In this illustration, rock debris, which is the fragments of a former, rocky planet, spirals inward towards a white dwarf. A NOIRLab geologist and an astronomer have discovered exotic rock types in white dwarf atmospheres. These results indicate that nearby rocky exoplanets may be more bizarre and diverse than previously thought. Credit: NOIRLab / NSF / AURA /J. da Silva / M. Zamani / M. Kosari, (NSF's NOIRLab).
NSF's NOIRLab astronomer has joined forces with a California State University Fresno geologist to estimate the types of rocks that are found on nearby planets. They have discovered that the majority of rocky planets orbiting near stars are more diverse than previously thought. The "polluted white dwarfs" contain a variety of rock types not found elsewhere in the Solar System.
Exoplanets are thousands of planets discovered by astronomers orbiting stars in the galaxy. It's not possible to determine what these planets are made from or if they resemble Earth. Siyi Xu, an astronomer at NSF's NOIRLab, teamed up with Keith Putirka, a geologist from California State University, Fresno to study the atmospheres known as polluted White dwarfs. These are dense, collapsed cores from once-normal stars such as the Sun that contain foreign matter from planets, asteroids or other rocky bodies. They "contaminate" the atmosphere of white dwarfs. Scientists can determine the origin of the rocky planetary objects and other elements that were found in the white dwarf's atmosphere.
Putirka, Xu and others looked at 23 polluted dwarfs located within 650 light years of the Sun. There, calcium, magnesium, iron, and silicon had all been measured with precision by the W. M. Keck Observatory, Hawai'i's Hubble Space Telescope and other observatories. They used the measurements of these elements to create the minerals and rocks they would form. These white dwarfs were found to have a greater variety of compositions than any other inner planets in the Solar System. This suggests that their planets may have had more rock types. Some of these compositions are so unique that Putirka & Xu had the task of creating new names (such "quartz pyroxenites", and "periclase dunenites) to identify the different rock types that must have existed in those places.
Xu stated that while some exoplanets once orbited polluted dwarfs look similar to Earth, many have rock types that are unique to the Solar System. They have no counterparts in our Solar System."
Putirka explains what these new types of rock might mean for the rocky planets they are part of. He explained that some of the rock types we have seen from the white dwarf data might dissolve more water than the rocks on Earth, which could impact the development of oceans. Some rock types may melt at lower temperatures, producing thicker crust than Earth rocks. Other rock types might be less strong, which could facilitate the development plate tectonics.
In earlier studies of polluted white dwarfs, elements were found in rocky bodies such as calcium, aluminum and lithium. Putirka and Xu say that these elements are minor elements, which typically make up a small portion of an Earth-rock. However, measurements of major elements, especially silicon, are required to determine what type of rock would have been found on those planets.
Putirka and Xu also state that the low levels of silicon and magnesium found in white dwarfs' atmospheres suggests that the rocky material detected was likely to have come from the interiors and not their crust. Previous studies of polluted white dimers found evidence that there was continental crust on the rocky worlds orbiting these stars. However, Putirka and Xu did not find any evidence of crustal rocks. The observations don't rule out the existence of continental crust on the planets. Putirka stated that crustal rock is not visible to us if it exists. This could be due to it being too small in comparison to other components of the planets, such as the core and mantle.
Xu believes that the combination of an astronomer with a geologist is the key to unlocking secrets in the atmospheres polluted white dwarfs. Keith Putirka was the one I met at a conference. He was eager to help me understand the systems I was seeing. He taught me geology, and I taught him Astronomy. Together we were able to understand these exoplanetary systems.
The results of the pair are published in Nature Communications, 2 November 2021.
Explore more Vaporized crusts of Earth-like Planets Found in Dying Stars
More information: Keith D. Putirka and colleagues, Polluted White dwarfs reveal exotic types of mantle rocks on exoplanets within our solar neighborhood, Nature Communications (2021). Information from Nature Communications Keith D. Putirka and colleagues, Polluted White Dwarfs Reveal Exotic Mantle Rock Types on Exoplanets In Our Solar Neighborhood, (2021). DOI: 10.1038/s41467-021-26403-8
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