October 24, 2021

Many exoplanets in our galaxy are probably made of diamonds and rocks

In the solar system we observe an interesting variety of planets. However, they are determined almost entirely by the composition of our Sun. Because planets, satellites, asteroids, and other cosmic bodies are formed from what is left after the formation of the Sun, their chemistry is thought to be directly related to the star.

However, not all stars are formed from the same material as our Sun. This means that somewhere out there – in the vast expanses of our galaxy – we can expect exoplanets that are extremely different from those in the small solar system.

For example, stars that are richer in carbon than our Sun (ie, have more carbon than oxygen) have probably led to the formation of exoplanets made up mainly of diamonds (with little silicon). As long as the right conditions are observed. And now, after crushing and heating silicon carbide in the laboratory, scientists have proven that such conditions are possible.

“These exoplanets are nothing like space objects in our solar system,” said geophysicist Harrison Allen-Sutter of the School of Earth and Space Research at Arizona State University.

The idea that stars with a higher carbon to oxygen ratio than the Sun could form diamond planets first emerged after the discovery of 55 Cancri, a super Earth orbiting another carbon-rich star about 41 light-years from us.

It later became clear that this star was not as rich in carbon as originally thought, and the idea of ​​diamond planets (at least for 55 Cancri) was abandoned.

However, between 12 and 17 percent of planetary systems could revolve around carbon-rich stars. And given the fact that to date we have identified thousands of exoplanets, the idea of ​​a diamond planet is not so impossible.

Scientists have already confirmed the idea that such planets are most likely formed mainly from carbides – mixtures of carbon and other planets. If such a planet is rich in silicon carbide and if there is water to oxidize silicon carbide and convert it to silicon and carbon, then (with enough heat and pressure) carbon could turn into diamond.

To confirm this hypothesis, scientists have resorted to the help of a cell with a diamond anvil – a device that crushes very small samples under extremely high pressure. They take miniature samples of silicon carbide and immerse them in water. They are then placed in a cage with a diamond anvil, which crushes them at a pressure of about 50 gigapascals – approximately half a million times the Earth’s atmospheric pressure at sea level. After the samples were crushed, the team heated them with lasers.

The researchers repeated the experiment 18 times and found that at high temperatures and pressures, their silicon carbide samples reacted to water and the end result was silicon and diamond.

This is how scientists came to the conclusion that at temperatures of 2500 Kelvin and pressures up to 50 gigapascals (and in the presence of water and silicon carbide) the planets could oxidize, which in turn could make their interior composition rich in silicon and diamonds.

The study was published in  The Planetary Science Journal.