How can planets be present around pulsars which are exploded stars?

You would think that when a star goes supernova to produce a neutron star, that a delicate thing like a planet would be blown away like a dandelion seed in a gale. That seems not to be the case in two separate instances. In 1991, after a four-year study, astronomers Alexander Wolszcan of the Pennsylvania State University and Dale Frail of National Radio Astronomy Observatory discovered among the details of the radio signals from the pulsar PSR B1257+12 that the signal's changes could be explained if two small planets orbited the pulsar. A few years later, the traces of a third planet emerged from the data. The planets would have to have masses of up to three times that of Earth, and orbits with distances of 9 to 40 million miles.

A possible Jupiter-sized body may also exist orbiting at Pluto's distance with a period of 170 years. Since then, a second pulsar has been added to the short list of neutron stars with orbiting planets. But how could the devastating supernova explosions that surely accompany the formation of neutron stars have left these planets intact? The orbits also seem to be exceedingly circular, as though not much has happened to disturb them in billions of years, yet the pulsars are probably less than 800 million years old of themselves. The origins of these planets remain very much a mystery today.

If a star looses more than half its mass, a sure bet for most supernovae, any orbiting planets will drift out of the star's gravitational grasp. These orbits should be highly elliptical, which in the case of the actual planets they are not, so this process could not have led to the formation of the planets. The planets could not have formed before the supernova detonation occurred. At the present time, there is no accepted explanation for how these planets formed after the supernova.

This answer was updated in 2011. See my books: The Astronomy Cafe (1998) and Back to the Astronomy Cafe (2003) for more FAQs in printed form. Author: Dr. Sten Odenwald, Copyright 2011

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