Astronomers detecting another pulsar‚ a whirling neutron star emitting radio waves‚ may be routine business, but their discovery that its companion star evolved into a "planet‚" is not . The recent discovery of such a pair has sparked broader public attention because a former star is likely composed of carbon and oxygen and the gravity of this one is powerful enough to suggest its massive quantity of carbon exists in the form of "diamond" .
The paper in Science  is based on the detection of PSR J1719-1438. Millisecond pulsars such as this one are thought to be neutron stars that have been spun-up by accretion of matter from a binary companion. In this discovery, the companion's mass is near that of Jupiter, but its minimum density of 23 grams per cubic centimeter suggests that it may be an ultra-low mass carbon white dwarf. The researchers suggest that the system may have once been an Ultra Compact Low-Mass X-ray Binary, where the companion narrowly avoided complete destruction during the evolution of the pulsar. They conclude that the companion's remains‚ which arguably fall within the definition of planet‚ appear to have a chemical composition, pressure, and dimensions indicating that it contains a crystallized form of carbon. At these conditions,"diamond" is expected to transform to denser forms of carbon.
Figure: Keck/LRIS 20-min R-band image centered on the location of PSRJ1719-1438. The image was constructed from five exposures taken during the expected maximum luminosity of the companion in a total integration time of 1200 s.