The glowing sphere at the center is the star; its color and brightness change with type. The translucent green band is the habitable zone (Goldilocks zone) — the range of orbital distances where a rocky planet's surface could sustain liquid water. The small dotted planets are fixed reference worlds; the larger probe planet is yours to drag through the system with the distance slider, and its surface visibly freezes, pools into an ocean, or boils into vapor depending on where it sits.
T_eq(K) ≈ 278 · L^0.25 / √d(AU)
zone_inner(AU) = 0.95·√L zone_outer(AU) = 1.37·√L
- L — the star's luminosity relative to the Sun (M dwarfs ≈ 0.01–0.05 L☉, G stars like the Sun = 1 L☉, A stars ≈ 15–25 L☉).
- Star type — moving from M to A increases luminosity and surface temperature, which pushes the whole habitable zone outward (a red dwarf's zone hugs the star; a hot A-type star's zone sits much farther out).
- Orbital distance — the probe planet's equilibrium temperature falls off with the square root of distance, so doubling the distance does not simply halve the temperature.
- Water state — below 273 K the surface is read as frozen (ice), between 273–373 K as a liquid ocean, and above 373 K as boiled off into vapor — a simplified stand-in for the real balance of pressure, greenhouse gases and albedo that actually decides a planet's climate.
Real-world relevance: this is why habitability estimates for exoplanets always start with the host star's type — Proxima Centauri b orbits a dim red dwarf at just 0.05 AU and still sits in that star's habitable zone, while a planet at the same distance from a Sun-like star would be a scorched cinder.