For decades, scientists have looked to the strange worlds beyond our solar system to learn more about our home planet. A team of researchers using the resources of the United States Department of Energy (DOE) Argonne National Laboratory recently discovered more about these planets without leaving Earth.
Over 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 the extrasolar planets are composed of and how they are structured is important for understanding how the planets of our galaxy form, behave and evolve.
“Thanks to the discovery of extrasolar planets, we have a whole new vision of what exists, the types of planets feasible and how they work,” said Thomas Duffy, professor of geosciences at Princeton University.
“Thanks to the discovery of extrasolar planets, we have a whole new vision of what exists, what types of planets are possible and how they can function. —Thomas Duffy, Princeton University
For example, some extrasolar planets are composed of the same silicate minerals that make up most of Earth, but are up to 10 times larger and, therefore, have much higher pressures and temperatures inside. Pressures in the mantle of large, rocky exoplanets can be three times greater than the pressure at Earth’s center, according to Duffy. He and his colleagues set out to better understand the physical properties that 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 ultra-bright X-ray beams from the Advanced Photon Source (APS), a DOE Installation of users of the Office of Science in Argonne. The team’s findings were recently published in the Proceedings of the National Academy of Sciences.
“None of this would have been possible without the state-of-the-art high-pressure beamlines of the APS“said Dutta.
Specifically, the scientists studied magnesium germanate, an analogue of the magnesium silicate minerals that make up most of the Earth’s mantle. By substituting a larger germanium ion for silicon, the team was able to 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 scientist at the Carnegie Institution for Science who participated in the research. Tracy and her colleagues assessed 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, scientists discovered that the mineral adopted the structure of a compound called thorium phosphide. This, they think, could be an important part of the deep interiors of large rocky extrasolar planets.
“It’s not like any crystalline structure you find on Earth or other planets in our solar system,” Duffy said.
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 ionic structure instead of having a distinct order.
The researchers were surprised by this disruption.“A structure where two different ions with vastly different size and valence substitute for each other goes against our intuition,” Tracy said.“The idea that this type of disordered structure can be stabilized at high pressure and temperature opens the door to thinking about other new mineral structures that could be viable under extreme conditions.
Disordered structures tend to incorporate impurities and defects more easily, which can affect physical properties. One of them is thermal conductivity, which influences how planets cool and change over time.
“The discovery of these phases has revolutionized our understanding of the deep earth,” Dutta said.
To learn more 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 among the brightest in the world.
These beamlines allowed the researchers to achieve extremely high pressure by pressing the sample between two diamonds. High temperatures have been achieved using advanced laser heating techniques. The samples were studied with an intense, tightly focused X-ray beam.
“We focused X-rays down to about three microns, or nearly 50 times thinner than a strand of hair, to probe a tiny sample under very extreme conditions,” said Vitali Prakapenka, co-author of the study and research professor. at the University of Chicago.
By analyzing the diffraction pattern created by shooting an X-ray beam through the mineral at high temperature and pressure, the scientists determined the structure and density of this new phase of thorium phosphide.
The researchers were able to operate the beamlines remotely, which was essential as the experiment began early in the COVID-19[feminine] pandemic.
“The discoveries made during this project lifted my spirits during a difficult time,” said Yue Meng, Argonne physicist and co-author of the study, noting that none of this would be possible without the staff at exceptional support from APS who ensured the proper functioning of the beamlines.
Scientists plan to further explore this new crystal structure to better understand the dynamics of extrasolar planets and learn more about our universe.“It’s curiosity-driven science,” Duffy said.“There are very strange worlds out there and we can discover exotic types of planets that we never dreamed of before.
About the Advanced Photon Source
Advanced Photon Source from the US Department of Energy Office of Science (APS) at Argonne National Laboratory is one of the most productive X-ray light source facilities in the world. The APS provides high-luminosity X-ray beams to a diverse community of researchers in materials science, chemistry, condensed matter physics, life and environmental sciences, and applied research. These X-rays are perfectly suited to the exploration of materials and biological structures; elementary distribution; chemical, magnetic, electronic states; and a wide range of technologically important engineering systems, from batteries to fuel injectors, all of which are the foundations of our nation’s economic, technological, and physical well-being. Each year, more than 5,000 researchers use the APS to produce more than 2,000 publications detailing impactful discoveries and solving more vital biological protein structures than users of any other x-ray light source research facility. APS scientists and engineers are innovating in the technology that is at the heart of advancing the operations of accelerators and light sources. This includes insertion devices that produce the extremely bright x-rays prized by researchers, lenses that focus x-rays down to a few nanometers, instrumentation that maximizes how x-rays interact with samples studied and the software that gathers and manages the massive amount of data resulting from discovery research at scale APS.
This research used the resources of the Advanced Photon Source, an American organization DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under contract No. DE-AC02-06CH11357.
Argonne National Laboratory seeks solutions to pressing national problems in science and technology. The nation’s first national laboratory, Argonne conducts cutting-edge basic and applied scientific research in virtually every scientific discipline. Argonne researchers work closely with researchers from hundreds of companies, universities, and federal, state, and municipal agencies to help them solve their specific problems, advance American scientific leadership, and prepare the nation for a better future. With employees from more than 60 nations, Argonne is operated by UChicago Argonne, SARL for the Office of Science of the United States Department of Energy.
U.S. Department of Energy Office of Science is the largest supporter of basic physical science research in the United States and strives to address some of the most pressing challenges of our time. For more information, visit https://energy.gov/science.