Neutron stars form from the core of a collapsing star and, as the supernova dissipates, often rotate rapidly, creating a pulsar. In less than a million years, however, their strong magnetic fields act as a brake, slowing them down considerably. In some cases, however, the neutron star will have a nearby companion, and its gravity is sufficient to start stripping mass off it. As the process continues, the neutron star will spin back up, creating what's called a millisecond pulsar. In most cases, these companions are still around, visible as a bright star locked in an orbital embrace with a pulsar. Now, researchers have spotted one where the star is still there, but not visible—the neutron star has stripped it down to a crystaline core the size of Jupiter.

The system in question, which has the catchy name PSR J1719−1438, was identified in a recent survey for pulsars. Careful timing observations revealed the influence of a nearby companion—very nearby, given that it orbited the neutron star with a period of only a bit over two hours. Given the orbital information and the typical mass of a neutron star, the authors were able to estimate that the orbiting body has a mass somewhere around that of Jupiter.

But that mass must be highly compact; otherwise, given the limited distance between the two, the neutron star would end up gravitationally disrupting its companion. The same goes for a helium-rich white dwarf star. The only thing that the authors calculate could fit into this uncomfortably close orbital configuration is a carbon white dwarf. So, they conclude that the "planet" orbiting the neutron star is simply the core of its previous stellar companion, stripped of most of its mass through the process that spun up the pulsar. And, in the last sentence of the paper, they drop a bit of a bombshell: "The chemical composition, pressure and dimensions of the companion make it certain to be crystallized (i.e., diamond)."

About 30 percent of the millisecond pulsars we know about don't have a stellar companion, which raises the possibility that there are other Jupiter-mass diamonds out there awaiting our discovery. However, other fates are possible; a bit closer, and the companion star would have been devoured completely, leaving no remnant at all. And, in at least one case, a companion star seems to have been torn apart in a way converting it into a disk that has formed three planets that now orbit the neutron star. With further observations, we should get a better sense of how common these odd companions are—and possibly find something else that's even stranger.

Science, 2011. DOI: 10.1126/science.1208890 (About DOIs).