On the Number of Confirmed Pulsar Planets: The Rule of Six

Abstract Since the first definitive exoplanet detection, the three planets around millisecond radio pulsar PSR B1257+12, few more pulsar planets (PPs) have been reported. The peer-reviewed literature is ambiguous about the identity of confirmed PPs, with the number 6 appearing repeatedly, although it is generally not the same six listed pulsars or planets. We examined all claims, finding eight confirmed planets distributed among six pulsars. These objects overlap with the black widow pulsar population, there being 12 millisecond pulsars with credible companion masses below 10 Jupiter masses and little reason to choose between them. If, on the other hand, evidence for a planetary disk origin were adopted as a defining criterion, then only PSR B1257+12 with its three planets would qualify. Other claimed PP candidates have been either retracted, ruled out by subsequent studies, doubted because of mutually exclusive planetary solutions, or based on alternative theories. Long-lived quasiperiodic timing residuals are major culprits with several claimed orbital periods closely matching the data durations. Widely used exoplanet repositories differ in their inclusion criteria for PPs, resulting in large numerical discrepancies ranging from 6/8 confirmed pulsars/planets (NASA Exoplanet Archive) to 71/74 (Extrasolar Planets Encyclopaedia), which more importantly obscure a crucial distinction between planetary systems and ablated low-mass substellar companions. Tentative evidence for a spin–orbit correlation is found for the confirmed PPs, whereas such coupling is ruled out for black widows as a class. Surveys show PPs are extremely rare, yet their existence and physical properties present major challenges to understanding supernova explosions and planet formation around millisecond pulsars.