Rocky exoplanets are even stranger than we thought


Rock debris, pieces of an ancient rocky planet that shattered, spiral inward toward a white dwarf in this illustration. By studying the atmospheres of white dwarfs “polluted” by such debris, a NOIRLab astronomer and geologist have identified exotic rock types that do not exist in our solar system. The results suggest that nearby rocky exoplanets must be even stranger and more diverse than previously thought. Credit: NOIRLab / NSF / AURA / J. da Silva, Image processing: M. Zamani and M. Kosari (NOIRLab from NSF)

A new astrogeology study suggests that most nearby rocky exoplanets are unlike anything in our solar system.

An astronomer from NSF’s NOIRLab has teamed up with a geologist at California State University, Fresno, to make the first estimates of the types of rocks that exist on planets orbiting nearby stars. After studying the chemical makeup of ‘polluted’ white dwarfs, they concluded that most rocky planets orbiting nearby stars are more diverse and exotic than previously thought, with rock types than the it is not found anywhere in our solar system.

Astronomers have discovered thousands of planets orbiting stars in our galaxy, called exoplanets. However, it’s unclear what exactly these planets are made of, or if any of them look like Earth. In an attempt to find out, astronomer Siyi Xu of NSF’s NOIRLab has teamed up with geologist Keith Putirka of California State University, Fresno, to study the atmosphere of so-called polluted white dwarfs. These are the dense, collapsed nuclei of once normal stars like the Sun that contain foreign matter from planets, asteroids, or other rocky bodies that once revolved around the star but eventually fell into the white dwarf and “contaminated” its atmosphere. By looking for elements that would not naturally exist in a white dwarf’s atmosphere (other than hydrogen and helium), scientists can find out what the rocky planetary objects that fell into the star were made of. .

Rocky exoplanets are even stranger than previously thought, according to new research by an astronomer and geologist at NOIRLab. By studying the atmosphere of stellar remnants called white dwarfs, the couple discovered types of rocks not found in our solar system. Each white dwarf is “polluted” by material from rocky bodies that originally gravitated around it, but which fell into the white dwarf and spilled their elements into its atmosphere. Some of the rock compositions are so rare that scientists had to come up with new names to classify the types of rocks that once made up these ancient planets.

Putirka and Xu examined 23 polluted white dwarfs, all located about 650 light years from the Sun, where calcium, silicon, magnesium, and iron were accurately measured using the WM Keck Observatory in Hawaii. ‘i, the Hubble Space Telescope, and other observatories. Scientists then used the measured abundances of these elements to replenish the minerals and rocks that would form from them. They discovered 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 greater variety of rock types. In fact, some of the compositions are so unusual that Putirka and Xu had to come up with new names (such as “quartz pyroxenites” and “periclase dunites”) to classify the new types of rocks that must have existed on these planets.[1]

“While some exoplanets that once revolved around polluted white dwarfs appear similar to Earth, most have rock types that are exotic to our solar system,” Xu said. “They don’t have direct counterparts in the solar system.”

Putirka describes what these new types of rocks could mean for the rocky worlds to which they belong. “Some of the types of rocks we see in the white dwarf data would dissolve more water than rocks on Earth and could impact the development of the oceans,” he explained. “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.”

Previous studies on polluted white dwarfs had found elements from rocky bodies, including calcium, aluminum and lithium. However, Putirka and Xu explain that these are minor elements (which are usually a small part of an earth rock) and that the measurements of the major elements (which make up a large part of an earth rock), in especially silicon, are necessary to really know what kind of rock types would have existed on these planets.

Additionally, 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 detected likely originated from inside the planets – from the mantle, not their crust. Some previous studies of polluted white dwarfs reported signs of the existence of a continental crust on the rocky planets that once revolved around these stars, but Putirka and Xu found no evidence of crustal rocks. However, observations do not completely rule out that planets have a continental crust or other types of crust. “We believe that if crustal rock exists, we cannot see it, possibly because it occurs in too small a fraction compared to the mass of other planetary components, like the core and mantle, to be measured,” Putirka said.

According to Xu, the association 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 I was delighted 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 to make sense of these mysterious exoplanetary systems.

The pair’s results are published in the November 2, 2021 issue of Nature Communication.


  1. The “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 of other rocky planets in our solar system). For many exoplanets, however, olivine might be absent and quartz present, or orthopyroxene might be absent and periclase is present, and a new classification nomenclature has therefore been developed. New rock type classifications proposed by Putirka and Xu include: “quartz pyroxenites”, each of which contains more than 10% orthopyroxene, clinopyroxene and quartz; “Quartz orthopyroxenites”, which contain more than 10% orthopyroxene and quartz and less than 10% clinopyroxene; “Periclase dunites”, each containing more than 10% periclase and olivine and less than 10% clinopyroxene; “Periclase wehrlites”, which each contain more than 10% periclase, olivine and clinopyroxene; and “periclase clinopyroxenites”, which contain less than 10% olivine and more than 10% each of periclase and clinopyroxene.

Reference: “Polluted White Dwarfs Reveal Alien Mantle Rock Types on Exoplanets in Our Solar Neighborhood” by Keith D. Putirka and Siyi Xu, November 2, 2021, Nature Communication.
DOI: 10.1038 / s41467-021-26403-8

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