All objects that are warm emit some radio waves.
That includes the Earth and moon, the other planets in our solar system, and ... you! The planets are heated by the Sun.
Now that you know this, you can predict. Choose which planet will emit more radio waves: Mercury or Venus? Mars or Jupiter?
If you answered Mercury and Mars, then you are thinking like a scientist. Astronomers reasoned that the closer the planet is to the Sun, the hotter it will be, and the more radio waves it will emit.
Boy, did they get a surprise when they observed the planets with radio telescopes!