An Expensive Planet

Title: Transformation of a Star into a Planet in a Millisecond Pulsar Binary
Authors: M. Bailes et al.

This past week's article comes our way via Science Magazine instead of the usual Astro-PH listing. The results made science headlines in a wide variety of popular science outlets as the "diamond planet". Normally, pop. sci. articles are filled with exaggerated claims and are mired by shaky conclusions. So, how did the science writers do on reporting these results? Quite well, actually.

After a star dies via a supernova explosion, it will form a neutron star (if it has less than ~20 solar masses), a super compact object which sustains itself against gravity via neutron degeneracy pressure. If you are familiar with electron degeneracy pressure, for instance, in the case of a white dwarf, then neutron degeneracy pressure is already known to you - except  you replace electrons by neutrons. It turns out, that when electron degeneracy pressure is not strong enough to support the gravitational collapse of matter, neutron degeneracy pressure is the next line of defense. Anyway, I digress. Post-supernova explosion, as stellar material collapses down to the core of the stellar remnant, it spins up the core due to angular momentum conservation (think figure skater pulling his/her arms in to speed up their spin). This produces a neutron star that is rapidly spinning (~1000 rotations per second) and one that has a very strong magnetic field (10¹²G) which spews material out in the form of bipolar jets; a (radio) millisecond pulsar.

Credit: European Space Agency & Francesco Ferraro (Bologna Astronomical Observatory)

Large magnetic fields, however, quickly help to spin-down the neutron star due to magnetic breaking - a process by which angular momentum is transferred away from the star to its surroundings via magnetic processes. The spin-down causes the pulsar to stop emitting at radio wavelengths. Interestingly, a "dead radio pulsar" can be revived if the pulsar is in a binary system with a stellar companion. Processes by which pulsars are revived are not fully understood, but we shall describe a couple of the possible scenarios later.

Now to the results of the paper. Bailes et al. have discovered one of these millisecond pulsars which has a binary companion. Not too uncommon (~70% are in binary systems), but based on fairly straight forward geometrical and dynamical arguments (think Kepler's laws), the authors were able to determine the radius and the average density of the pulsar's companion. They find that the radius is actually smaller than Jupiter's radius! Fantastic. This means there is a planet sized object orbiting the pulsar. Then, using some clever techniques, the authors are able to derive a lower limit to the average density of the object. If the object were gas planet, one would expect to find densities around 2 grams per cm³, instead, they find the lower limit is 23 g per cm³! This is a HUGE density!

After running a few more tests to be sure of the robustness of their findings and to limit, even more, the parameters of the binary companion to the pulsar, the team discovered that, indeed, they found a pulsar with a planetary sized white dwarf companion, supported by electron degeneracy pressure mentioned earlier. What is most interesting, is that the white dwarf did not form in the typical fashion of a lower mass star evolving until its subsequent death. This is evidenced by the low mass (slightly more massive than Jupiter) and tiny radius (smaller than Jupiter) of the white dwarf. A typical evolutionary scenario would be very, very unlikely to produce a remnant of these proportions. Instead, the white dwarf must have been formed by interactions with the primary (aka: the pulsar).

Here there are a couple different scenarios which could lead to the observed configuration. The first is that the companion white dwarf, once a helium burning star, was converting helium to carbon in it's core. However, it may have transferred mass to the neutron star - a process known as accretion - which occurs when material is gravitationally removed from the star by the neutron star. This material would then be either accreted onto the neutron star or expelled in the jets which are generated by the magnetic field. Either way, this would act to spin-up the neutron star and bring it back to life (aka: emit at radio wavelengths).

This process of accretion by the neutron star can happen in a couple ways, so it's thought. The first is if the companion star to the pulsar is fairly far away from the pulsar, but is itself very large. Material can then stream away from the companion. However, this would tend to create white dwarfs of varying size depending on the distance. If, instead, the star is close to the pulsar to start with, matter can be transferred more readily, allowing more easily for the creation of a smaller white dwarf. In either case, for this particular planet to be observed, the mass transfer would have to be at a sufficient rate in order to remove enough mass from the companion BEFORE it has time to generate a typical carbon core. This close-in binary scenario also raises questions about possible interactions during the primary stars life before it became a neutron star. It presumably would have been rather large, thus possibly encompassing the secondary, or at least allowing for mass transfer at this point, as well. Complicated physics, to say the least.

Credit: An artist whose name is not associated with this image on several prominent websites.

In the end, the planet-sized object that remains in the system at hand is a very dense carbon object. Very dense carbon inevitably leads to the image of diamonds. So the popular science articles were not too far off in their analysis and spin on the results. We have here a planet that would fetch a very high price on the market and one that would make the Hope Diamond seem like pebble.