🌡️ The Habitable Zone

About this simulation

The circumstellar habitable zone is the range of orbital distances around a star where a rocky planet could hold liquid water on its surface — not so hot that oceans boil away in a runaway greenhouse, not so cold that they freeze solid. Because a star's radiated energy falls off with the square of distance, the habitable zone's inner and outer edges scale with the square root of the star's luminosity.

How it works

• Inner edge — where absorbed sunlight is intense enough to trigger a runaway or moist greenhouse effect, boiling the oceans away.
• Outer edge — where sunlight is too weak to keep CO₂-driven greenhouse warming from freezing the surface.
• Both edges move outward as √L (luminosity in solar units), so a brighter star pushes its whole habitable band farther out.
• A planet's equilibrium temperature also depends on its albedo (reflectivity) — a shinier planet absorbs less starlight and runs cooler at the same distance.

Controls

• Stellar luminosity — the star's brightness in solar luminosities (L☉), on a log scale from a dim red dwarf to a bright giant.
• Orbital distance — the planet's distance from the star in astronomical units (AU).
• Planet albedo — the fraction of starlight the planet reflects (0 = pitch black, 0.9 = near-mirror ice).
• HZ model — switch between the conservative (empirically anchored) and optimistic (theoretical, wider) habitable-zone boundaries.

Did you know?

Earth sits comfortably inside the Sun's conservative habitable zone at 1 AU, but climate models suggest the inner edge may lie as close as 0.95 AU and the outer edge near 1.7 AU — meaning our own planet has less margin than it might seem.