Most rocky exoplanets are exotic in composition and mineralogy, according to an analysis of the chemical composition of the so-called ‘polluted’ white dwarfs in the solar neighborhood.
Putirka & Xu present the first estimates of rock types on exoplanets that once orbited polluted white dwarfs — stars whose atmospheric compositions record the infall of formerly orbiting planetary objects. Image credit: NOIRLab / NSF / AURA / J. da Silva / M. Zamani & M. Kosari, NSF’s NOIRLab.
White dwarfs are stars that have left the main sequence, having used up all their fuel; the stars first expand to form red giants, and then contract, to a size that is about that of Earth.
Planets orbiting these stars may disintegrate due to the strong gravitational pull of their hosts, with the resulting debris falling into the stellar atmospheres.
These polluted white dwarfs act as ‘cosmic mass spectrometers’ that provide near-direct analyses of exoplanet compositions.
The pollution sources may consist of entire planets or the broken bits of planets like the Solar System’s main asteroid belt.
By looking for elements that wouldn’t naturally exist in a white dwarf’s atmosphere (anything other than hydrogen and helium), astronomers can figure out what these rocky objects were made of.
In the new study, California State University geologist Professor Keith Putirka and Dr. Siyi Xu from Gemini Observatory and NSF’s NOIR Lab 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 NASA/ESA Hubble Space Telescope, and other observatories.
The astronomers 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 the authors 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,” Dr. Xu said.
“Some of the rock types that we see from the white dwarf data would dissolve more water than rocks on Earth and might impact how oceans are developed,” Professor Putirka added.
“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 development of plate tectonics.”
“Earlier studies of polluted white dwarfs had found elements from rocky bodies, including calcium, aluminum, and lithium.”
“However, 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 existed on those planets.”
“In addition, high levels of magnesium and low levels of silicon measured in the white dwarfs’ atmospheres 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 the team 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 components, like the core and mantle, to be measured,” Professor Putirka said.
The findings were published this week in the journal Nature Communications.
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K.D. Putirka & S. Xu. 2021. Polluted white dwarfs reveal exotic mantle rock types on exoplanets in our solar neighborhood. Nat Commun 12, 6168; doi: 10.1038/s41467-021-26403-8
