Final answer:
Planets, including Earth, emit thermal energy back into space primarily in the form of infrared radiation, which is central to the concept of the greenhouse effect.
Step-by-step explanation:
Heat escapes from a planet's surface into space by thermal radiation. Planets radiate almost entirely in the wavelength range of the infrared radiation. This form of heat transfer occurs when electromagnetic waves such as infrared radiation are emitted or absorbed.
The Earth, for instance, radiates energy back into space mainly in the form of infrared radiation after absorbing energy from the sun which arrives in various forms including ultraviolet rays and visible light. This re-radiation in the infrared is a fundamental aspect as to why some gases in planetary atmospheres can trap heat, leading to what is known as the greenhouse effect.