Rocky exoplanets are even stranger than we thought

November 2, 2021

(Nanowerk news) An astronomer from NSF’s NOIRLab has teamed up with a geologist from California State University, Fresno, to make the first estimates of rock types found on planets orbiting stars. After studying the chemical composition of “polluted” white dwarfs, they have concluded that most rocky planets orbiting nearby stars are more diverse and exotic than previously thought, with types of rocks found nowhere in our solar system.

Astronomers have discovered thousands of planets orbiting stars in our galaxy – known as exoplanets. However, it is difficult to know what exactly these planets are made of, or whether anyone resembles Earth. To try to figure it out, astronomer Siyi Xu of NSF’s NOIRLab teamed up with geologist Keith Putirka of California State University, Fresno, to study the atmosphere of what are known as polluted white dwarfs. These are the dense, collapsed nuclei of once normal stars like the Sun, which contain foreign matter from planets, asteroids or other rock bodies that once orbited the star but eventually fell into the white dwarf and “polluted” its atmosphere.

By looking for elements that would not naturally exist in a white dwarf atmosphere (anything other than hydrogen and helium), scientists can find out what the rocky planetary objects that fell into the star were made of. Illustration of stone debris around a white dwarf Rock debris, the pieces of a previously rocky planet that has been broken up, spiral inwards towards a white dwarf in this illustration. By studying the atmosphere of white dwarfs that have been “polluted” by such waste, a NOIRLab astronomer and a geologist have identified exotic rock types not found in our solar system. The results suggest that nearby rocky exoplanets may be even stranger and more diverse than previously thought. (Image: NOIRLab / NSF / AURA / J. Da Silva Image processing: M. Zamani and M. Kosari (NSF’s NOIRLab))

Putirka and Xu looked at 23 polluted white dwarfs, all within about 650 light-years of the Sun, where calcium, silicon, magnesium and iron had been measured with precision using the WM Keck Observatory in Hawai’i, the Hubble Space Telescope, and other observatories. . The researchers then used the measured amounts of these elements to reconstruct the minerals and rocks that would be formed from them.

They found that these white dwarfs have a much wider range of compositions than any of the inner planets in our solar system, suggesting that their planets had a wider range of rock types. In fact, some of the compositions are so unusual that Putirka and Xu had to create new names (such as “quartz pyroxenites” and “periclase dunites”) to classify the new rock types that must have existed on these planets.[1]

“While some exoplanets that once orbited contaminated white dwarfs resemble Earth, most have rock types that are exotic to our solar system,” Xu said. “They have no direct counterparts in the solar system.”

Putirka describes what these new rock types can mean for the rock worlds to which they belong. “Some of the rock types we see from the white dwarf data would dissolve more water than rocks on Earth and could affect how oceans evolve,” he explained. “Some rock types can melt at much lower temperatures and produce thicker crusts than earth stones, and some rock types can be weaker, which can facilitate the development of plate tectonics.”

Previous studies of contaminated white dwarfs had found elements from rock bodies, including calcium, aluminum and lithium. However, Putirka and Xu explain that it is smaller elements (which typically make up a small part of a ground stone), and measurements of larger elements (which make up a large part of a ground stone), especially silicon, are necessary to really know what kind rock types would have existed on those planets.

In addition, Putirka and Xu state that the high levels of magnesium and low levels of silicon measured in the atmospheres of the white dwarfs suggest that the rock debris discovered probably came from the interior of the planets – from the mantle, not their crust. Some previous studies of polluted white dwarfs reported evidence of continental crust on the rocky planets that once orbited these stars, but Putirka and Xu found no evidence of crustal plague. However, the observations do not completely rule out that the planets had continental crust or other crust types.

“We believe that if the crustal plague exists, we are not able to see it, probably because it occurs in too small a fraction compared to the mass of other planetary components, such as the nucleus and mantle, to be measured,” said Putirka.

According to Xu, the mating of an astronomer and a geologist was the key to unlocking the secrets hidden in the atmosphere of the polluted white dwarfs. “I met Keith Putirka at a conference and was excited that he could help me understand the systems I was observing. He taught me geology, and I taught him astronomy, and we figured out how we could make sense of it. these mysterious exoplanetary systems. ”

The couple’s results are published in Nature communication (“Polluted white dwarfs reveal exotic mantle types on exoplanets in our solar system”).


[1] “Normal” or existing rock classification methods are based on the fact that olivine and orthopyroxene are the dominant minerals in the Earth’s mantle (and the mantles on other rocky planets in our solar system). For many exoplanets, however, olivine may be absent and quartz present, or the orthopyroxene may be absent and periclase present, and therefore a new classification nomenclature was developed. The new rock classifications proposed by Putirka and Xu include: “quartz pyroxenites”, each having more than 10% of the orthopyroxene, clinopyroxene and quartz; “quartz orthopyroxenites,” having more than 10% orthopyroxene and quartz, and less than 10% clinopyroxene; “periclase dunites,” having more than 10% each of periclase and olivine, and less than 10% of clinopyroxene; “periclase wehrlites,” which contains more than 10% each of periclase, olivine and clinopyroxene; and “periclase clinopyroxenites”, which have less than 10% olivine and more than 10% each of periclase and clinopyroxen.

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