New research led by Yale University scientists suggests that a rocky planet twice Earth's size orbiting a nearby star is a diamond planet. "This is our first glimpse of a rocky world with a fundamentally different chemistry from Earth," said lead researcher 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 granite¹（花岗岩）."

 

The paper reporting the findings has been accepted for publication in the journal Astrophysical Journal Letters.

 

The planet -- called 55 Cancri e -- has a radius² twice Earth's, and a mass eight times greater, making it a "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 constellation³ of Cancer（巨蟹座）.

 

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 Fahrenheit⁴, researchers said, a far cry from a habitable world.

 

The planet was first observed transiting⁵ its star last year, allowing astronomers⁶ to measure its radius for the first time. This new information, combined with the most recent estimate of its mass, allowed Madhusudhan and colleagues to infer its chemical composition using models of its interior and computing⁷ all possible combinations of elements and compounds that would yield those specific characteristics.

 

Astronomers had previously⁸ reported that the host star has more carbon than oxygen, and Madhusudhan and colleagues confirmed that substantial amounts of carbon and silicon⁹ 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 makeup¹⁰ was similar to Earth's, 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 silicates¹¹. 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," says co-author and Yale geophysicist Kanani Lee.

 

The identification of a carbon-rich super-Earth means that distant rocky planets can no longer be assumed to have chemical constituents¹², interiors, atmospheres, or biologies similar to those of Earth, 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 thermal¹³ evolution and plate tectonics, for example, with implications for volcanism, seismic¹⁴ activity, and mountain formation.

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