Alien worlds contain minerals like nothing in our solar system, scientists say
There is a lot we don’t know about planets outside of the solar system.
They’re small, dark, and distant, which means we don’t have a lot of detailed information about their makeup. This is especially true for rocky exoplanets, like Earth, Venus, and Mars, the surfaces of which we currently cannot see.
There is, however, a way to peer into the bowels of rocky worlds – and it suggests that some of the minerals they are made of bear no resemblance to the minerals in the solar system. These minerals are so foreign, in fact, that scientists have had to invent new terms to classify them.
The method does this by analyzing the atmosphere of white dwarf stars, which can be âpollutedâ by minerals from planets and asteroids that have fallen into the stars. The study of these destroyed exoplanets is called necroplanetology.
“The polluted white dwarfs reveal greater planetary variety in our solar neighborhood than is currently appreciated, therefore with unique planetary accretion and differentiation paths that have no direct counterparts in our solar system. “, write the researchers in their article.
âThese require new rock classification schemes. “
White dwarfs are what happens to a star like the Sun when it reaches the end of its main sequence lifespan, causing its core to collapse into an ultra-dense glowing object of waste heat. Meanwhile, its outer skin stretches across its solar system in the form of a vast bubble of hot gases.
Surprisingly, exoplanets can survive this process – but their orbits can change, become unstable, causing tidal disturbance (this is when the star’s gravitational field separates the exoplanet) and accretion ( when debris from the jagged exoplanet falls on the star).
When this happens, elements of the exoplanets are incorporated into the star, altering the light emitted by the star. Planetologists can then analyze this light, looking for elements that would not normally be found in the atmosphere of a white dwarf, to determine what the rocky bodies were made of. It is the science of necroplanetology.
Geologist Keith Putirka of California State University and astronomer Siyi Xu of the National Science Foundation’s NOIRLab performed such analyzes on 23 white dwarfs, all located within 650 light years of the Sun. For each of these stars, previous observations have shown the presence of elements such as calcium, silicon, magnesium and iron.
Because white dwarfs are so dense, heavier elements like these should not be present in the atmosphere, but attracted inside the star, where they would not be detectable. Their presence suggests a relatively recent accretion of rock material.
Purtika and Xu analyzed the abundance of these elements in the atmospheres of white dwarfs in an attempt to reconstruct the mineral composition of the parent rock bodies. What they found was surprising.
âWhile some exoplanets that once revolved around polluted white dwarfs appear similar to Earth, most have rock types that are exotic to our solar system,â says Xu. “They don’t have direct counterparts in the solar system.”
Researchers have developed a number of new terms to classify these rocks and their exotic compositions, including quartz pyroxenites, quartz orthopyroxenites, periclastic dunites, periclastic wehrlites, and periclastic clinopyroxenites.
These rocks could tell us a lot about the types of exoplanets they come from and how they have evolved, the researchers say. And this information could also have implications for assessing the habitability of exoplanets.
âSome of the rock types we see in the white dwarf data would dissolve more water than rocks on Earth and could impact the development of the oceans,â Purtika explains.
“Certain types of rock could melt at much lower temperatures and produce a thicker crust than terrestrial rocks, and certain types of rock could be weaker, which could facilitate the development of plate tectonics.”
Additionally, learning more about the compositions of rocky exoplanets via necroplanetology could help us answer some existential questions about our own place in the Universe. For example, we might find that certain regions of the galaxy are more likely to form Earth-like planets than other regions.
“Studies of exoplanets also force us to face still unresolved questions about why Earth is so completely different from its immediate planetary neighbors, and whether such contrasts are typical or inevitable,” the researchers explain.
The research was published in Nature Communication.