Donaldson R F Habitable Zone Calculator
The range of orbital distances where a planet could have liquid water on its surface — for any star.
How this is worked out
This tool uses the widely-cited model from Kopparapu et al. (2013), which works out, for a star of a given temperature, how much stellar flux (relative to what Earth receives from the Sun) corresponds to five specific climate limits — from "Recent Venus" (any closer and a planet likely loses its oceans the way Venus is thought to have) out to "Early Mars" (any further and a planet can no longer keep liquid water even with a thick CO₂ atmosphere). Two standard pairs bracket the habitable zone: the conservative zone (Moist Greenhouse inner edge, Maximum Greenhouse outer edge) and the wider optimistic zone (Recent Venus inner edge, Early Mars outer edge) — the same two brackets the paper itself highlights. Each limit's stellar flux is converted to an orbital distance using the star's luminosity: distance (AU) = √(L / Seff).
This is a one-dimensional, Sun-like-atmosphere model: it says nothing about a specific planet's actual atmosphere, greenhouse gases, rotation or clouds, which can shift where liquid water is actually possible. It's a starting estimate for where to look, not a guarantee either way.
Frequently asked questions
Why does Earth sit inside the conservative zone but close to the inner edge?
Because the model is tuned so the conservative zone roughly brackets the Sun's own inhabited Earth and (on the outer side) an early Mars that may once have had liquid water — it's meant to be a reasonable, physically-grounded reference, not an exact fit to any one planet.
What temperature range does this work for?
2,600–7,200 K, the range the original fit was calculated over (covering M dwarfs through to hot F-type stars); outside that range the polynomial isn't a validated fit, so this tool won't show a result.
Related tools: Exoplanet Equilibrium Temperature Calculator, Star Luminosity, Radius & Temperature Calculator, Kepler's Third Law Calculator.
Source / Last verified: Kopparapu, R. K. et al. (2013), "Habitable Zones Around Main-Sequence Stars: New Estimates", The Astrophysical Journal, 765, 131 (arXiv:1301.6674), Table 1 — all five limits' coefficients read directly from the paper, 1 October 2026. This tool reproduces the paper's own Sun-based figures: for Teff = 5772 K and 1 L☉, the conservative zone comes out at 0.99–1.71 AU and the optimistic zone at 0.75–1.77 AU, matching the commonly-quoted values for the Solar System from this model.