A rocky planet discovered by astronomers in 2004 is made of diamonds, new research suggests. A thick layer of the precious stone lies just beneath its surface and makes up a third of its rocky mass. 

The planet, which is twice the diameter of the Earth, orbits a star that lies only 40 light-years away in the constellation of Cancer, the Crab.

It is one of five alien worlds found going round the Sun-like star, which is known as 55 Cancri and can be seen with the naked eye in the night sky.

The planet, dubbed 55 Cancri e, is a rocky world eight times denser than our own, known as a “super-Earth”.

It zips around the star so quickly that its “year” lasts only 18 hours compared to the Earth’s 365 days. But it is unlikely to be inhabited because it has a surface temperature of a searing 2,150 C (3,900 F).

The astronomers are unable to see the surface of 55 Cancri e directly. But it was observed passing in front of its star in an event called a transit last year, which allowed them to measure its diameter.

By combining that figure with the best estimates of its mass, they were able to use computer models to work out its chemical composition, or what it was likely to be made of.

Leader of the research team Dr Nikku Madhusudhan, of Yale University, Connecticut, said: “This is our first glimpse of a rocky world with a fundamentally different chemistry from Earth. The surface of this planet is likely covered in graphite and diamond rather than water and granite.”

The discovery, reported jointly with Olivier Mousis, a planetary scientist at the Institut de Recherche en Astrophysique et Planetologie in Toulouse, France, will appear in the journal Astrophysical Journal Letters.

Astronomers had previously discovered that the planet’s home star contains more carbon than oxygen, and Madhusudhan and his colleagues confirmed that substantial amounts of carbon and silicon carbide, but very little water ice, were available when the planet was forming.

It had been thought that 55 Cancri e contained a substantial amount of super-heated water, based on the assumption that it had a similar chemical makeup to Earth’s.

But the new research suggests the planet has no water at all. Instead it appears to be made mainly of carbon, in the form of graphite and diamond, plus iron, silicon carbide, and, possibly, some silicates. The study estimates that at least a third of the planet’s mass — the equivalent of about three Earth masses — could be diamond.

Co-author and Yale geophysicist Kanani Lee said: “By contrast, Earth’s interior is rich in oxygen, but extremely poor in carbon — less than a part in thousand by mass.”