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).

Pulsars

As mentioned in the last post, pulsars are a specific type of neutron stars. Neutron stars have very strong magnetic fields, that are not necessarily lined up with their axes of rotation. Pulsars give off radiation as energy is directed along the axis of the magnetic poles and "beamed" towards Earth. These beams or jets are very energetic and can be seen for thousand of light years. When the jet of energy is pointed in the direction of Earth, we see them as a pulsar.

A very badly drawn schematic of a pulsar

A pulsar acts kind of a like a lighthouse in that the beam sweeps around as the light source rotates. Unlike a lighthouse, however, a pulsar's period can be extremely short, on the scale of milliseconds. The faster pulsar discovered has a period of about 3 milliseconds, and for something as large as a neutron star (about 10 km in diameter), the pulsar has to be rotating very fast.

We are unsure whether or not all neutron stars are pulsars, though we do know that most pulsars are neutron stars. Since pulsars can only be detected if the jet of energy is pointed directly towards Earth, we don't know if all neutron stars do this.

Neutron Stars

Neutron stars are the final stage of a medium-high mass star. They come about after a medium-high mass star ends in a violent Supernova Type II explosion. What exactly are neutron stars?

Neutron stars are a second type of degenerate stars, much like white dwarfs. However, unlike white dwarfs, the degeneracy comes from neutrons, not electrons, hence the name. But neutron degeneracy only arises when the neutrons are much more closely packed than the electrons. Whereas a white dwarf is on the scale on the Earth, in terms of radius, neutron stars are only 10 km in diameter, about as big as the size of Manhattan Island. Masses for neutron stars are larger than the Chandrasekhar limit, but only goes up to two (2) solar masses. Anything larger than that, gravity breaks the neutron degeneracy, and the neutron star will collapse into a black hole, which is ever wackier.

Neutron stars can also spin rapidly, and if they are aimed correctly, they can beam jets of energy along magnetic field lines towards Earth. When we see a neutron star rotating in this manner and we detect the beam of energy, the neutron star is called a pulsar, even though the star does not pulsate. The beams of energy are similar to a lighthouse shining light out to the ocean at regular intervals. However, pulsars rotate so fast that they can have periods on the scale of milliseconds (rotating almost a thousand times a second). We will talk more about pulsars in the next post.