Free Astronomy Magazine January-February 2022

52 JANUARY-FEBRUARY 2022 ASTRO PUBLISHING normal stars like the Sun that contain foreign material from planets, aster- oids, or other rocky bodies that once or- bited the star but eventually fell into the white dwarf and “contami- nated” its at- mosphere. By looking for elements that wouldn’t natu- rally exist in a white dwarf’s a t m o s p h e r e (anything other than hydrogen and helium), scientists can figure out what the rocky planetary ob- jects that fell into the star were made of. Putirka and Xu looked at 23 pol- luted white dwarfs, all within about 650 light-years of the Sun, where calcium, silicon, magnesium, and iron had been measured with preci- sion using the W. M. Keck Observa- tory in Hawai’i, the Hubble Space Telescope, and other observatories. The scientists then used the meas- ured 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 compo- sitions than any of the inner plan- ets in our Solar System, suggesting their planets had a wider variety of rock types. In fact, some of the com- positions are so unusual that Putir- ka and Xu had to create new names (such as “quartz pyroxenites” and “periclase dunites”) to classify the novel rock types that must have ex- isted on those planets. “While some exoplanets that once ! orbited polluted white dwarfs ap- pear 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 dis- solve 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 develop- ment 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 ex- isted on those planets. In addi- tion, Putirka and Xu state that the high levels of magnesium and low levels of silicon measured in the white dwarfs’ atmos- pheres 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 compo- nents, 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 myste- rious exoplanetary systems.” B y studying the atmospheres of stellar remnants called white dwarfs, two scien- tists discovered types of rocks not found in our Solar System. This video sum- marises the discovery. [NOIRLab/NSF/AURA/J. da Silva/NASA’s Goddard Space Flight Center/Scott Wiessinger/ESO/M. Kornmesser]