55 Cancri E: Diamond Planet 40 Light Years Away

Yale researchers have discovered that a rocky super-Earth orbiting a nearby star is a diamond planet.

This illustration shows the interior of 55 Cancri e, an extremely hot planet with a surface of mostly graphite surrounding a thick layer of diamond, below which is a layer of silicon-based minerals and a molten iron core at the center (Haven Giguere)

“This is our first glimpse of a rocky world with a fundamentally different chemistry from Earth,” said Dr Nikku Madhusudhan, a Yale researcher in physics and astronomy and lead author of a paper to be published in the Astrophysical Journal Letters (arXiv.org version).

“The surface of this planet is likely covered in graphite and diamond rather than water and granite.”

The planet, called 55 Cancri e, has a radius twice Earth’s, and a mass eight times greater, making it a so-called super-Earth. It is one of five planets orbiting a sun-like star, 55 Cancri, that is located 40 light years from Earth yet 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. It is also blazingly hot, with a temperature of about 3,900 degrees Fahrenheit, researchers said, a far cry from a habitable world.

55 Cancri e was first observed transiting its star last year, allowing astronomers to measure its radius for the first time. This new information, combined with the most recent estimate of its mass, allowed the Yale team to infer its chemical composition using models of its interior and by computing all possible combinations of elements and compounds that would yield those specific characteristics.

Astronomers had previously reported that the host star has more carbon than oxygen, and the team confirmed that substantial amounts of carbon and silicon carbide, and a negligible amount of water ice, were available during the planet’s formation.

“Astronomers also thought 55 Cancri e contained a substantial amount of super-heated water, based on the assumption that its chemical makeup was similar to Earth’s,” Dr Madhusudhan said.

But 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.

“By contrast, Earth’s interior is rich in oxygen, but extremely poor in carbon — less than a part in thousand by mass,” said co-author Dr Kanani Lee, geophysicist with Yale University.

“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,” Dr Madhusudhan said.

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


Bibliographic information: Nikku Madhusudhan et al. 2012. A Possible Carbon-rich Interior in Super-Earth 55 Cancri e. Accepted for publication in ApJ Letters; arXiv: 1210.2720