Sentence:  The study of the radio emission of the moon and <WSD>planets</WSD> began with the detection of the thermal radiation of the moon at 1.25 - cm wave length by Dicke and Beringer ( 1946 ) . 
Meaning of the word marked with <WSD> tags:  (astronomy) any of the nine large celestial bodies in the solar system that revolve around the sun and shine by reflected light; Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto in order of their proximity to the sun; viewed from the constellation Hercules, all the planets rotate around the sun in a counterclockwise direction