atmosphere factor 1.5: ≈ 415 K (about 142 °C), above the boiling point, so the planet is likely too hot with a thick atmosphere; with an Earth-like atmosphere (factor 1.0) it would sit at about 277 K, just above freezing.
Frequently asked questions
What does the habitable zone mean for an exoplanet?
The habitable zone (HZ) is the range of orbital distances around a star where a rocky planet could maintain liquid water on its surface under suitable atmospheric pressure. It is not a guarantee of life — it simply identifies where the stellar energy flux is neither too intense nor too weak. The inner edge is set by runaway greenhouse warming and the outer edge by carbon-dioxide condensation. Earth sits comfortably within the Sun's HZ at 1 AU.
How does atmosphere type affect an exoplanet's surface temperature?
Atmospheric composition and thickness govern how much stellar radiation is retained through the greenhouse effect. A thick atmosphere traps more infrared radiation, raising surface temperatures well above the bare-rock equilibrium — Venus is a dramatic example. A thin atmosphere like Mars's allows most heat to escape, keeping surfaces cold. This calculator models that effect with a crude multiplier on the equilibrium temperature: 1.5× for thick, 1.0× for Earth-like, and 0.7× for thin atmospheres.
Why is stellar luminosity more important than stellar temperature for habitability?
Luminosity determines the total energy output of a star and therefore the flux received by an orbiting planet at any given distance. Temperature affects the spectrum of that radiation but its direct role in surface heating is secondary once luminosity and distance are known. A star twice as luminous requires a planet to orbit roughly 1.41× farther away to receive the same energy flux as Earth does from the Sun, and the equilibrium temperature rises only as L^(1/4). This is why low-luminosity red dwarfs have habitable zones very close in, raising additional concerns about tidal locking and stellar flares.