A planet's equilibrium temperature scales roughly with
1/√distance from its star. Move it closer and insolation rises —
clouds thin, oceans dominate, ice caps retreat; move it away and
the planet cools, growing reflective polar ice that further
cools the climate (ice-albedo feedback).
Insolation follows an inverse-square law with distance and
scales directly with the star's luminosity: S ∝ L / d².
A simple blackbody estimate then gives an equilibrium temperature
T_eq ∝ S^(1/4), which the greenhouse effect boosts
further to produce the actual surface temperature.
- Orbital distance — how far the planet sits from its
star; farther away means less insolation and a colder,
icier world.
- Star type — sets the star's luminosity. A red dwarf
emits far less energy than the Sun, while a blue giant emits
many times more, both changing insolation at the same
distance.
- Greenhouse gas — traps outgoing heat independently
of insolation, raising surface temperature and shrinking ice
caps even without moving the planet.
This mirrors the real "habitable zone" concept: Venus sits in
a warm orbit but its runaway greenhouse effect pushes its surface
to roughly 460 °C, while Earth's much milder greenhouse effect
keeps it comfortably liquid-ocean temperate.