Unlike Anything in Our Solar System: Rocky Exoplanets Are Even Stranger Than We Thought

Rocky Debris Around a White Dwarf

Rocky debris, the pieces of a former rocky planet that has broken up, spiral inward toward a white dwarf in this illustration. Studying the atmospheres of white dwarfs that have been “polluted” by such debris, a NOIRLab astronomer and a 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 (NSF’s NOIRLab)

A new astrogeology study suggests that most nearby rocky exoplanets are quite unlike anything in our Solar System.

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 that exist on planets orbiting nearby 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 not found anywhere in our Solar System.

Astronomers have discovered thousands of planets orbiting stars in our galaxy — known as exoplanets. However, it’s difficult to know what exactly these planets are made of, or whether any resemble Earth. To try to find out, astronomer Siyi Xu of NSF’s NOIRLab partnered with geologist Keith Putirka of California State University, Fresno, to study the atmospheres of what are known as polluted white dwarfs. These are the dense, collapsed cores of once-normal stars like the Sun that contain foreign material from planets, asteroids, or other rocky bodies that once orbited the star but eventually fell into the white dwarf and “contaminated” its atmosphere. By looking for elements that wouldn’t naturally exist in a white dwarf’s atmosphere (anything other than hydrogen and helium), scientists can figure out what the rocky planetary objects that fell into the star were made of.

According to new research by a NOIRLab astronomer and a geologist, rocky exoplanets are even stranger than previously thought. By studying the atmospheres of stellar remnants called white dwarfs, the pair has discovered types of rocks not found in our Solar System. Each white dwarf is “polluted” with material from rocky bodies that originally orbited it but fell into the white dwarf and spread their elements through its atmosphere. Some of the rock compositions are so rare, the scientists had to create new names to classify the types of rocks that once made up these ancient planets.

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 W. M. Keck Observatory in Hawai‘i, the Hubble Space Telescope, and other observatories. The scientists then used the measured abundances of those elements to reconstruct the minerals and rocks that would form 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 their planets had a wider variety 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 novel rock types that must have existed on those planets.[1]

“While some exoplanets that once orbited polluted white dwarfs appear similar to Earth, most have rock types that are exotic to our Solar System,” said Xu. “They have no direct counterparts in the Solar System.”

Putirka describes what these new rock types might mean for the rocky worlds they belong to. “Some of the rock types that we see from the white dwarf data would dissolve more water than rocks on Earth and might impact how oceans are developed,” he explained. “Some rock types might melt at much lower temperatures and produce thicker crust than Earth rocks, and some rock types might be weaker, which might facilitate the development of plate tectonics.”

Earlier studies of polluted white dwarfs had found elements from rocky bodies, including calcium, aluminum, and lithium. However, Putirka and Xu explain that those are minor elements (which typically make up a small part of an Earth rock) and measurements of major elements (which make up a large part of an Earth rock), especially silicon, are needed to truly know what kind of 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 white dwarfs’ atmospheres suggest that the rocky debris detected likely came from the interiors of the planets — from the mantle, not their crust. Some previous studies of polluted white dwarfs reported signs that continental crust existed on the rocky planets that once orbited those stars, but Putirka and Xu found no evidence of crustal rocks. However, the observations do not completely rule out that the planets had continental crust or other crust types. “We believe that if crustal rock exists, we are unable to see it, probably 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 stated.

According to Xu, the pairing of an astronomer and a geologist was the key to unlocking the secrets hidden in the atmospheres of the polluted white dwarfs. “I met Keith Putirka at a conference and was excited that he could help me understand the systems that 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 Communications.


  1. “Normal” or existing rock classification methods are based on the fact that olivine and orthopyroxene are the dominant minerals in Earth’s mantle (and the mantles of other rocky planets in our Solar System). For many exoplanets, though, olivine might be absent and quartz present, or orthopyroxene could be absent and periclase is present, and so a new classification nomenclature was developed. The new rock type classifications proposed by Putirka and Xu include: “quartz pyroxenites,” which have more than 10% each of orthopyroxene, clinopyroxene, and quartz; “quartz orthopyroxenites,” which have more than 10% orthopyroxene and quartz, and less than 10% clinopyroxene; “periclase dunites,” which have more than 10% each of periclase and olivine, and less than 10% clinopyroxene; “periclase wehrlites,” which contain 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 clinopyroxene.

