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Back in the 1920s, the great Jazz Age writer F. Scott Fitzgerald published a story titled The Diamond as Big as the Ritz — the Ritz-Carlton hotel, that is, which even in those pre-high-rise days was a pretty hefty chunk of real estate. Fitzgerald wasn't a science-fiction writer, so he didn't have to explain how such a thing could possibly exist. Lucky thing too since it couldn't. Not on earth, anyway.

But the universe is a vast and strange place, where all sorts of seemingly impossible things happen routinely. Still, a paper just published in Science seems to teeter on the edge of utter fantasy: 20 quadrillion miles away lies a star more massive than the sun but only 15 miles across, spinning around more than 100 times a second — and orbiting that star is a diamond the size, not of a mere luxury hotel, but of the planet Jupiter. Oh, and the diamond used to be a star too, before it turned into a planet.

Surprisingly, perhaps, most of that story is old news to astronomers. The fast-spinning star is a pulsar, a superdense chunk of matter left over when a massive star explodes, then collapses in on itself. If what's left over weighs more than three times as much as the sun, it collapses forever, forming a black hole. But if it's a bit smaller, it turns into a whirling neutron star whose intense magnetic field generates a beacon of radio waves that sweeps across the universe like the beam of a lighthouse — in this case, flashing more than 10,000 times every minute. When pulsars, as they became named, were discovered in the 1960s, they were nicknamed LGM for "little green men." Nobody could imagine a natural force that could generate such a rapid, precisely timed series of radio blasts.

The natural explanation wasn't long in coming though, and astronomers have since found hundreds upon hundreds of pulsars. They've also found that slight variations in the timing of the pulses can be indirect evidence for objects orbiting a pulsar. The gravitational pull of, say, a planet, will make the radio flashes arrive closer together, then farther apart, then closer, in a regularly changing rhythm. In fact, the first planets ever discovered beyond our solar system were found this way in 1992, but thanks to the intense radiation coming off a pulsar, there's no chance life could exist on them.

All of this has long since become a standard part of astronomy textbooks. What's not standard at all is the idea of a star actually turning into a planet — but that, says Matthew Bailes, astronomer at Australia's Swinburne University of Technology and lead author of the Science paper, isn't as crazy as it sounds. He suggests that originally this odd couple was a pair of ordinary stars orbiting each other. One was more or less like the sun, the other perhaps 10 times as massive. The bigger star exploded, leaving behind a neutron star. Meanwhile, the sunlike star aged in the normal way, eventually swelling, blowing off its outer layers and collapsing to the white hot ember known as a white dwarf star. That's just what will happen to our sun in 5 billion years or so.

But the sun isn't right next to a neutron star. If it were, its outer gas layers would have been sucked in by the fierce gravity of its companion. The sun itself — or its white dwarf corpse, anyway — could have spiraled in as well, eventually coming in so close that its "year" was only a little more than two hours long. The neutron star's gravity would now be so powerful that the white dwarf star would lose even more layers, leaving behind only its inner core — about the mass of Jupiter and most likely made largely of oxygen and carbon, two elements that are forged in the nuclear fires at the heart of an aging star.

Bailes and his team couldn't actually detect the carbon or oxygen, but given the mass of the "planet" and their understanding of the lifecycle of stars, there's not much else it could be. And because a Jupiter's worth of carbon would have a pretty powerful gravity of its own, it would almost certainly have crushed itself into crystalline form — in other words, diamond. "We can't uniquely say what percentage of the planet would be diamond," says Bailes, since the details of the process aren't absolutely clear. But it would likely be a lot.

Purists might raise their eyebrows at calling the nameless object a planet, given that it once looked just like the sun. But when the International Astronomical Union created its formal definition of the word planet in 2006 — and demoted Pluto by doing it — there was nothing in the fine print about how the object had formed. It's clearly not a star now, and it's about the mass of Jupiter. If it looks like a planet (albeit a very weird one) and acts like a planet, then it probably should be called a planet.

The discovery also raises the question of whether there are other diamond planets studding the Milky Way like jewels on a tiara. And the answer, says Bailes, is an absolute "maybe." "This is the only one like it so far," he adds. But the find was part of a major international search designed to look for and study pulsars across the sky — and that search is far from over.

• Michael Lemonick
• Astronomers have discovered that some dying stars can collapse in on themselves -- transforming into giant diamonds