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Illustration of a carbon-rich planet with diamonds and silica as the main minerals. Water can turn a carbide planet into a diamond rich planet. Inside, the main minerals would be diamond and silica (a layer with crystals in the illustration). The core (dark blue) can be an alloy of iron and carbon. Credit: Shim / ASU / Vecteezy
Like missions like POTit is hubble space telescope, TESS, and Kepler continues to provide information on the properties of exoplanets (planets around other stars), scientists are increasingly able to piece together what these planets look like, what they are made of, and whether they could be habitable or even inhabited.
In a new study recently published in The Journal of Planetary Sciences, a team of researchers from Arizona State University and the Chicago University have determined that some carbon-rich exoplanets, given the right circumstances, could be made of diamonds and silica.
“These exoplanets are unlike anything in our solar system,” said lead author Harrison Allen-Sutter of ASU’s School of Earth and Space Exploration.
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
Diamond exoplanet formation
When stars and planets form, they do so from the same gas cloud, so their overall composition is similar. A star with a lower carbon to oxygen ratio will have planets like Earth, made up 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 rich in carbon. 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 turn into diamond and silicate, if water ( which is abundant in the universe) were present, creating a diamond-rich composition.
In a diamond anvil cell, two gem-quality single crystal diamonds are formed on anvils (flat top in photo) and then pitted against each other. The samples are loaded between the culets (flat surfaces), then the sample is compressed between the anvils. Credit: Shim / ASU
Diamond anvils and X-rays
To test this hypothesis, the research team needed to mimic the interior of carbide exoplanets using high temperatures and high pressures. To do so, they used high-pressure diamond anvil cells in the lab of co-author Shim’s Lab for Earth and Planetary Materials.
First, they dipped silicon carbide in water and compressed the sample between diamonds at a very high pressure. Then, to monitor the reaction between silicon carbide and water, they conducted laser heating at Argonne National Laboratory in Illinois, taking X-ray measurements as the laser heated the sample to high pressures.
As predicted, with high heat and pressure, the silicon carbide reacted with water to turn into diamonds and silica.
The cylinder-shaped objects in this photo are diamond anvil cells. Diamond anvil cells are mounted on copper brackets and then inserted into the synchrotron’s laser / X-ray path. The photo shows the diamond anvil cells and holders before aligning them for X-ray / laser experiments. Credit: Shim / ASU
Habitability and habitability
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 correct properties and the correct location around their stars where life could exist.
However, for the carbon-rich planets that are the focus of this study, they likely do not have the properties necessary for life.
While the Earth is geologically active (an indicator of habitability), the results of this study show that carbon-rich planets are too difficult to be geologically active and this lack of geological activity can make the atmospheric composition uninhabitable. Atmospheres are critical to life, providing 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 an additional step to help us understand and characterize our ever-increasing and improved observations of exoplanets,” Allen-Sutter said. “The more we learn, the better we can interpret new data from upcoming future missions such as the James Webb Space Telescope and the Roman Nancy Grace Space Telescope to understand worlds beyond our own solar system. “
Reference: “Oxidation of the interior of carbide exoplanets” by H. Allen-Sutter, E. Garhart, K. Leinenweber, V. Prakapenka, E. Greenberg and S.-H. Shim, August 26, 2020, The Journal of Planetary Sciences.
DOI: 10.3847 / PSJ / abaa3e
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