Astronomers studying gas giant WASP-12b with NASA's Spitzer Space Telescope have found it loaded with an elevated amount of carbon, perhaps in the form of diamond or graphite in its interior.

The 1.4 Jupiter-mass planet was originally found last year by the UK's Wide-Angle Search for Planets (WASP) project, but further investigation with the space-based Spitzer telescope revealed the planet's unusual composition, finding it dominated by carbon and depleted of oxygen. The measurements were made when the planet passed behind its star, a technique known as secondary eclipse, and the data combined with previously published observations obtained with the ground-based Canada-France-Hawaii Telescope at Mauna Kea, Hawaii.

Methane and carbon monoxide were also found in the planet’s atmosphere, and the science team speculate that WASP-12b could even harbour an interior of graphite or diamond, or perhaps an exotic form of carbon.

“This planet reveals the astounding diversity of worlds out there,” says Nikku Madhusudhan of the Massachusetts Institute of Technology. “Carbon-rich planets would be exotic in every way – formation, interiors and atmospheres.”

On Earth, carbon is a key ingredient for life, but there is much more oxygen than carbon – our Sun has a carbon-to-oxygen ratio of about one to two, which means it has about twice as much oxygen than carbon. On WASP-12b the situation is reversed; it is the first planet ever to have its carbon-to-oxygen ratio measured at greater than one.

“When the relative amount of carbon gets that high, it’s as though you flip a switch, and everything changes,” says Marc Kuchner of NASA's Goddard Space Flight Center, who helped develop the theory of carbon-rich rocky planets but is not associated with this study. “If something like this had happened on Earth, your expensive engagement ring would be made of glass, which would be rare, and the mountains would all be made of diamonds.”

Although WASP-12b is a hot gas planet like Jupiter, smaller rocky planets could also form in such planetary systems. “The theorists will have fun with this one,” says Professor Joe Harrington of the University of Central Florida. Could life thrive in such an environment, with little oxygen or water? That might not be so far-fetched given last week’s announcement by NASA of bacteria that can survive by using arsenic in place of phosphorus, previously thought to be essential”.

WASP-12b is located roughly 1,200 light-years away from Earth, orbiting its star in just over a day, with one face permanently locked towards the star. It is one of the hottest exoplanets known to exist, with temperatures reaching a scorching 2,600 kelvin on its sun-facing side.

The discovery is reported in this week's edition of Nature.