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)
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.
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.
“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.”