Carbon-rich planets made of diamonds may exist beyond our solar system, study says

Exoplanets, or planets outside of our solar system, that contain more carbon than Earth could be made out of diamonds, according to a new study.

Data provided by NASA’s planet-hunting TESS mission, the Hubble Space Telescope and retired Kepler mission have shown that our galaxy is filled with exoplanets that are very different from the planets in our own solar system.

As the NASA Exoplanet Travel Bureau posters created by artists collaborating with planetary scientists point out, these weird and wonderful exoplanets are filled with things that are hard to imagine. Some are so hot that they host oceans of lava with sparkly silicate skies while others orbit two suns like planets from Star Wars. Others are rogue planets with no host star.

Planets form from a disk of gas and dust around a star, called the circumstellar disk. This material was originally used to form the star, and its leftovers form planets that then orbit the star. This means that the elements included in the planets are unique to their stars.

This illustration shows a night-side view of the exoplanet WASP-76b. The ultra-hot giant exoplanet has a day side where temperatures climb above 2400 degrees Celsius, high enough to vaporise metals. Strong winds carry iron vapour to the cooler night side where it condenses into iron droplets. To the left of the image, we see the evening border of the exoplanet, where it transitions from day to night.

Iron rain could be falling on this hot exoplanet

The sun has a lower carbon-to-oxygen ration, so Earth has silicates and oxides, or oxygen and silica compounds bound with other elements, and only about 0.001% of diamonds.

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But the exoplanets found around stars with a higher ration of carbon to oxygen would have more carbon content. Some of these exoplanets containing more carbon could actually be composed of diamonds and silica if water is present. Silica is a natural compound in Earth’s crust, rocks, sand and clay.

“These exoplanets are unlike anything in our solar system,” said Harrison Allen-Sutter, lead study author and graduate associate in Arizona State University’s School of Earth and Space Exploration, in a statement.

The study published last week in The Planetary Science Journal.

Turning carbon into diamonds

The research team, including scientists from the University of Chicago, tested their hypothesis by simulating how the interiors of carbon-rich exoplanets could create diamonds through high levels of heat and pressure.

High-pressure diamond anvil cells were used at study coauthor Dan Shim’s Lab for Earth and Planetary Materials at Arizona State University.

This illustrations shows hydrocarbon compounds split into carbon and hydrogen inside ice giants, such as Neptune, which turns into a "diamond [rain] shower."

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A diamond anvil cell is a high-pressure device used in geology to subject a small amount of material to extreme pressure.

In a diamond-anvil cell, two gem quality single crystal diamonds are shaped into anvils. Samples are loaded between the culets and compressed between the anvils.

The scientists placed silicon carbide, which includes silicon and carbon, in water. Silicon carbide can be found in the rare mineral moissanite.

This sample was compressed between diamonds at high pressure.

A laser at the Argonne National Laboratory in Illinois was used to heat the samples. The researchers took X-ray measurements of the laser heating to determine the reaction between the silicon carbide and water.

The heat and pressure caused the silicon carbide to react with the water, resulting in the creation of diamonds and silica.

The quest for exoplanets

Although there is nothing like this in our solar system, scientists hypothesized in a 2017 study that high-pressure conditions on Uranus and Neptune could squeeze hydrogen and carbon together, creating diamond rain.

Much of the intrigue around finding exoplanets is the quest to find evidence of life on another planet. But carbon-rich planets are not likely to support life, according to the researchers.

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Earth is geologically active, one of the aspects that contributes to its habitability.

For instance, Earth is the only known planet to have plate tectonics, which are crucial to every aspect of our planet. The world’s oceans and continents sit on 15 different blocks of crust that move and shift.

An unaltered carbon planet (left) with a silicon carbide-dominated mantle can transform into a silica and diamond-dominated mantle (right).

While we largely think of them as responsible for creating mountains and earthquakes, the movement of these plates also contributed to creating environments with the right conditions to support life, chemically and physically. When these plates move, they not only form ocean basins and mountain ranges. The plates also expose different rocks to Earth’s atmosphere, which releases chemicals and causes reactions.

An artistic cross-section through forming crust approximately 3 billion to 4 billion years ago. The presence or absence of plate tectonics during this time is a topic of vigorous scientific debate. A study led by Harvard geologists has found evidence that the crust moved quickly over Earth's surface in the deep past, a hallmark of modern plate tectonics. This suggests that plate motion could have been a meaningful process in early Earth history. (Alec Brenner/Harvard University)

Tectonic plates helped early Earth evolve 3.2 billion years ago, and that shaped how life developed

These reactions likely helped stabilize Earth’s surface temperature over a time period of billions of years, and that allowed life to evolve.

But planets rich in carbon would be too hard to have geological activity, making the composition of its atmosphere hostile to life.

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

“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 on our own solar system.”