Against all expectations, the first confirmed planetary system beyond our own was found, in 1991, in orbit around a pulsar. The object in question is a millisecond pulsar known as PSR 1257+12. The discoveries of several other planets around millisecond pulsars, made using the pulsar timing method, have since been announced, though in most cases these are awaiting confirmation. The nature and origin of these strange worlds is a matter of debate.1, 2, 3 One possibility is that the pulsar planets formed in the normal way (see planetary systems, formation) before their host star exploded as a supernova. However, it is hard to see how this could be so. The problem is not that the supernova would destroy any nearby planets (though it would certainly incinerate any surface life), but that it would effectively loosen the gravitational glue holding the planetary system together. A planet orbiting a star that suddenly lost a large fraction of its mass would fly off into space. The alternative, and more likely scenario, is that pulsar planets formed after the pulsar came into existence. Millisecond pulsars are believed to spin so fast because they have acquired material from a companion star. Planets could condense from some of this material as it entered an accretion disk in orbit around the pulsar. However they were made, these strange and unexpected worlds are of little direct biological interest. The main sequence lifespan of massive stars which are the precursors of pulsars is probably too short to allow any kind of life to develop, even given the availability of suitable planets nearby. On the other hand, planets which formed around a pulsar would be permanently strafed by high-energy radiation, including X-rays and gamma rays, leaving them barren and inhospitable.
The table below shows those pulsar planets currently confirmed or suspected.4
|PSR 1257+12||1,000||0.015||0.19||25.3 d||0.0|
|PSR B1620-26||12,400||?||~38||~100 y||-|
1. Banit, M., Ruderman, M. A., Shaham, J., and Applegate, J. H. "Formation
of Planets Around Pulsars," Astrophysical Journal, 415,
2. Phillips, J. A., and Thorsett, S. E. "Planets Around Pulsars: A Review," Astrophysics & Space Science, 212, 91 (1994).
3. Phinney, E. S., and Hansen, B. M. S. "The Pulsar Planet Production Process." In Planets Around Pulsars, proceedings of the conference, California Institute of Technology, Pasadena, ASP Conference Series, p. 321, April 30-May 1, 1992.
4. Wolszczan, A. "Conformation of Earth-Mass Planets Orbiting the Millisecond Pulsar PSR1257+12," Science, 264, 538 (1994).