“Carbon super-Earths” – giant terrestrial planets that contain up to 50% diamond – may possibly exist in our galaxy, a new study has suggested. But if they exist, those planets are likely to be devoid of life.
The finding comes from a laboratory experiment at Ohio State University, where researchers recreated the temperatures and pressures of Earth’s lower mantle to study how diamonds form there.
Wendy Panero, associate professor in the School of Earth Sciences at Ohio State, and doctoral student Cayman Unterborn, used what they learned from the experiments to construct computer models of the minerals that form in planets composed with more carbon than Earth.
“It’s possible for planets that are as big as fifteen times the mass of the Earth to be half made of diamond,” Unterborn said.
“Our results are striking, in that they suggest carbon-rich planets can form with a core and a mantle, just as Earth did,” Panero added.
“However, the cores would likely be very carbon-rich – much like steel – and the mantle would also be dominated by carbon, much in the form of diamond.”
Diamonds transfer heat so readily, however, that a carbon super-Earth’s interior would quickly freeze. That means no geothermal energy, no plate tectonics, and – ultimately – no magnetic field or atmosphere.
“We think a diamond planet must be a very cold, dark place,” Panero said.
She and former graduate student Jason Kabbes subjected a tiny sample of iron, carbon, and oxygen to pressures of 65 gigapascals and temperatures of 2,400 Kelvin (close to 9.5 million pounds per square inch and 3,800 degrees Fahrenheit – conditions similar to the Earth’s deep interior).
As they watched under the microscope, the oxygen bonded with the iron, creating iron oxide – a type of rust – and left behind pockets of pure carbon, which became diamond.
Based on the data from that test, the researchers made computer models of Earth’s interior, and verified what geologists have long suspected – that a diamond-rich layer likely exists in Earth’s lower mantle, just above the core.
That result wasn’t surprising. But when they modelled what would happen when these results were applied to the composition of a carbon super-Earth, they found that the planet could become very large, with iron and carbon merged to form a kind of carbon steel in the core, and vast quantities of pure carbon in the mantle in the form of diamond.