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Published online by Cambridge University Press: 12 April 2016
This paper discusses the distinction between extra-solar planets and low-mass secondaries, in principle as well as in practice. Adopting a distinction based on the presumed different processes of formation, the paper compares the characteristic features of the giant planets in our solar system with those of the low-mass secondaries in spectroscopic binaries. The discussion reveals that there is no a priori obvious feature that can identify planets. Instead, this work considers the extremely small emerging population of discovered extra-solar planets. Based on the nine “planet-candidates” discovered as of mid-1998, it was found that their mass distribution is remarkably different from the distribution of low-mass secondaries. The transition between the two populations probably occurs at 10−30 Jupiter masses. This transition could reflect the borderline between planet and brown dwarf secondary masses.