Technically, thermal radiation is the heat emitted by anything that is not at absolute zero; absolute zero being the coldest possible temperature where all activity at molecular level stops (also called zero K or roughly -273 degrees C and -460 degrees F). Examples of thermal radiation are sunlight and the light from an incandescent bulb. Note that radiation in these forms can be visible to the human eye. Humans also emit thermal radiation, but it is within the infrared region and hot stars do so too, at the ultraviolet region.

Thermal radiation is generated when heat energy is converted into the radiative energy and is released in the form of electromagnetic waves. The term ‘thermal radiation’ is often used so electromagnetic radiation from radiant heat is distinguished from other forms such as gamma and X rays. Conversion into electromagnetic radiation occurs at the molecular level, by the movement of electrons and protons in the gas or object although the energy generated is not always visible to the human eye as in the case of infrared.

Thermal radiation information may be used to acquire information on astronomical phenomena and meteorological conditions of planets. The sun is usually the first thing that comes to mind when thermal radiation is the subject but the fact is, thermal radiation has been observed from the atmosphere (in the case of gas planets) and surfaces (in the case of terrestrial planets) of ALL planets.

Note that when thermal radiation strikes, it is absorbed by the object or body, reflected from the object or transmitted throughout the object. In the case of the planet Earth, the balance of incoming thermal radiation and outgoing radiation via heat emission plays a major role in over-all temperature.