A neutron star is a superdense object made almost entirely of neutrons packed together as they would be in an atom nucleus, but somehow able to group together in vast numbers in the absence of charged protons and electrons. Since neutrons have mass they are massive objects, typically 1.2-2.0 times the mass of the sun, but are very compact at most a few tens of thousands of kilometers across. Neutron stars are formed in the cores of massive stars as they go supernova; as the outer parts are blown away by the explosion the inner core is forced in upon itself and compressed into the neutron star. Since the stars that form them are invariably rotating, so are the neutron stars when they are formed, and because they have lost volume while retaining mass they spin much faster than their parent stars, just as an ice-skater spins faster when she pulls in her arms.
A pulsar is a neutron star which emits a beam of powerful electromagnetic radiation. This comes from a fixed point on the star, along its magnetic axis, and therefore spins with the star. This is only detectable while it is pointing at us, so many (some think all) objects that we see as neutron stars may be pulsars.
Two beams of radiation being emitted by a Pulsar. Unless the Pulsar is rotating at 180° to its polar access only one of these beams will be visible from any one fixed point in space.
Pulsars are named using the letters PSR (Pulsating Source of Radio) followed by their right ascension (celestial longitude) and declination (celestial latitude). If they are close to other pulsars in the sky (not necessarily close in space, but in a straight line from seen from Earth), then letters can be appended to the designation to help distinguish them, such as the J in PSR J1719-1438.
The pulsar PSR J1719-1438 was discovered in 2009 during a survey using the Parkes 64 m Radio Telescope in New South Wales, as part of the Parkes High Time Resolution Universe Legacy Survey for pulsars. It is a Millisecond Pulsar (MSP) a pulsar that spins faster than can be accounted for by the (theoretical) original mass of the star which produced it, in the case of PSR J1719-1438 rotating once every 5.7 milliseconds. Pulsars of this sort are thought to have had their rotation accelerated by the accretion of mass from a binary companion. Previous studies have shown many to have white dwarf companions (white dwarfs are stars that have stopped undergoing nuclear fusion, but which are still producing heat and light due to gravitational effects). PSR J1719-1438 is approximately 4000 light years from Earth, in the constellation of Serpens.
Image centered on PSR J1719-1438, taken with the Keck LRIS instrument.
This week a paper appeared in the 25 August edition of the journal Science, in which a team lead by Matthew Bailes of the Centre for Astrophysics and Computing at Swinburne University of Technology in Melbourne and the Department of Astronomy at the University of California, Berkeley, in which they describe the results of further study of PSR J1719-1438 using the Parkes Telescope and the Lovell Telescope at Jodrell Bank in Cheshire, England as well as the Keck 10 m Telescope in Hawaii, and the conclusions drawn from them.
Bailes et al. were able to detect a very slight wobble in the motion of PSR J1719-1438, regular enough to be the product of an orbiting body, and from this were able to make significant deductions about both bodies in the system.
For the sake of convenience standard astronomical nomenclature is used here, so that the pulsar is referred to as PSR J1719-1438A (a for the first body discovered in the system, capitalized because it is a star) and the orbiting body referred to as PSR J1719-1438b (b for the second body discovered in the system, but not capitalized, indicating a planet), although Bailes et al. do not use this nomenclature and the terms 'star' and 'planet' need to be used reservedly in this instance.
Bailes et al. calculate that PSR J1719-1438A is a body approximately 20 km across with a mass 1.4 times that of the sun. PSR J1719-1438b orbits this body once every 2.17 hours, at a distance of about 600 000 km, slightly less than the radius of the sun; the whole system could fit within the volume of our son. It has a volume roughly equal to that of Jupiter, but is roughly 20 times as dense, making it the densest planet yet discovered.
Bailes et al. propose that PSR J1719-1438b might by made up of crystalline carbon - diamond, which would fit the known data. Such an object could in theory be formed from the core of a white dwarf star, stripped of its outer layer.
PSR J1719-1438 is not the first pulsar known to host planets. The first confirmed planets to be discovered outside our own orbited the pulsar PSR B1257+12, approximately 2000 light years from Earth in the constellation of Virgo. This has three known planets, and one suspected.
PSR B1257+12A has a radius of about 15 km, and a mass of roughly 1.5 times that of our sun. PSR B1257+12b orbits this every 25 days at a distance of approximately 28 400 000 km. It has a mass of roughly 0.02 times that of the Earth, or a third that of Mercury. PSR B1257+12c orbits at a distance of 54 000 000 km, slightly closer to PSR B1257+12A than Mercury is to the sun. It has a mass of 4.3 times that of the Earth. PSR B1257+12d orbits at 69 000 000 km, slightly further out than Mercury, and has a mass of 3.9 times that of the earth. A possible fourth planet, PSR B1257+12e may orbit at roughly 450 000 000 km, which would be between the orbits of Mars and Jupiter in our system. This planet, if it exists has a mass of only 0.0004 times that of the Earth, roughly twice that of Ceres or Pluto in our system, and much smaller than our moon.
Comparison between the sizes of bodies in our solar system and the PSR B1257+12 system, and the distances between them (distances and size of planets are not to scale).
See also New Exoplanet: TrES-5, TrES-2b, the black planet and Exoplanets and Stars on Sciency Thoughts YouTube.