Fifteen years of the neutron star planet research

Abstract

Developments in the investigations of the known planets around two millisecond radio pulsars, PSR B1257+12 and PSR B1620−26, and in the searches for dusty disks around pulsars are discussed and related to relevant issues in astronomy of the planets around normal stars. The unambiguous determination of true masses and orbital inclinations of two of the three planets in the PSR B1257+12 system has been achieved by means of modeling their mean motion resonance condition. An approximate coplanarity of the orbits of these terrestrial-mass planets points to a disk origin of the system. A tentative Spitzer discovery of a cool, ∼10 M⊕ dusty disk around the x-ray pulsar, 4U 0142+61, further suggests that disks which may form planets can survive in a neutron star environment. Currently available upper limits on infrared emission from dust around other pulsars do not exclude a possible existence of circumpulsar disks with masses reaching up to a few hundred M⊕. The direct detection of a white dwarf companion to PSR B1620−26 has resulted in tighter constraints on the nature of a giant planet orbiting this binary system. The existence of such a planet around this neutron star, in a metal-poor environment of the globular cluster M4, raises questions concerning the established relationship between stellar metallicity and the occurence of planets around normal stars.