Never-before-seen rocks found in these exoplanet graveyards

Astronomers have discovered never-seen-before rock types, made up of unusual ratios of minerals, within the remains of alien worlds ripped apart by their dying host stars. The research suggests that such exoplanets are built from a much wider array of materials than previously thought. 

In the new study, researchers looked at 23 white dwarfs — the small, dense remains of dead low- and medium-mass stars — within 650 light-years of the sun. As these stars were dying and transitioning into white dwarfs, they ripped apart their orbiting exoplanets. And so, the atmospheres of these white dwarfs contain the guts from the alien worlds they destroyed. 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 researchers found that only one of the white dwarfs contained the remains of exoplanets with a similar geological make-up to Earth. Within the rest of the dead stars, the researchers found the remains of exoplanets made of alien rocks never seen on our planet or the rest of the solar system. The rocks were so different from those known to science that the researchers even had to create brand new names to classify them. 

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"While some exoplanets that once orbited white dwarfs appear similar to Earth, most have rock types that are exotic to our solar system," lead author Siyi Xu, an astronomer at the National Optical-Infrared Astronomy Research Laboratory (NOIRLab) in Arizona, said in a statement. "They have no direct counterparts in the solar system."

Exoplanet graveyards 

White dwarfs form when main sequence stars, like the sun, 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. During this process, these dying stars release a cloud of superheated gas that engulfs the planets that orbit them. 

Some exoplanets can withstand this cosmic barrage, but most get knocked out of their orbit and subsequently ripped apart by the white dwarf's strong gravitational field. This is known as tidal disruption; and once the planet is ripped apart, the white dwarf pulls the planetary remains inward in a process known as accretion. 

Normally, the atmosphere of a white dwarf contains only hydrogen and helium, because any heavier elements sink into the star's super-dense core. So, when the light the stars give off shows the presence of other heavier elements, researchers assume that those must come from exoplanet accretion.

Scientists have estimated that about 25% of all white dwarfs contain the remains of dead exoplanets or are so-called polluted white dwarfs. These exoplanet graveyards have become a hot topic of research among astronomers because scientists can use them to infer properties about the bodies that once circled them.

Reconstructing rock types 

In the new study, 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 cores and mantles of exoplanets, which make up the majority of planets below the outer crust. By calculating the ratio of these elements, scientists can reverse-engineer the minerals that would have made up the planets' rocky guts. 

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 Keith Putirka, a geologist at California State University, told Live Science.

However, the results revealed that a "surprising" majority of the minerals that made up these exoplanets were very different from what they expected, Putirka said.

"On Earth, rocks that occur in the mantle consist of mostly three minerals, olivine, orthopyroxene and clinopyroxene," Putirka said. But the ratio of elements in most polluted white dwarfs showed that some of these minerals would have been unlikely to form, he added.

Instead, other minerals made up of different formulations 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, Putirka said. This goes against past assumptions that exoplanets would be more similar to those we see in the solar system.

These minerals are so different from those we know that the researchers had to create new names to classify them, including "quartz pyroxenites" and "periclase dunites." However, it is unclear exactly how many new minerals exist in these white dwarfs. "New experiments to fully understand the mineralogy of the new compositions" are required, Putirka said.

Interior vs. exterior 

In the past, studies into the atmospheres of polluted white dwarfs have focused on whether or not exoplanets were likely to have a continental crust like that of Earth. A continental crust, scientists think, is vital for sustaining life on a planet because it provides a stable structure for evolution to occur. The likelihood of exoplanets having crusts could therefore answer questions about the possibility of alien life or the chances of finding an Earth-like exoplanet.

In a paper published in February in the journal Nature Astronomy, researchers claimed to have found evidence of an Earth-like continental crust in the atmospheres of polluted white dwarfs. Like the newest study, this paper noted that a large part of the exoplanets' compositions was different from Earth's, Live Science previously reported. But rather than focus on the differences between the planets' overall compositions, the authors of that study focused on a specific set of elements as evidence to conclude the presence of continental crust. 

However, the authors of the new paper are not convinced. "We disagree that their identifications are valid examples of continental crust," Putirka said. Their assumptions rely too heavily on the presence of individual elements like aluminium and lithium, and not enough on the mineral they were from, he added.

The researchers also think that it may not even be possible to detect continental crusts within a polluted white dwarf because they make up such a small fraction of an exoplanet's mass. "Earth's crust is less than 0.5% of its total mass," Putirka said. "If planets are being assimilated wholesale into white dwarf atmospheres, it will be impossible to see crustal compositions."

But this doesn't mean that there is no hope in the search for continental crusts among exoplanets. Instead, 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 crust or even plate tectonics, which are overlapping sections of a continental crust which move and collide with each other leading to earthquakes and volcanic activity. 

"If we have a mantle that contains no olivine but has quartz, or a mantle that contains no orthopyroxene but has periclase, the thermodynamic and physical properties could be quite different and could affect the type, thickness and extent of crust," Putirka said. "New experiments are needed to truly understand the kinds of geological histories that might be possible."

The study was published online Nov. 2 in the journal Nature Astronomy.

Originally published on Live Science.