This super-Earth is a diamond planet

Posted by Trent Nouveau

Scientists now believe that a rocky planet twice Earth's size orbiting a nearby star can actually be classified as a diamond planet.

"This is our first glimpse of a rocky world with a fundamentally different chemistry from Earth," explained Nikku Madhusudhan, a Yale postdoctoral researcher in physics and astronomy. "The surface of this planet is likely covered in graphite and diamond rather than water and granite."



According to Madhusudhan, 55 Cancri e boasts a radius twice the size of Earth and a mass eight times greater - essentially making the planet a "super-Earth."

Interestingly, it is one of five planets orbiting a sun-like star known as 55 Cancri which is located some 40 light years from Earth - yet is also visible to the naked eye in the constellation of Cancer.



"The planet orbits at hyper speed - its year lasts just 18 hours, in contrast to Earth's 365 days," said Madhusudhan. "It is also blazingly hot, with a temperature of about 3,900 degrees Fahrenheit, a far cry from a habitable world."

55 Cancri e was first observed transiting its star last year, allowing astronomers to accurately measure its radius. This new data, combined with the most recent estimate of its mass, permitted Madhusudhan and colleagues to decipher its chemical composition using models of its interior, while computing all possible combinations of elements and compounds that would yield those specific characteristics.

Astronomers had previously hypothesized that the host star is loaded with more carbon than oxygen, and Madhusudhan and colleagues confirmed that substantial amounts of carbon and silicon carbide, along with a negligible amount of water ice, were available during the planet's formation.

Astronomers also originally believed 55 Cancri e contained a substantial amount of super-heated water, based on the assumption that its chemical makeup was similar to Earth's. 

However, the new research suggests the planet has no water at all, and appears to be composed primarily of carbon (as graphite and diamond), 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.

"In contrast, Earth's interior is rich in oxygen, but extremely poor in carbon - less than a part in thousand by mass," noted Yale geophysicist Kanani Lee.



According to Madhusudhan, the identification of a carbon-rich super-Earth means that distant rocky planets can no longer be assumed to have chemical constituents, interiors, atmospheres, or biologies similar to those of Earth.



Indeed, the discovery also opens new avenues for the study of geochemistry and geophysical processes in Earth-sized alien planets. For example, a carbon-rich composition could influence the planet's thermal evolution and plate tectonics, with implications for volcanism, seismic activity and mountain formation.

"Stars are simple — given a star's mass and age, you know its basic structure and history," added David Spergel, professor of astronomy and chair of astrophysical sciences at Princeton University. 

"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."