Astronomers have discovered thousands of planets outside the Solar System, most of which orbit stars that will eventually evolve into red giants and then into white dwarfs. During the red giant phase, any close-orbiting planets will be engulfed by the star, but more distant planets can survive this phase and remain in orbit around the white dwarf. Some white dwarfs show evidence for rocky material floating in their atmospheres, in warm debris disks or orbiting very closely, which has been interpreted as the debris of rocky planets that were scattered inwards and tidally disrupted. Recently

Observational evidence s suggest a co-evolution of the central supermassive black holes and their host galaxies. In some of them, the black hole is ingesting the material surrounding it at a very high rate, emitting a large quantity of energy. In those cases we say that the galaxy has an active nucleus (AGN). Studying the mechanisms which control the relation between the active nucleus and the rest of the galaxy is essential to understand how these objects form and evolve, and to be able to throw light on this question we need to compare active and non-active galaxies. Here, we first identif