Sunday, September 4, 2011

A Pulsar’s Diamond Planet

A pulsar is a rapidly spinning and strongly magnetized neutron star which emits an intense bipolar beam of electromagnetic radiation that generally does not coincide with the spin axis of the pulsar. This results in the ‘lighthouse effect’ when the beam of electromagnetic radiation happens to be orientated towards the Earth; leading to an apparent pulsed nature as the beam of emission periodically sweeps past the Earth. Hence, the pulsar derives its namesake from this observable behaviour. For some pulsars, the periodicity of its pulsed nature can be as precise as an atomic clock. A pulsar forms out from the ultra-compressed core of a massive star during a supernova explosion and a typical pulsar contains an entire Sun’s worth mass of matter packed into an incredibly small volume with a diameter of no more than a few tens of kilometres. On average, each cubic centimetre of a pulsar’s material holds on the order of a few hundred billion metric tons of mass.

PSR J1719-1438 is a pulsar with a spin period of 5.7 milliseconds, which means that it spins 175 times each second. The very rapid spin rate of PSR J1719-1438 means that it is categorized under a unique group of pulsars called millisecond pulsars. The high spin rate of a millisecond pulsar is believed to be cause by the spinning-up of a pulsar by the accretion of matter from a binary companion which transfers angular momentum to the pulsar. A recent paper by M. Bailes et al that is titled “Transformation of a Star into a Planet in a Millisecond Pulsar Binary” describes the discovery of a Jupiter-mass companion in a 2.2 hour orbit around the pulsar PSR J1719-1438. The paper also investigates the possibility that PSR J1719-1438 was once an ultra compact low-mass X-ray binary (UC LMXB), whereby matter was accreted by the pulsar from a binary companion star. Almost all of the material from the binary companion star was accreted by the pulsar, leaving behind a Jupiter-mass remnant of what was once the companion star.

The Jupiter-mass companion orbiting around PSR J1719-1438 was detected from slight pulse-timing variations observed in the pulsar’s extremely regular pulses. These pulse-timing variations are caused by the gravitational tugging of the pulsar by its Jupiter-mass companion. The Jupiter-mass companion is orbiting so close to PSR J1719-1438 that it cannot be anywhere as large in size as Jupiter because at that size, its own gravity will not be sufficiently strong enough to prevent it from being gravitationally torn apart by the nearby pulsar. For this reason, the Jupiter-mass companion must be much more compact in nature to avoid being gravitationally torn apart. A lower limit of 23 grams per cubic centimetre is required for the mean density of the Jupiter-mass companion such that its overall physical size is compact enough for its own gravity to be sufficiently strong to prevent it from being gravitationally torn apart. In comparison, the mean density of Jupiter is less than 2 grams per cubic centimetre.

Such a high density means that the Jupiter-mass companion orbiting around PSR J1719-1438 cannot be a gas-giant planet like Jupiter as it is too dense to be made up of just hydrogen and helium like Jupiter. Instead, the Jupiter-mass companion is theorized to be what remains of the degenerate core of the companion star whose material was stripped away and accreted by the pulsar. In this scenario, the Jupiter-mass companion was once a white dwarf star in a tight orbit around PSR J1719-1438. The white dwarf star was basically what remained of a star like our Sun after it had extinguished its hydrogen and helium in its core through fusion of these elements into carbon. So, if the Jupiter-mass companion is what remains of the core of the white dwarf star, it is expected be comprised of heavier elements such as carbon. This will allow it to have a mean density of 23 grams per cubic centimetre or more, making it compact enough such that it will not be gravitationally torn apart by the nearby pulsar.

The Jupiter-mass companion around PSR J1719-1438 is a very unique case because most white dwarfs stars tend not to survive the transfer of matter to their companion pulsars. A typical white dwarf star tends to be too close to its companion pulsar by the time it starts transferring material to the pulsar. This leads to the complete destruction of the white dwarf star, possibly leaving behind a disk of remnant material orbiting around the pulsar. Such a disk of material is expected to coalescence into a planetary system of Earth-mass planets rather than a Jupiter-mass object. Therefore, a Jupiter-mass companion around a pulsar will require a low mass white dwarf star so that the transfer of matter to its companion pulsar occurs at a further distance away. This can allow the transfer of matter to cease just in time before the complete destruction of the white dwarf star, leaving behind the remnant core as a Jupiter-mass object.

Of all the pulsars discovered so far, only a handful of them have planetary-mass companions. This means that the formation of planetary-mass companions around pulsars is the exception rather than the rule. Furthermore, the companion around PSR J1719-1438 is the only known Jupiter-mass object around a pulsar. All previously discovered planetary-mass objects around pulsars are around the mass of the Earth. The case for PSR J1719-1438 requires a very unusual combination of white dwarf mass and composition. This unique combination allows PSR J1719-1438 to transform its companion into very rare and exotic type of planet. The Jupiter-mass companion around PSR J1719-1438 is likely to be entirely composed of crystallized carbon, which is also known on Earth as diamond.