|It is important to know that every
body emits its own radiation, even if this radiation is
This emission follows approximately a physical formula (called Planck's law) which describes the intensity of the emission of a Black Body (or in other words, a perfect emittor). This Black Body radiation depends mainly on the temperature of the body (on the right, the intensity versus wavelength for 3 black body radiators at temperatures of 4000K, 6000K and 8000K).
Normally, the temperature of "cold"
celestial bodies (by this we mean objects that are not star-like,
such as planets, asteroids...) make this emission maximum in the
Infrared band of the spectrum. For this reason, this radiation
essentially due to the heat of the body is called Infrared
An interesting example is the thermal (or infrared ) emission of the Earth. In the picture below, we can see the real emission of our planet, compared to a black body radiation of a body at a temperature of 280K.
In the picture it is also possible to see the absorption spectral lines of oxygen and CO2.