Energy In, Energy Out
A planet's temperature is set by a simple energy balance — it absorbs a fraction (1-A) of incoming stellar radiation and re-emits thermal infrared radiation to space. When absorption equals emission, the planet reaches equilibrium temperature T_eq = [L(1-A)/(16 pi sigma d²)]^(1/4). This elegant equation contains all the essential physics: stellar luminosity L, orbital distance d, albedo A, and the Stefan-Boltzmann constant sigma.
The Role of Albedo
Albedo determines what fraction of sunlight a planet reflects back to space unused. Fresh snow reflects 80-90% of sunlight; ocean water absorbs 94%. A planet covered in ice would reflect so much light that it cools further, making more ice — a positive feedback loop called the ice-albedo feedback that may have caused Snowball Earth episodes 700 million years ago. Cloud albedo is Earth's largest uncertainty in climate modeling.
Greenhouse Warming
Earth's atmosphere is transparent to visible sunlight but partially opaque to outgoing infrared radiation. Greenhouse gases (CO₂, H₂O, CH₄) absorb and re-emit thermal photons, effectively insulating the surface. This natural greenhouse effect adds 33 K to Earth's temperature, making it habitable. Venus demonstrates the extreme: its 90-atmosphere CO₂ blanket adds over 500 K, creating surface temperatures hot enough to melt lead.
Habitable Zone Science
The habitable zone is defined as the orbital distance range where a planet with reasonable atmospheric conditions could support liquid surface water. Too close, and water vapor triggers a runaway greenhouse. Too far, and CO₂ condenses, collapsing the greenhouse. This concept guides the search for Earth-like exoplanets with missions like Kepler, TESS, and the James Webb Space Telescope.