Rocky exoplanets turn out to harbor rocks that we don’t know in our solar system

It seems that rocky exoplanets are even stranger than thought.

Meanwhile, astronomers have already discovered thousands of planets orbiting other stars in our galaxy. What researchers often don’t know, however, is what these planets are made of. For example, do they resemble the Earth, or do they have a completely different composition? Researchers have studied the chemical makeup of so-called “polluted” white dwarfs in a new study. And that leads to the surprising conclusion that most of the rocky exoplanets orbiting nearby stars are much more exotic than thought.

Dirty White Dwarfs

As you may know, a white dwarf is a star that has reached the end of its life. They are actually dense, collapsed cores of once very normal stars. Our own sun will also transform into a white dwarf in five billion years. However, many white dwarfs in the Milky Way galaxy appear to be ‘contaminated’ with strange material, from for example planets, asteroids or other rocky celestial bodies that once orbited that star. This material polluted the white dwarf’s atmosphere as it died, drawing in everything that circled it. That is why we call these phenomena ‘polluted’ white dwarfs.

Study

In the new study, the researchers came up with a good idea. Because by looking for elements not naturally found in a white dwarf’s atmosphere—anything but hydrogen and helium—scientists can figure out what made up the rocky planetary objects that were drawn toward the dying star. The team studied 23 polluted white dwarfs, all about 650 light-years away from the sun. Previous studies had already revealed that these white dwarfs contain trace amounts of calcium, silicon, magnesium and iron. The researchers then used the measured abundances of those elements to reconstruct the original minerals and rocks from which the elements may have originated.

Exotic

It leads to a surprising discovery. Because the polluted white dwarfs appear to have a wide range of chemical compositions. This suggests that the exoplanets that once orbited the white dwarfs were made up of many different rocks. And some rocks are completely foreign to us themselves. It means the researchers had to come up with new names to classify the new rock types. “Although some exoplanets that once orbited polluted white dwarfs resemble Earth, most of the rocks we don’t know in our solar system are home to,” said study researcher Siyi Xu.

Stranger than expected

So it seems that rocky planets may be even stranger than thought. Because apparently the lion’s share consists of rocks that do not occur anywhere in our solar system. This can then also have consequences for the rocky worlds in question. “Some rocks are already melting at much lower temperatures or producing thicker crusts than Earth-like rocks,” explains Keith Putirka. “Some rocks can also be softer, which in turn influences the development of plate tectonics.”

Crust

Furthermore, in their study, the researchers have in some cases even managed to decipher where the detected rocky debris must have come from. High concentrations of magnesium and little silicon measured in the atmospheres of polluted white dwarfs suggest that the detected rocky material comes from a planet’s interior. This means that it originates in the mantle and thus does not come from the crust. And that’s interesting. In previous studies of polluted white dwarfs, the researchers thought they had spotted traces of continental crusts. But Putirka and Xu found no evidence of this, although the new observations also do not rule out the possibility that the exoplanets did not have a crust. “We just think that if they had a crust, we can’t see the traces of it now,” Putirka explains. “In that case, it would be a very small fraction compared to the mass of other planetary components, such as the core and the mantle.”

All in all, we are beginning to learn more and more about other exotic and unusual exoplanets that are outside our solar system. And that even the rocky specimens can look very different from the Earth, seems conclusively proven with this study. Apparently there are many more types of stone than we think. And we will probably learn more about that in the future. The upcoming James Webb telescope, due to launch in December, will be able to study stars and exoplanets in even more detail. And then we may learn a lot more about the extraordinary diversity that our universe is rich in.