"In this case, something with the mass of our sun has evolved to be something the mass of a planet - quite extraordinary," astronomer Michael Keith, with the Australia Telescope National Facility, wrote in an email to Discovery News.
The companion to J1719-1438 never got big enough to produce elements much heavier than carbon, so after its lighter-weight hydrogen and helium were stripped away that would leave a solid core of carbon - diamond.
"Due to the immense pressure, the carbon will be in a dense crystal-like structure, although much more closely packed than in a diamond on Earth," Keith said.
The system is now stable, with no evidence that it will change for billions of years.
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"Of course, this also means that it could well have been around for a long time, just waiting for us to find it. Since it's likely to last for longer than the Earth or the sun, I would say that in this case, a diamond really is forever," Keith said.
The diamond planet was found as part of an ongoing search for pulsating stars, known as pulsars, which scientists like to use as probes.