Reference: “Polluted white dwarfs reveal exotic mantle rock types on exoplanets in our solar neighborhood” by Keith D. Putirka and Siyi Xu, 2 November 2021, Nature Communications.
DOI: 10.1038/s41467-021-26403-8

10 Comments on "Unlike Anything in Our Solar System: Rocky Exoplanets Are Even Stranger Than We Thought"

  1. Amateur astronomer | November 4, 2021 at 5:33 pm | Reply

    Wow, someone funded this nonsense? If you use the exact same principle on our star you still get a giant list of compounds that are not hydrogen or helium. When a meteor crashes on the moon is all the ejecta ejected away from the moon, or does it rain back down? Same thing with the cloud given off by a dieing star, some of those heavier elements fall back onto the new white dwarf. Also, white dwarfs don’t just suddenly get more mass when they explode. If anything they loose mass, so why does this research suggest the planets fall into the white dwarf? They would either be consumed by the growing star and sucked to the core before you could get a cloud to dirty, be launched off into space by the “exploding” host star, or they would survive and keep orbiting. This article seems to think we are slowly moving towards the sun and will one day fall into it. If gravity worked that way the moon would be slowly falling towards Earth.

    • in other words, you don’t understand it, so you attack it. you definitely fit the casual definition of “amateur”
      it made sense to me
      with your attitude, I’ll bet I can accurately guess who you voted for.
      btw, please point out where the article said anything about white dwarves exploding?
      however, maybe with a lot more study of the physics of white dwarves, understanding will come and you will realize the fundamental mistakes you have made

    • Torbjörn Larsson | November 8, 2021 at 7:15 am | Reply

      If you didn’t read the paper and understood the science, how can you of all people tell if it is nonsense?

      They picked the white dwarfs because planets (or asteroids) had collided with them, not because it is inevitable. Our own system isn’t guaranteed to be long term stable [ http://www.scholarpedia.org/article/Stability_of_the_solar_system ]:

      “The main surprise that comes from the numerical simulations of the recent years is that the probability for this catastrophic events to occur is relatively high, of the order of 1%, and thus not just a mathematical curiosity with extremely low probability values. At the same time, 99% of the trajectories will behave in a similar way as in the recent past millions of years, which is coherent with our common understanding that the Solar System has not much evolved in the past 4 Gyr. What is more surprising is that if we consider a pure Newtonian world, starting with the present initial conditions, the probability of collisions within 5 Gyr grows to 60%, which can thus be considered as an additional indirect confirmation of general relativity.”

  2. the free out the side friend in add face new driving the car the bus stop it card a be friend a family the teacher the toking moth staffthing trip on the town a on friend a me back later a friend me back later we selft the sun a me a out the side we on time.

  3. The tone in this article “describes”, in a humble way, the heart of serious space science. This is not Popular Science. When the Nasa James Webb Space Telescope’s added in December, we’ll have one more effective tool in order to get more knowledge about our surroundings or environment. Space science is A and O. Great in all ways. And 🤞for the Webb Telescope’s launch in Dec 🤞!

  4. As interesting as the findings are, the researchers are jumping to conclusions by saying that most exoplanets must be composed of rock types that are not only exotic to our Solar System but that they have no direct counterparts in the Solar System.

    This is a surprising assertion, given that their data shows that most of the rocky material they found was mantle type rock.

    After all, we still have no real knowledge of what kinds of rock are deep in the Earth, let alone in the other planets in our system. If we could actually examine them, we might find surprisingly exotic rocks deep under our very feet.

    • Torbjörn Larsson | November 8, 2021 at 7:17 am | Reply

      I thought so too, but the paper uses ternary composition diagram to define their mineral types, not the other way around.

      “Bulk Silicate Planets (Mg + Si + Ca + Fe) of PWDs are recast as mineral components and plotted in a the classic ternary ultramafic rock classification39, and two new ternary diagrams (b, c) that can describe PWDs as a set of positive mineral components.”

      Tt is (a geological) choice of terminology, is all.

  5. … Well, when will rocky play rock and roll…

  6. Torbjörn Larsson | November 8, 2021 at 7:11 am | Reply

    Interesting, and it ties into similar observations of our system rarity [ https://phys.org/news/2021-10-planets-protoplanetary-discs.html ]:

    “Since different minerals in planetesimals condense under different conditions, times, and places, the overall situation is complex, making it hard to understand the observed chemistry of planets.

    CfA geologist Michail Petaev and his colleagues simulated the collapse of a molecular cloud core and the formation of the star, disk and planets, and analyzed the evolving distribution of temperatures across the disk to infer the mineral condensation sequence. They find that the properties of the initial cloud core significantly affect the maximum temperatures reached in the disk and the resultant compositions of the planets and asteroids; …

    Significantly, they conclude that in order to reproduce the composition seen in Solar system meteorites and the terrestrial planets either the initial core had rare properties like temperatures near 2000 kelvin (well above the expected median value of 1250 kelvin), or else some other source of heating must have raised the protoplanetary disk’s temperature.”

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