Diamond planets could be born from carbon-rich worlds – just add water

Diamonds are relatively rare here on Earth, but perhaps on other planets they’d be so common they’d be as worthless as a handful of dirt. Astronomers suggest that some planets might actually be largely made of diamonds, and now a team has calculated how such a planet could form and how it would be structured.

As varied as the planets in our solar system are, they don’t come close to capturing the full range of weird worlds that the universe can conjure up. There are rogue planets drifting around untethered to stars, planets with ruby and sapphire clouds, scorching worlds where it rains molten iron and, of course, planets that may be mostly made of diamond.

And it’s the last one on that list that was the focus of the new study by astronomers from Arizona State University and the University of Chicago. The team hypothesized that exoplanets that form in systems with a high carbon-to-oxygen ratio could end up with compositions rich in diamond, with the right amount of water, heat and pressure.

To test the idea, the researchers used high-pressure diamond-anvil cells to simulate the conditions these exoplanets might form under. They immersed silicon carbide in water, then squeezed the sample in the instrument to very high pressures. Then they heated the sample with a laser and took X-ray measurements to study the reaction. And sure enough, the silicon carbide reacted with the water and formed diamonds and silica.

A close-up of the pressing surfaces of the diamond-anvil cell
A close-up of the pressing surfaces of the diamond-anvil cell

Dan Shim/ASU

Next they simulated how this would work on a planetary scale. The team says that a carbon-rich planet with no water would start off with a core made of an iron-carbon alloy, wrapped in a silicon carbide mantle and topped off with a thin layer of graphite covering the planet’s surface.

Once water was added, through asteroid collisions for example, the composition would change. A thick layer of diamond and silica would form in the upper mantle above the silicon carbide, while the crust would be made of silicate instead.

Before you go dreaming of alien life thriving on these diamond-dominated worlds, the team says that such planets are unlikely to be habitable. That’s because, unlike Earth’s liquid mantle, a diamond one would be too hard to be geologically active. That in turn wouldn’t be conducive to a habitable atmosphere.

“Regardless of habitability, this is one additional step in helping us understand and characterize our ever-increasing and improving observations of exoplanets,” says Harrison Allen-Sutter, lead author of the study. “The more we learn, the better we’ll be able to interpret new data from upcoming future missions like the James Webb Space Telescope and the Nancy Grace Roman Space Telescope to understand the worlds beyond our own solar system.”

The research was published in the Planetary Science Journal.

Source: Arizona State University

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