For decades, scientists have looked to the strange worlds beyond our solar system to understand more about our home planet. A team of researchers using the resources of the U.S. Department of Energy's (DOE) Argonne National Laboratory recently discovered more about those planets without leaving Earth.
More than 5,000 extrasolar planets have been discovered since 1992. These planets are large astronomical bodies that occur outside our solar system and orbit stars other than the sun. Studying what minerals extrasolar planets are composed of and how they are structured is important for understanding how planets in our galaxy form, behave and evolve.
"Through the discovery of extrasolar planets, we have a whole new vision of what is out there, what type of planets are feasible, and how they may operate," said Thomas Duffy, a geosciences professor at Princeton University.
For instance, some extrasolar planets are composed of the same silicate minerals that make up the bulk of the Earth, but are up to 10 times larger in size, and as a result have much higher pressures and temperatures in their interiors. The pressures in the mantle of large, rocky exoplanets can be three times more than the pressure at the center of the Earth, according to Duffy. He and his colleagues set out to understand more about the physical properties these minerals take on under such pressures.
Duffy and a team of scientists led by Rajkrishna Dutta, a postdoctoral fellow at the Carnegie Institution for Science, conducted experiments on specific minerals under extremely high pressure and temperature. They used the ultrabright X-ray beams of the Advanced Photon Source (APS), a DOE Office of Science user facility at Argonne. The team's results were recently published in the Proceedings of the National Academy of Sciences.
"None of this would have been possible without the cutting-edge high-pressure beamlines at the APS," said Dutta.
Specifically, the scientists studied magnesium germanate, an analogue for the magnesium silicate minerals that make up the bulk of Earth's mantle. By substituting a larger germanium ion for silicon, the team could study transitions between chemical phases at lower temperatures and pressures in the laboratory.
"If we want to understand larger planets that have similar chemical compositions to our Earth, this mineral is a good place to start," said Sally June Tracy, a staff scientist at the Carnegie Institution for Science, who assisted with the research. Tracy and her colleagues evaluated how the atomic structure of magnesium germanate changes under extremely high pressures.
By using two X-ray beamlines at the APS to create these extreme conditions, the scientists discovered that the mineral adopted the structure of a compound called thorium phosphide. This, they think, could be an important component of the deep interiors of large, rocky extrasolar planets.
"It's not like any crystal structure that you find in the Earth or other planets in our solar system," said Duffy.
This structure is interesting for several reasons. First, the number of oxygen atoms surrounding each germanium atom increases from four to eight under high pressure and temperature. Second, the new crystal structure has a disordered ion structure instead of having a distinct order.
The researchers were surprised by this disordering. "A structure where two different ions with largely different size and valence substitute for each other goes against our intuition," said Tracy. "The idea that this type of disordered structure can be stabilized at high pressure and temperature opens the door to thinking about other novel mineral structures that could be viable under extreme conditions."
Disordered structures tend to incorporate impurities and defects more readily, which can affect physical properties. One of those is thermal conductivity, which influences how planets cool and evolve over time.
"The discovery of these phases revolutionized our understanding of the deep earth," Dutta said.
To learn about the properties of the new crystal structure, the team relied on the capabilities of two beamlines at the APS: the High-Pressure Collaborative Access Team (HPCAT), operated by Argonne, and GeoSoilEnviroCARS (GSECARS), operated by the University of Chicago. These X-ray sources are some of the brightest in the world.
These beamlines allowed the researchers to achieve extremely high pressure by squeezing the sample between two diamonds. High temperatures were achieved with advanced laser heating techniques. The samples were studied with an intense, tightly focused X-ray beam.
"We focused the X-ray down to around three microns, which is almost 50 times thinner than a strand of hair, to probe a tiny sample at very extreme conditions," said Vitali Prakapenka, a co-author of the study and a research professor at the University of Chicago.
By analyzing the diffraction pattern created by shining an X-ray beam through the mineral at high temperature and pressure, the scientists determined the structure and density of this novel thorium phosphide phase.
The researchers were able to operate the beamlines remotely, which was essential, as the experiment was started at the beginning of the COVID-19 pandemic.
"The discoveries made during this project lifted my spirit during a challenging moment," said Yue Meng, a physicist at Argonne and co-author of the study, noting that none of this would be possible without the outstanding support staff for the APS who kept the beamlines running smoothly.
The scientists plan on further exploring this new crystal structure to better understand the dynamics of extrasolar planets and learn more about our universe. "It's curiosity-driven science," said Duffy. "There are very strange worlds out there and we may discover exotic types of planets that we never dreamed of before."
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