Definition
Earth is built from rocks rich in silicon and oxygen, but scientists think some alien planets could form in places unusually rich in carbon. A diamond planet is a hypothetical world where extreme heat and crushing pressure packed that carbon tight, turning a large part of its interior into diamond crystal. No confirmed example exists yet.
Built from different ingredients than Earth
Ever imagined a giant world made of the black graphite in a pencil? If you dug into Earth, you would mostly find silicon and oxygen (the stuff of sand and glass), plus iron. Carbon, the element that makes gems on Earth, is less than 0.1 percent of what Earth is made of. It's very rare here.
But out in the vast universe, there may be a few places where gas and dust clouds hold more carbon than oxygen. Those planetary systems would be built from completely different materials than our solar system. Scientists think carbon planets could be born in places like that, with carbon instead of rock as the main ingredient.
Such a planet would grow with graphite and silicon carbide packed into its body from the start. Think of a castle made not of sand and soil, but of charcoal and black pencil dust layered on top of each other, on a planetary scale.
The biggest pressure cooker in the universe
Lots of carbon doesn't automatically become clear, hard gems. The graphite in a pencil and a sparkling diamond are made of the exact same carbon atoms, but the atoms are linked in totally different ways. Turning thin, slippery layers of graphite into a tough 3D lattice takes a huge amount of energy.
The deeper you go underground, the more weight presses down from above, so pressure and heat keep rising. In Earth's mantle, diamonds form at around 150 kilometers (about 93 miles) down, where pressure reaches tens of thousands of atmospheres. On a carbon planet, carbon below a certain depth could exist as diamond, and a bigger planet could have an even thicker diamond layer.
Under these harsh conditions, carbon atoms can't hold out any longer and grab onto each other tightly, with no gaps left. The idea is that the whole giant body works like a supersized natural gem factory, building a thick diamond mantle inside the planet.
To be more precise
The name "diamond planet" makes it easy to picture a world whose whole surface shines as clear as glass beads. But the real look would be nothing like a jewelry store window.
The surface is thought to be covered in dark red lava, coal-black graphite dust, and sticky tar-like material. To see the sparkle, you'd have to drill down hundreds of kilometers below the surface before reaching a hard layer of crystalline diamond.
One planet once seen as a leading candidate is 55 Cancri e. It hugs its star so closely that its year lasts less than a day. In theory its day side was expected to top 2,000 degrees, but 2024 observations from the James Webb Space Telescope found about 1,500 degrees, lower than predicted. Researchers think a thick atmosphere rich in carbon monoxide or carbon dioxide may be carrying heat around the planet. So it's less a romantic jewel world and more a rough alien place shaped by extreme physics.
Common misconceptions
A diamond planet is a gem world that sparkles clear and bright from the surface down.
The surface is thought to be covered in graphite dust or dark hydrocarbon material. Diamond would form in the deep mantle, under intense heat and pressure.
Where you meet it
55 Cancri e, an exoplanet about 40 light-years from Earth, drew attention as a diamond planet candidate after a 2012 study suggested its interior might be carbon-rich. Later observations raised doubts.
Deep inside Uranus and Neptune in our own solar system, some studies suggest methane is squeezed so hard that it produces diamond rain.
In one sentence
A diamond planet is a hypothetical world rich in carbon, where extreme gravity and pressure may have built a giant layer of diamond inside.