An illustration of rocks around a dwarf. The image is from NOIRLab.
Astronomers have discovered rock types that were never seen before in the remains of alien worlds. The research suggests that the exoplanets are built from a lot of different materials.
The researchers looked at 23 white dwarfs that were within 650 light-years of the sun. These stars ripped apart their exoplanets as they transitioned into white dwarfs. The guts from alien worlds they destroyed are contained in the atmospheres of white dwarfs. Researchers worked out the ratio of different elements in the white dwarf atmospheres by analyzing the light given off by the stars; then, they calculated the most likely makeup of the minerals that would have formed the obliterated alien worlds.
The remains of exoplanets with a similar geological make-up to Earth were found in one of the white dwarfs. The remains of exoplanets made of alien rocks were found within the rest of the dead stars. The rocks were so different that the researchers had to create new names for them.
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The National Optical-Infrared Astronomy Research Laboratory (NOIRLab) in Arizona has a lead author who said that most of the exoplanets that once circled white dwarfs are exotic. They have no direct counterparts in the solar system.
White dwarfs form when main sequence stars run out of fuel to burn and begin to swell up into red giants before collapsing under their own weight into super-condensed and cooled stellar cores. The dying stars release a cloud of superheated gas that surrounds the planets.
Some exoplanets can survive this onslaught, but most get knocked out of their orbits and ripped apart by the white dwarf's strong gravity. Once the planet is ripped apart, the white dwarf pulls the planetary remains inward in a process known as accretion.
The atmosphere of a white dwarf is usually made of hydrogen and helium, because heavier elements sink into the star's super-dense core. When the stars give off light that shows the presence of other heavier elements, researchers assume that those are from exoplanet accretion.
Scientists estimate that 25% of white dwarfs contain the remains of dead exoplanets. Scientists can use the exoplanet graveyards to infer properties about the bodies that once circled them.
Reconstructing rock types.
The researchers focused on polluted white dwarfs that already had precise measurement data showing their ratio of magnesium, calcium, Silicon and iron in their atmospheres. Astronomers think these elements are common in the core and mantles of exoplanets, which make up the majority of planets below the outer crust. Scientists can reverse engineer the minerals that would have made up the planets' rocky guts by calculating the ratio of these elements.
To do this, the researchers used a set of calculations that had previously worked remarkably well when used to "classify rocks on Earth" with similar data, co-author Putirka told Live Science.
The surprising majority of the minerals that made up these exoplanets were very different from what they expected, Putirka said.
An artist's interpretation. The image is from Shutterstock.
Putirka said that the rocks that occur in the mantle consist of mostly three minerals. The ratio of elements in white dwarfs showed that some of the minerals would not have formed.
Putirka said that other minerals made up of magnesium-rich periclase and quartz, which is a crystalline mineral made of silica, would have formed instead, which are different from those predicted within the other inner planets in the solar system. Past assumptions that exoplanets would be similar to the solar system are incorrect.
The researchers had to create new names for the minerals, including "periclase dunites" and "quartz pyroxenites." It's not clear how many new minerals exist in these white dwarfs. Putirka said that new experiments are needed to fully understand the mineralogy of the new compositions.
The interior and exterior are different.
In the past, studies into the atmospheres of polluted white dwarfs have focused on whether or not they had a continental crust like that of Earth. Scientists think a continental crust is vital for sustaining life on a planet because it provides a stable structure for evolution to occur. The chances of finding an Earth-like exoplanet are related to the likelihood of having crusts.
In a paper published in the journal Nature Astronomy, researchers claimed to have found evidence of an Earth-like continental crust in the atmospheres of polluted white dwarfs. The paper noted that a large part of the exoplanets' compositions were different from Earth's. The authors of the study focused on a specific set of elements as evidence to conclude that continental crust is present on the planets.
The authors of the new paper are not convinced. Putirka disagreed that their identifications were valid examples of continental crust. They rely too much on the presence of individual elements and not enough on the mineral they were from, he said.
The researchers think that continental crusts within a polluted white dwarf may not be possible because they make up a small fraction of an exoplanet's mass. Putirka said that the Earth's crust is less than half of its mass. It will be impossible to see crustal compositions if planets are being assimilated wholesale into white dwarf atmospheres.
There is still hope in the search for continental crusts among exoplanets. The researchers believe that learning more about the minerals within a planet's mantle could tell them more about how likely it is that those worlds could have supported a plate tectonics.
"If we have a mantle that contains no olivine but also has periclase, it could affect the type, thickness and extent of the crust," Putirka said. New experiments are needed to understand geological histories.
The study was published in Nature Astronomy.
Live Science published the original article.
