Carbon-RIch Exoplanets May Be Made of Diamonds

Illustration of a carbon-rich planet with diamond and silica as main minerals. Water can convert a carbide planet into a diamond-rich planet. In the interior, the main minerals would be diamond and silica (a layer with crystals in the illustration). The core (dark blue) might be iron-carbon alloy. 

Credit: Shim/ASU/VecteezyDownload Full Image

“These exoplanets are unlike anything in our solar system,” said lead author Harrison Allen-Sutter of ASU’s School of Earth and Space Exploration.
Diamond exoplanet formation

When stars and planets are formed, they do so from the same cloud of gas, so their bulk compositions are similar. A star with a lower carbon-to-oxygen ratio will have planets like Earth, comprised of silicates and oxides with a very small diamond content (Earth’s diamond content is about 0.001%).

But exoplanets around stars with a higher carbon-to-oxygen ratio than our sun are more likely to be carbon-rich. Allen-Sutter and co-authors Emily Garhart, Kurt Leinenweber and Dan Shim of ASU, with Vitali Prakapenka and Eran Greenberg of the University of Chicago, hypothesized that these carbon-rich exoplanets could convert to diamond and silicate, if water (which is abundant in the universe) were present, creating a diamond-rich composition.

Diamond-anvils and X-rays

To test this hypothesis, the research team needed to mimic the interior of carbide exoplanets using high heat and high pressure. To do so, they used high pressure diamond-anvil cells at co-author Shim’s Lab for Earth and Planetary Materials.

First, they immersed silicon carbide in water and compressed the sample between diamonds to a very high pressure. Then, to monitor the reaction between silicon carbide and water, they conducted laser heating at the Argonne National Laboratory in Illinois, taking X-ray measurements while the laser heated the sample at high pressures.

An unaltered carbon planet (left) transforms from a silicon carbide dominated mantle to a silica and diamond dominated mantle (right). The reaction also produces methane and hydrogen

Credit: Harrison/ASU

As they predicted, with high heat and pressure, the silicon carbide reacted with water and turned into diamonds and silica. 

Habitability and inhabitability

So far, we have not found life on other planets, but the search continues. Planetary scientists and astrobiologists are using sophisticated instruments in space and on Earth to find planets with the right properties and the right location around their stars where life could exist.

For carbon-rich planets that are the focus of this study, however, they likely do not have the properties needed for life.

In a diamond-anvil cell, two gem-quality single crystal diamonds are shaped into anvils (flat top in the photo) and then faced towards each other. Samples are loaded between the culets (flat surfaces), then the sample is compressed between the anvils.

 Credit: Shim/ASU 

While Earth is geologically active (an indicator of habitability), the results of this study show that carbon-rich planets are too hard to be geologically active and this lack of geologic activity may make atmospheric composition uninhabitable. Atmospheres are critical for life as it provides us with air to breathe, protection from the harsh environment of space and even pressure to allow for liquid water.

“Regardless of habitability, this is one additional step in helping us understand and characterize our ever-increasing and improving observations of exoplanets,” said Allen-Sutter. “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.”