planet pulsarThe millisecond pulsar PSR 1257+12, which is orbited by at least two planetlike objects. The pulsar has a spin period of 6.2 milliseconds, and the two inferred planets have masses of a few Earth masses and periods of 66 and 98 days, respectively. Because these orbits are close to a 3:1 resonance, their mutual perturbations produce detectable variations in the pulse arrival times. Careful monitoring of the system has confirmed these predicted variations, unambiguously proving the planet hypothesis. In addition, it is likely that a third planet has now been identified in the system with an orbital period of 25 days and a mass similar to that of the Moon. It is presently not known how these ‘planets' have formed. One of the more probable possibilities is that the system was a binary millisecond pulsar in which the companion star, a low-mass star or a white dwarf, was dynamically disrupted (see Roche limit). The material of the disrupted star then formed a protoplanetary disk in which planets were able to form (see protoplanet).
Collins Dictionary of Astronomy © Market House Books Ltd, 2006