Forget the diamond as big as the Ritz - astronomers believe they have found one bigger than Earth.

Orbiting a star that is visible to the naked eye, astronomers have discovered a planet twice the size of our own made largely out of diamond.

The rocky planet, called 55 Cancri e, orbits a sun-like star in the constellation of Cancer and is moving so fast that a year there lasts a mere 18 hours.

Discovered by a US-Franco research team, its radius is twice that of Earth's but it is much more dense with a mass eight times greater.

It is also incredibly hot, with temperatures on its surface reaching 1,648 degrees Celsius.

"The surface of this planet is likely covered in graphite and diamond rather than water and granite," said Yale researcher Nikku Madhusudhan, whose findings are due to be published in the journal Astrophysical Journal Letters.

The study - with Olivier Mousis at the Institut de Recherche en Astrophysique et Planetologie in Toulose, France - estimates that at least one-third of the planet's mass, the equivalent of about three Earth masses, could be diamond.

Diamond planets have been spotted before but this is the first time one has been seen orbiting a sun-like star and studied in such detail.

"This is our first glimpse of a rocky world with a fundamentally different chemistry from Earth," Mr Madhusudhan said.

He said the discovery of the carbon-rich planet meant distant rocky planets could no longer be assumed to have chemical constituents, interiors, atmospheres or biologies similar to Earth.

David Spergel, an astronomer at Princeton University, said it was relatively simple to work out the basic structure and history of a star once you know its mass and age.

"Planets are much more complex," he said.

"This 'diamond-rich super-Earth' is likely just one example of the rich sets of discoveries that await us as we begin to explore planets around nearby stars."

55 Cancri e is about 40 light years from Earth.

Reuters