Scientists at Yale University said the planet, first observed in 2004, has been determined to consist mostly of carbon in the form of diamond and graphite.
"This is our first glimpse of a rocky world with a fundamentally different chemistry from Earth," lead researcher Nikku Madhusudhan, a postdoctoral researcher in physics and astronomy, said. "The surface of this planet is likely covered in graphite and diamond rather than water and granite."
The planet with a radius twice Earth's and a mass eight times greater is one of five planets orbiting a sun-like star, 55 Cancri, 40 light years from Earth but visible to the naked eye in the constellation of Cancer, a Yale release said Thursday.
At least a third of the planet's mass -- the equivalent of about three Earth masses -- could be diamond, astronomers estimate.