The Drake Equation: The fp Term fp: The fraction of these stars having a planetary system. The gas and dust clouds from which stars are formed rotate. They have angular momentum - that is rotational energy - which must be conserved. If, as generally seems the case, the stars themselves are not rotating rapidly, then the angular momentum must reside elsewhere. Around half of all stars are in binary or multiple star systems where the motion of the stars around their common centre of gravity carries the angular momentum. It is suspected that where the rotation of a star is slow and it has no stellar companions, then the angular momentum might reside in one or more unseen planets. It thus seems that comparatively many stars would have planetary systems. We now have good observational evidence that this is indeed the case. Not only have about 70 Jupiter-sized planets been detected around Sun-like stars, but infra-red observations and direct imaging by the ground- and space-based telescope have also shown that in the Orion Nebula region, nearly half of all young stars are surrounded by so called "protoplanetary discs". These observations imply that about a half of all new stars will be accompanied by planets.