Is our Earth an oddity in space?
When you think of the rocks that make up our home planet (but also all the other rocky planets in the solar system), you probably immediately think of the fact that olivine and orthopyroxene are the predominant minerals in the Earth’s mantle. If not, you are probably not a geologist. I confess, I didn’t know that either. But is this actually normal? Are all rocky planets in the universe composed primarily of these minerals? That’s an important question, because other rock types absorb more water than Earth’s rocks, for example, which would affect the development of oceans. Others melt at much lower temperatures, forming a thicker crust (making plate tectonics more difficult), and still others are less hard, which would facilitate the development of plate tectonics.
But how do you test what’s in all those other planets? After all, it’s hard enough to drill a meter-deep hole in Mars. But if a planet is a few light-years away, we don’t even have a chance to send a rover there yet. Fortunately, there is a process that turns the insides of planets inside out. Eventually, it will hit the Earth, too: The sun will inflate into a red giant and swallow our planet in the process. What is left of it, extraterrestrial astronomers can control later, if they analyze the white dwarf, which was once the sun, spectroscopically.
In its light, traces of all the atomic species that made up the Earth can be found. That’s exactly what astronomer Siyi Xu of NSF’s NOIRLab did in collaboration with geologist Keith Putirka of California State University. Putirka and Xu studied 23 polluted white dwarfs, all within about 650 light-years of the Sun, where calcium, silicon, magnesium and iron were accurately measured using the W. M. Keck Observatory in Hawai’i, the Hubble Space Telescope and other observatories.
Using the measured abundances of these elements, scientists then reconstructed the minerals and rocks that would form from them. They found that these white dwarfs have a much wider range of compositions than the inner planets of our solar system, suggesting that their planets – when they existed – had a much wider variety of rock types. Some of the compositions are so unusual, in fact, 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 these planets. The Earth, therefore, is really almost an exotic – just the right composition to give rise to oceans and plate tectonics.