On the occurrence and nature of planets outside the solar system

Abstract

The topic of the occurence of planets outside the solar system is discussed in detail from the point of view of its potential in future astronomical research. It is found that in spite of the speculative nature of most considerations to date, many independent approaches are possible. They stem from present astrometric observations, future observations from space, physical theories on the evolution of submassive stars and planets, theories on the origin of the solar system, problems of the unseen mass in the solar neighborhood and in certain galaxies, statistics of the mass-frequency function of stars and of solar system bodies, and statistics and theories of the angular momenta of stellar systems. It is concluded that as our knowledge of these approaches increases it will eventually be possible to derive meaningful results as to the frequency and nature of planets in the universe. From the astrometric investigations alone it is now known that at least six of the nearest one hundred stars have planetary companions of mass greater than that of Jupiter.