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    Stars & exoplanets

    Habitable Zone Estimator

    Conservative and optimistic habitable-zone limits around a star, plus a planet's equilibrium temperature.

    S_eff = S☉ + aT★ + bT★² + cT★³ + dT★⁴ · d = √(L / S_eff) · T_eq = (L(1−A) / 16πσd²)^¼

    Inputs

    Solar luminosities
    K
    ×

    Results

    Planet location
    Inside the conservative habitable zone
    Conservative HZ (runaway → max greenhouse)
    0.9509 AU – 1.677 AU
    Optimistic HZ (recent Venus → early Mars)
    0.7507 AU – 1.769 AU
    Equilibrium temperature
    254.6 K (-18.56 °C)
    Insolation relative to Earth
    1 S⊕
    • Being inside the habitable zone does not mean a planet is habitable. Atmosphere, mass, magnetic field, stellar activity and water inventory all matter.
    Worked solution
    1. T★ = 5772 − 5780 = -8 K
    2. Runaway greenhouse S_eff → d = √(1 / S_eff) = 0.9509 AU
    3. T_eq = (L(1 − A) / 16πσd²)^¼ = 254.59 K

    How it works

    The habitable zone is the range of distances where a rocky planet with an Earth-like atmosphere could keep liquid water on its surface. The inner edge is set by a runaway greenhouse; the outer edge by the point where CO₂ can no longer warm the planet.

    Variables

    • S_eff — stellar flux at a boundary, relative to Earth's
    • T★ — T_eff − 5780 K
    • A — planet's Bond albedo

    Assumptions and limits

    • Kopparapu et al. (2014) 1-Earth-mass climate limits
    • Equilibrium temperature assumes full heat redistribution and no greenhouse effect (Earth's is 255 K, versus a real mean of about 288 K)

    For education only. These models are simplified and are not suitable for spacecraft navigation or safety-critical engineering. See our scientific accuracy policy.