In fact, this is not the first diamond planet ever discovered, but it is the first found orbiting a sun-like star and whose chemical makeup has been specified.

The discovery means that distant rocky planets can no longer be assumed to have chemical constituents, interiors, atmospheres or biologies similar to those of Earth, said lead researcher Nikku Madhusudhan, a Yale postdoctoral researcher in physics and astronomy.

The planet was first observed last year — but researchers initially assumed it was similar in its chemical make-up to Earth.

It was only after a more detailed analysis that the French-American research team determined the planet, dubbed 55 Cancri e, is vastly different from our own.

The planet "appears to be composed primarily of carbon (as graphite and diamond), iron, silicon carbide, and, possibly, some silicates," the authors wrote in a statement ahead of their findings' publication in the US journal Astrophysical Journal Letters.

"The surface of this planet is likely covered in graphite and diamond rather than water and granite," Madhusudhan noted.

In fact, the planet appears to have no water at all. And as much as a third of the planet's substantial mass could be made of diamond, a super-dense compound of carbon.

In comparison, the Earth's interior is rich in oxygen and very poor in carbon, explained study co-author Kanani Lee of Yale.

The researchers estimated the planet's radius with data collected while it was transiting in front of its star.

That information, combined with an estimate of its mass, was used to model the planet's chemical composition, based on a calculation of just what elements and compounds could result in that specific size and mass.

The planet's orbit around its star is lightning fast — a year lasts just 18 hours. And because it is so close to its star, surface temperatures average 3,900 degrees Fahrenheit (2,148 Celsius), rendering it completely inhospitable to life.

But the planet — just 40 lightyears away from Earth in the Cancer constellation — opens new avenues for studying geochemical and geophysical processes of Earth-sized planets outside our solar system.

The high levels of carbon may have implications on how volcanoes and earthquakes work and how mountains form — and add to the growing body of evidence that planets are far more numerous and varied than initially imagined.

"Stars are simple — given a star's mass and age, you know its basic structure and history," said David Spergel, a Princeton University astronomer.

"Planets are much more complex. 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."