Every so often, a headline announces that astronomers have found a planet made of diamond. It's an irresistible idea: a whole world of glittering gemstone floating in space. But is it real, or just a sparkly bit of wishful thinking?
The honest answer is that no planet has been confirmed to be made of diamond. Scientists have proposed a type of world called a carbon planet, where intense pressure deep inside could squeeze carbon into diamond. A couple of real planets have been suggested as candidates, but the evidence is still uncertain, and some researchers doubt carbon planets are common at all.
What a Carbon Planet Would Be Like
Earth, Mars, and Venus are built mostly from compounds of silicon and oxygen. A carbon planet would be different, with more carbon than oxygen. The term was coined in 2005 by astronomers Marc Kuchner and Sara Seager.
Such a world would probably have an iron-rich core. Above that, scientists imagine layers of carbon, possibly including a diamond layer kilometers thick if the pressure is high enough. Volcanoes might even push diamonds up to the surface. It wouldn't be a pretty place, though. The surface could be covered in tar and methane, with almost no water, under a sky full of carbon smog.
The Most Famous Candidate
The world most often called a diamond planet is 55 Cancri e, officially named Janssen. It orbits a Sun-like star about 41 light-years away. It has about eight times Earth's mass and roughly twice its width, and it whips around its star in less than 18 hours. Its day side tops 3,000 kelvins, or more than 4,900 degrees Fahrenheit.
Some scientists proposed that it could be a solid, carbon-rich world. In that case, about a third of its mass would be carbon, much of it possibly in the form of diamond. But they also stressed that more observations were needed. In 2024, the James Webb Space Telescope found evidence of a thick atmosphere rich in carbon dioxide or carbon monoxide. The planet's true makeup is still being worked out.
The Strange Case of the Pulsar Companion
An even odder candidate was announced in 2011. Called PSR J1719−1438 b, it orbits a pulsar, the tiny, spinning remains of an exploded star, about 4,000 light-years away. It's slightly more massive than Jupiter and was the densest planet known when it was found.
Scientists think it is made largely of carbon and oxygen, and the crushing pressure suggests the carbon has crystallized, much like diamond. But this may not be a true planet at all. It's believed to be the leftover core of a star that had most of its mass stripped away by the pulsar.
Diamonds Really Are Out There
Even if a diamond planet remains unproven, space is surprisingly full of diamonds. Tiny diamonds, each containing only about 2,000 carbon atoms, are common in meteorites, and some formed in stars before our solar system existed.
Closer to home, high-pressure experiments suggest large amounts of diamond may form from methane deep inside Uranus and Neptune. And a dead star called BPM 37093, about 2,500 miles across, may have a diamond core, earning it the nickname Lucy.
So the next time you see a headline about a diamond world, enjoy the sparkle, but keep the question mark. The universe may well be hiding a gem-filled planet. Astronomers just haven't proven it yet.
