The Proportion of Stars with Planets

Abstract

Estimates of the proportion of Sun-like stars with accompanying planets vary widely; the best present estimate is that it is about 0.34. The capture theory of planet formation involves an interaction between a condensed star and either a diffuse protostar or a high-density region in a dense embedded cluster. The protostar, or dense region, is tidally stretched into a filament that is gravitationally unstable and breaks up into a string of protoplanetary blobs, which subsequently collapse to form planets, some of which are captured by the star. A computational model, in which the passage of collapsing protostars, with initial radii 1000, 1500 and 2000 au, through a dense embedded cluster are followed, is used to estimate the proportion of protostars that would be disrupted to give planets, in environments with star number-densities in the range 5000–25,000 pc−3. It is concluded from the results that the capture theory might explain the presently-estimated proportion of stars with exoplanet companions, although other possible ways of producing exoplanets are not excluded.