Extrasolar Planets: Change of Paradigm During the Twentieth Century

The question of the plurality of worlds in the universe has been present in philosophers’ discussions for over 2000 years. The idea had also been present to the public for centuries, as can be seen, for example, in the iconography of eighteenth-century astronomical atlases or the frescoes of religious buildings from the same century (Figs. 1 and 2). However, this idea was rejected by the astronomical community in the first half of the twentieth century (Dick, In: Heidmann J, Klein MJ (eds) Bioastronomy: “the search for extraterrestrial life”. Springer, New York, pp 356–363, 1991). Observation of the weak rotations of low main-sequence stars led to a fundamental change in this idea (Struve, Observatory 72:199–200, 1952) and proposed that planetary systems must be very common in the galaxy. This paradigm shift was amply verified by the discovery of planet 51 Pegasi b in 1995 and the countless discoveries that followed. The astonishing period of this planet also demonstrated the importance of orbital migration during planetary formation (Lin, Nature 380:606–607, 1996). This discovery also stimulated the search for exoplanetary systemsExoplanetary systems and their astonishing diversity.