PSR J1719−1438 is a pulsar, which is a rapidly spinning neutron star. Such an object is called a pulsar because its powerful magnetic field shoot jets of energetic particles out along its magnetic poles. This also makes the spinning neutron star appear to pulse as its jet sweeps past an astronomer on Earth.
Slight variations in the timing of the neutron star's pulses can indicate the presence of an unseen planet that is also orbiting the pulsar. Indeed, the first known extra-solar planet was discovered via pulsar timing variations. Matthew Bailes (Swinbourne University in Australia) discovered the timing variations in pulsar PSR J1719−1438, and they are due to a Jupiter-mass planet in a very close two-hour orbit about the neutron star. He and his colleagues also show that this planet must be very small and dense to avoid having been ripped apart by the neutron star's gravitational tide. This planet's minimum density is about 20 times that of Jupiter's, and the planet's core is likely made of carbon. If so, then carbon at the center of this very dense planet will have crystallized, possibly into one giant planetary core-sized diamond. See this press release for more details.