The astronomical study of planets is as old as Galileo's telescope, but in a profound way it was reborn with the advent of the Space Age. By constructing probes capable of leaving the surface of the Earth and traveling to other places in our solar system, sending back data collected from the very places that the astronomers wished to study, for the first time we were freed from the restrictions of observing astronomical objects from afar. These in-situ measurements, in their turn, have inspired countless new research projects back on Earth, from laboratory studies of materials to telescopic observations, of objects and in wavelengths now known to be of astronomical interest, thanks to those probes.