Rock Chemistry of Worlds That Once Were (Planetary Science)

The analysis of the spectral lines of the gas atmospheres of 23 white dwarfs contaminated by the remains of rocky planets – detected by Keck, Hubble and other telescopes – allowed a US astronomer and geologist to reconstruct the chemical composition of the planets themselves. Composition turned out to be quite different from that encountered in the rocky worlds of the solar system, and much more varied than expected. The results in Nature Communications

“I met Keith Putirka at a conference, and the fact that he could help me understand the systems I was observing thrilled me. He taught me geology and I taught him astronomy ”, remembers today Siyi Xu , an astrophysicist at the NoirLab of the US National Science Foundation,“ and we figured out how to make sense of these mysterious exoplanetary systems ”.

The “mysterious exoplanetary systems” ended up under the bi-disciplinary – astronomical and geological – lens of Xu and Keith Putirka(geologist at California State University, Fresno) are those who once surrounded 23 Sun-like stars, and who when these stars have reached the end of their life – becoming white dwarfs, as will happen to the Sun in a few billion years – were in part destroyed. The planets closest to each of these stars, in particular, have been incinerated and torn to shreds by the star itself. The material they were made of was thus mixed with the gas surrounding the star, contaminating it: it is no coincidence that they are called in jargon “polluted white dwarfs”. And it is precisely by analyzing the spectrum of the gaseous atmosphere of these 23 polluted white dwarfs – all within 650 light years of us – that Putirka and Xu were able to trace the material that those now lost rocky worlds must have been made of.

“Some exoplanets that once orbited polluted white dwarfs look similar to Earth, but most have rock types that are ‘exotic’ to us – rock types with no direct counterpart in the Solar System,” observes Xu. Exotic rocks to the point of forcing the two scientists to invent new names to classify them. Names like ” pyroxenite of Quartz ” (for rocks that contain more than 10 percent each ortopirosseni, clinopyroxene and quartz), for example, or dunite of periclase “(when periclase and olivine exceed 10 percent, and clinopyroxene are instead below 10 percent).

Previous studies conducted on polluted white dwarfs had shown the presence of some characteristic elements of the rocky worlds, such as calcium, aluminum and lithium. But these were elements usually present in small quantities, at least on terrestrial rocks. To understand what these worlds were actually made of, it was necessary to find traces – and quantify the concentration – of much more abundant elements, such as silicon, magnesium and iron. And it was precisely the spectral lines of these elements – detected by Keck, Hubble and other telescopes – that provided some clues, reported yesterday in an article in Nature Communications . Also valuable clues to imagine what these planets might once have looked like.

“Some rock types may melt at much lower temperatures and produce a thicker crust than terrestrial rocks,” Putirka speculates, “and some rock types may be weaker, which could facilitate the development of plate tectonics.”

Having been conducted around white dwarfs, these observations are useful not only for discovering the geological variety of the rocky worlds in the Milky Way, but also for getting an idea of what will happen to the rocky planets of the Solar System when our star also becomes one. white dwarf. “The bad news is that, from the solar system’s dynamic simulations, it appears that the Earth is too small and too close to the Sun, and may not survive our star’s white dwarf stage,” Xu says of this. The eventual discovery of a surviving Earth-like planet around a white dwarf, on the other hand, would offer us some hope for the survival of our planet. “We have not yet found any Earth-like planets around the white dwarfs, but we will continue to search,” promises the astronomer.

Featured image: Artist’s impression of rock debris – fragments of a shattered rocky planet – spiraling attracted to the white dwarf in the center. By studying the atmospheres of white dwarfs that have been “polluted” by such debris, a NoirLab astronomer and a geologist have identified types of exotic rocks that do not exist in the solar system. The findings suggest that rocky exoplanets are stranger and more varied than previously thought. Credits: NoirLab / Nsf / Aura / J. da Silva

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