Rocky exoplanets are more diverse and exotic than previously thought

Most rocky exoplanets have an exotic composition and mineralogy, according to an analysis of the chemical composition of so-called “polluted” white dwarfs in the solar neighborhood.

Putirka & Xu present the first estimates of rock types on exoplanets that once revolved around polluted white dwarfs – stars whose atmospheric compositions record the arrival of planetary objects once in orbit. Image credit: NOIRLab / NSF / AURA / J. da Silva / M. Zamani & M. Kosari, NOIRLab of NSF.

White dwarfs are stars that have left the main sequence, having used up all their fuel; the stars first expand to form red giants, then contract to a size close to that of Earth.

Planets orbiting these stars can disintegrate due to the strong gravitational pull of their hosts, the resulting debris falling into stellar atmospheres.

These polluted white dwarfs act as “cosmic mass spectrometers” which provide almost direct analyzes of the compositions of exoplanets.

Sources of pollution can be entire planets or fragments of planets such as the main asteroid belt in the solar system.

By looking for elements that would not naturally exist in a white dwarf’s atmosphere (other than hydrogen and helium), astronomers can understand what these rocky objects were made of.

In the new study, California State University geologist Professor Keith Putirka and NSF’s Gemini Observatory and BLACK Lab Dr. Siyi Xu examined 23 polluted white dwarfs, all located approximately 650 light years from the Sun, where calcium, silicon, magnesium, and iron were accurately measured using the WM Keck Observatory in Hawai’i, the NASA / ESA Hubble Space Telescope, and other observatories .

Astronomers then used the measured abundances of these elements to replenish the minerals and rocks that would form from them.

They discovered that these white dwarfs have a much wider range of compositions than any of the inner planets in our solar system, suggesting that their planets had a greater variety of rock types.

In fact, some of the compositions are so unusual that the authors had to come up with new names – such as quartz pyroxenites and periclase dunites – to classify the new types of rocks that must have existed on these planets.

“While some exoplanets that once circled around polluted white dwarfs appear similar to Earth, most have rock types that are exotic to our solar system,” Dr. Xu said.

“Some of the types of rocks we see in the white dwarf data would dissolve more water than rocks on Earth and could impact the development of the oceans,” Professor Putirka added.

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

“Previous studies on polluted white dwarfs had found elements in rocky bodies, including calcium, aluminum and lithium.”

“However, these are minor elements (which usually make up a small part of an earth’s rock) and measurements of major elements (which make up a large part of an earth’s rock), especially silicon, are needed to really find out what kind of rock types would have existed on these planets.

“Additionally, high levels of magnesium and low levels of silicon measured in the atmosphere of the white dwarfs suggest that the detected rock debris likely originated from inside the planets – from the mantle, not from their crust.”

Some previous studies of polluted white dwarfs reported signs of the existence of a continental crust on the rocky planets that once revolved around these stars, but the team found no evidence of crustal rocks.

However, observations do not completely rule out that planets have a continental crust or other types of crust.

“We think if crustal rock exists, we can’t see it, possibly because it occurs in too small a fraction of the mass of other planetary components, like the core and mantle, to be measured.” , said Professor Putirka.

The results were published this week in the journal Nature Communication.

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KD Putirka & S. Xu. 2021. Polluted white dwarfs reveal exotic mantle rock types on exoplanets in our solar neighborhood. Nat Common 12, 6168; doi: 10.1038 / s41467-021-26403-8


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