55 Cancri e: The Planet That Could Be a Diamond

Forty light-years away, in the constellation Cancer, a planet completes a full year every eighteen hours. Its surface is hot enough to melt iron. And as much as a third of its mass may be diamond.

55 Cancri e is a super-Earth: about twice our planet's diameter and eight times its mass. The star it orbits is called 55 Cancri A, and you can see that star from your backyard. It's faint, but on a clear, dark night, between Gemini and Leo, it's there with the naked eye. The planet itself is far too small and too close to its star to see. But the star is visible. And whatever is happening on that planet, you can hold its star in your gaze right now.

What's happening on it is hard to picture.

A year on 55 Cancri e lasts eighteen hours. The planet sits so close to its star that its entire orbit would fit well inside Mercury's. The dayside temperature is around 3,900 degrees Fahrenheit, roughly five times hotter than Venus, the hottest world in our own solar system. The planet is almost certainly tidally locked, meaning one face is permanently aimed at its star and the other permanently dark. The bright side is an ocean of molten rock. The dark side, nobody is sure.

A different recipe

To understand what might be hiding inside 55 Cancri e, start with Venus.

Venus is the closest thing our solar system has to a hot, hostile, rocky world. It's built from the same materials as Earth: an iron core, a silicate mantle, a basalt crust. Silicates are minerals where silicon and oxygen sit at the heart of nearly every molecule. Oxygen is the abundant ingredient. There is more of it floating around in the leftover dust of our solar system than there is carbon. So our rocky planets are oxygen-rich. They are made of rock.

Now imagine a star that formed somewhere different. A cloud of dust where the ratio was flipped, where carbon was the abundant ingredient and oxygen the scarce one. The planets condensing out of that dust wouldn't be built from silicates. They'd be built from carbon compounds. And carbon, when it is squeezed hard enough by the weight of a planet sitting above it, doesn't stay as graphite or coal. It crystallises.

It's a process we've already caught at work inside our own solar system. Deep inside Neptune and Uranus, pressure breaks methane apart and locks the carbon atoms into tiny crystals. They sink through the planet's interior for thousands of years, a slow rain of diamond falling through the dark. On 55 Cancri e, scale that principle up to a hot, dense, carbon-rich super-Earth and you don't get weather. You get architecture. A whole layer of the planet built from crystallised carbon.

A team led by the astronomer Nikku Madhusudhan ran the numbers in 2012 and proposed exactly this. They estimated that as much as a third of the planet's mass could be diamond, sitting in a layer beneath a graphite crust, above a mantle of silicon carbide and a core of iron.

A third of the mass. Roughly three Earths' worth of diamond, locked inside one planet.

The catch

Other studies have walked some of this back. To make the carbon-rich planet idea work, the host star itself needs to have more carbon than oxygen by a clear margin. Different teams, looking at the same star with different methods, have reached different answers about that ratio. Some say it's high enough. Others say it isn't. In 2024, the James Webb Space Telescope found that the planet has a thick atmosphere of carbon dioxide or carbon monoxide. Plenty of carbon, but possibly not enough excess for the full diamond model.

So 55 Cancri e might be a partly diamond world. It might have a narrow diamond layer buried deep beneath something else. It might turn out to be stranger again. Twenty years of debate, and we still don't quite know.

What we do know is that planets like this should exist somewhere. The chemistry isn't speculation. The temperatures and pressures inside super-Earths are more than enough to forge diamond from carbon. The only real question is whether 55 Cancri e is the one we found first.

The idea that whole worlds can be built from a different recipe than our own isn't theoretical anymore. In 2025, a comet drifted through our solar system that had formed around a different star, carrying chemistry no local object should have. Different star, different starting ingredients, different chemistry. We have a sample now.

55 Cancri e, if the early models are right, is what that kind of chemistry looks like when you scale it up to a planet.

That's the strange part to sit with. Somewhere out there, possibly visible from your back garden tonight, is a planet that may be part jewel. We haven't seen it. We've calculated it. And the calculation could be right.