Exoplanets around Pulsar PSR 1257+12

Back in 1991, the only planets known to astronomers, and to an extent, all of humanity, were the nine known planets of our Solar System. In 1992, that all changed.


A while ago, we learned how planets formed around a star. A stellar nebula begins to rotate, and pockets of the gas and dust collapse and consolidate to form planetessimals. In turn, these planetessimals collide with each other, growing ever bigger, until they reach full-fledged planetary status. The one thing that astronomers were certain of, were that planets would most likely be around main sequence stars and possibly some white dwarfs. However, the first exoplanets discovered were not around either of these objects. They were found around Pulsar PSR 1257+12 by Polish astronomer Aleksander Wolszczan and his team. Why is this strange?


What do we remember about pulsars? Pulsars are rapidly rotating neutron stars, which we know were formed when a medium-high mass star supernovas. And supernovas are extreme events in a high mass star's life. The supernova explosion itself should have obliterated any planets that orbited around that star. So how exactly were planets found around a pulsar?


There are a couple of theories.

1. The planets formed from the supernova remnant. Possible, as long as the remnant did not disperse too quickly
2. The planets were actually lone planets, without a parent star. The planets passed too close to the pulsar and were captured by the pulsar's gravity.
3. The planets somehow survived the supernova event and instead of being obliterated, rode out of the supernova and were pushed farther away.

The most likely theory is the second one, lone planets captured by the pulsar. We do know one thing for sure, however. Though the planets are roughly Earth-sized (A ~ 0.020 Earth masses, B ~ 4.3, and C ~ 3.9), they cannot be hospitable to life. They are all within one AU of the pulsar which means they are bathed in extreme radiation from the pulsar.


A nice thing about pulsars is that they do have extremely precise periods, and any perturbations from a companion planet could easily be measured. The radial velocity technique works well for planets around pulsars (if there are any).