A new astrogeology study suggests that most nearby rocky exoplanets are quite unlike anything in our Solar System
From:
Association of Universities for Research in Astronomy (AURA)
November 2, 2021 -- 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."
Putirka describes what these new rock
types might mean for the rocky worlds they belong to. "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," he explained.
"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, Putirka and Xu explain that 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, Putirka and Xu state that
the 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 Putirka and Xu 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," Putirka stated.
According to Xu, the pairing of an
astronomer and a geologist was the key to unlocking the secrets hidden in the
atmospheres of the polluted white dwarfs. "I met Keith Putirka at a
conference and was excited that he could help me understand the systems that I
was observing. He taught me geology and I taught him astronomy, and we figured
out how to make sense of these mysterious exoplanetary systems."
The pair's results are published in the
2 November 2021 issue of Nature Communications.
https://www.sciencedaily.com/releases/2021/11/211102180527.htm
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