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Rocky exoplanets are even stranger than we thought

An astronomer from NSF’s NOIRLab has teamed up with a geologist from California State College, Fresno, to make the primary estimates of rock sorts that exist on planets orbiting close by stars. After finding out the chemical composition of “polluted” white dwarfs, they’ve concluded that the majority rocky planets orbiting close by stars are extra various and unique than beforehand thought, with sorts of rocks not discovered anyplace in our Photo voltaic System.

Astronomers have found hundreds of planets orbiting stars in our galaxy — often known as exoplanets. Nonetheless, it is tough to know what precisely these planets are product of, or whether or not any resemble Earth. To attempt to discover out, astronomer Siyi Xu of NSF’s NOIRLab partnered with geologist Keith Putirka of California State College, Fresno, to review the atmospheres of what are often known as polluted white dwarfs. These are the dense, collapsed cores of once-normal stars just like the Solar that include international materials from planets, asteroids, or different rocky our bodies that after orbited the star however ultimately fell into the white dwarf and “contaminated” its environment. By on the lookout for parts that would not naturally exist in a white dwarf’s environment (something aside from hydrogen and helium), scientists can determine what the rocky planetary objects that fell into the star have been product of.

Putirka and Xu checked out 23 polluted white dwarfs, all inside about 650 light-years of the Solar, the place calcium, silicon, magnesium, and iron had been measured with precision utilizing the W. M. Keck Observatory in Hawai’i, the Hubble Area Telescope, and different observatories. The scientists then used the measured abundances of these parts to reconstruct the minerals and rocks that may kind from them. They discovered that these white dwarfs have a a lot wider vary of compositions than any of the internal planets in our Photo voltaic System, suggesting their planets had a greater diversity of rock sorts. The truth is, a number of the compositions are so uncommon that Putirka and Xu needed to create new names (resembling “quartz pyroxenites” and “periclase dunites”) to categorise the novel rock sorts that will need to have existed on these planets.

“Whereas some exoplanets that after orbited polluted white dwarfs seem just like Earth, most have rock sorts which might be unique to our Photo voltaic System,” stated Xu. “They don’t have any direct counterparts within the Photo voltaic System.”

Putirka describes what these new rock sorts would possibly imply for the rocky worlds they belong to. “A number of the rock sorts that we see from the white dwarf information would dissolve extra water than rocks on Earth and would possibly impression how oceans are developed,” he defined. “Some rock sorts would possibly soften at a lot decrease temperatures and produce thicker crust than Earth rocks, and a few rock sorts could be weaker, which could facilitate the event of plate tectonics.”

Earlier research of polluted white dwarfs had discovered parts from rocky our bodies, together with calcium, aluminum, and lithium. Nonetheless, Putirka and Xu clarify that these are minor parts (which generally make up a small a part of an Earth rock) and measurements of main parts (which make up a big a part of an Earth rock), particularly silicon, are wanted to actually know what sort of rock sorts would have existed on these planets.

As well as, Putirka and Xu state that the excessive ranges of magnesium and low ranges of silicon measured within the white dwarfs’ atmospheres recommend that the rocky particles detected seemingly got here from the interiors of the planets — from the mantle, not their crust. Some earlier research of polluted white dwarfs reported indicators that continental crust existed on the rocky planets that after orbited these stars, however Putirka and Xu discovered no proof of crustal rocks. Nonetheless, the observations don’t utterly rule out that the planets had continental crust or different crust sorts. “We imagine that if crustal rock exists, we’re unable to see it, most likely as a result of it happens in too small a fraction in comparison with the mass of different planetary parts, just like the core and mantle, to be measured,” Putirka said.

In line with Xu, the pairing of an astronomer and a geologist was the important thing to unlocking the secrets and techniques hidden within the atmospheres of the polluted white dwarfs. “I met Keith Putirka at a convention and was excited that he may assist me perceive the methods that I used to be observing. He taught me geology and I taught him astronomy, and we found out easy methods to make sense of those mysterious exoplanetary methods.”

The pair’s outcomes are revealed within the 2 November 2021 problem of Nature Communications.


[1] “Regular” or present rock classification strategies are primarily based on the truth that olivine and orthopyroxene are the dominant minerals in Earth’s mantle (and the mantles of different rocky planets in our Photo voltaic System). For a lot of exoplanets, although, olivine could be absent and quartz current, or orthopyroxene may very well be absent and periclase is current, and so a brand new classification nomenclature was developed. The brand new rock kind classifications proposed by Putirka and Xu embrace: “quartz pyroxenites,” which have greater than 10% every of orthopyroxene, clinopyroxene, and quartz; “quartz orthopyroxenites,” which have greater than 10% orthopyroxene and quartz, and fewer than 10% clinopyroxene; “periclase dunites,” which have greater than 10% every of periclase and olivine, and fewer than 10% clinopyroxene; “periclase wehrlites,” which include greater than 10% every of periclase, olivine, and clinopyroxene; and “periclase clinopyroxenites,” which have lower than 10% olivine and greater than 10% every of periclase and clinopyroxene.