All matter with a temperature above absolute zero emits EM radiation because of molecular agitation. Planck’s law of radiation describes the amount of emitted radiation for black bodies. Real object radiance is lower than the black body radiance.
The Sun behaves similarly to a black body. It is a prime source of the EM radiation that plays a role in Earth Observation, but it is not the only source. The global mean temperature of the Earth’s surface is 288 K and over a finite period the temperatures of objects on the Earth rarely deviate much from this mean. The surface features of the Earth therefore emit EM radiation. Solar radiation constantly replenishes the energy that the Earth radiates into space. The Sun’s temperature is about 6000 K.