Solar radiation: The primary energy source for most planetary bodies.

Re-radiation to space: The primary loss mechanism for most planetary bodies.

Heat affects the molecular composition of atoms, and the amount of time necessary to reach chemical equilibrium; gradients in temperature and pressure produce atmospheric winds, ocean currents, and convective cells that mix both atmospheres and interiors; heat can even affect orbital trajectories (e.g. the Yarkovski effect).

Bodies tend to radiate almost the same amount of energy to space as they absorb from the Sun (Jupiter, Saturn and Neptune radiate more, as their interiors are cooling or becoming more centrally condensed). However, energy can be stored from day to night, from summer to winter, or from perihelion to aphelion.

Blackbody (thermal) radiation: Most objects emit a continuous spectrum of EM flux, which we call thermal radiation. A blackbody is an object which absorbs all flux which falls on it, at all frequencies and at all angles of incidence (i.e. none is reflected or scattered). An object's capacity to emit radiation at a given frequency is the same as its capability to absorb radiation at that same frequency. The emitted flux goes as

and is defined as the specific intensity, in units of erg / cm² / Hz / s /steradian. The blackbody curve of the Sun peaks in the optical (5700 K), while that of the planets tends to peak in the IR (40 - 700 K).

[NMSU, N. Vogt]

The Rayleigh-Jeans Limit:
Consider the Taylor Series expansion below.

In the limiting case where h << kT,

This limit applies to radio wavelengths, for planets.

Wien's Law:
Consider the following identity.

In the limiting case where h >> kT,

We can find the peak frequency of a blackbody curve by setting the first derivative with respect to frequency to zero and searching for a local maximum.

We next determine the flux density, in units of ergs / s / cm² / Hz, where _s is the solid angle into which the radiation is emitted.

The flux is thus

where T is the effective temperature (the temperature of a blackbody which emits the same amount of flux F).