Physicists scramble to understand the extreme crystals hiding inside giant, alien planets
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An artist illustrated this image of an exoplanet in a distant star system.(NASA)
Deep in the heart ofalien worlds, crystals form under pressures up to 40 million times more intense than the atmospheric pressure on Earth, and as much as 10 times more intense than the pressure in our planet’s core. Understanding them better could help us search for life elsewhere in our galaxy.
Right now, scientists know almost nothing about these mysterious crystals. They don’t know how and when they form, what they look like or how they behave. But the answers to those questions could have enormous implications for the surfaces of those worlds — whether they are covered either in flowing magma or ice, or are bombarded with radiation from their host stars. The answer, in turn, could affect the possibility of these planets harboring life.
The interiors of these exoplanets are mysterious to us because, in our solar system, planets tend to be either small and rocky, like Earth and Mars, or large and gassy, like Saturn and Jupiter. But in recent years, astronomers have found that so-called “super-Earths” — giant rocky planets — and “mini-Neptunes” — smaller gas planets than exist in our solar system — are more common in the rest of our galaxy. [9 Most Intriguing Earth-Like Planets]
Because these planets can be seen only as faint flickers in the light coming from their host stars, much about them remains mysterious. Are they superdense or superwide? What are their surfaces made of? Do they have magnetic fields? The answers to those questions, it turns out, depend heavily on how the rock and iron in their ultrapressurized cores behave.
Right now, our understanding of exoplanets is based mostly on scaling up or down what we know about planets in our own solar system, said Diana Valencia, a planetary scientist at the University of Toronto in Canada, who called at the March meeting of the American Physical Society (APS) for mineral physicists to explore these exotic exoplanetary materials.
The problem with the scaling-up approach is you can’t really understand how iron will behave at 10 times the pressure of Earth’s core just by multiplying, she said. At those enormous pressures, the properties of chemicals fundamentally change.
“We would expect to find crystals inside super-Earths that don’t exist in Earth, or anywhere else in nature, for that matter,” said Lars Stixrude, a theoretical mineral physicist at the University of California, Los Angeles, who has done basic theoretical work to calculate the properties of these extreme materials. “These would be unique arrangements of the atoms that only exist at very high pressure.”
These different arrangements happen, he told Live Science, because enormous pressures fundamentally change how atoms bind together. On Earth’s surface and even deep inside our planet, atoms link up using only the electrons in their outer shells. But at super-Earth pressures, electrons closer to the atomic nucleus get involved and completely change the shapes and properties of materials.
And those chemical properties could affect the behavior of whole planets. For example, scientists know that super-Earths trap a lot of heat. But they don’t know how much — and the answer to that question has major implications for those planets’ volcanoes and plate tectonics. At Earth’s internal pressures, lighter elements get mixed in with the iron core, impacting the planet’s magnetic field — but that might not happen at higher pressures. Even the physical size of super-Earths depends on the crystal structure of compounds in their cores.
But without planets of this sort to study up close in our own solar system, Valencia said, scientists have to turn to basic physical calculations and experiments to answer these sorts of questions. But those calculations often turn up open-ended answers, Stixrude said. As for the experiments?
“Those pressures and temperatures are beyond the capability of most of the technology and experiments we have today,” he said.
Building a super-Earth on regular Earth
On Earth, the most extreme pressure experiments involve crushing tiny samples between the sharpened points of two industrial diamonds.
But those diamonds tend to shatter long before reaching super-Earth pressures, Stixrude said. To get around the limitations of diamonds, physicists are turning to dynamic-compression experiments, of the sort performed by the mineral physicist Tom Duffy and his team at Princeton University.
These experiments produce more super-Earth-like pressures, but only for fractions of a second.
“The idea is, you irradiate a sample with a very high-powered laser, and you rapidly heat the surface of that sample and you blow off a plasma,” Duffy, who chaired the APS session where Valencia spoke, told Live Science.
“It’s really like a rocket ship effect,” Duffy said.
The samples involved are tiny — nearly flat, and just about a millimeter square in surface area, he said. And the whole thing lasts a matter of nanoseconds. When the pressure wave reaches the back of the sample, the whole thing shatters. But through careful observations during those brief pulses, Duffy and his colleagues have figured out the densities and even the chemical structures of iron and other molecules under previously unheard-of pressures.
There are still many unanswered questions, but the state of knowledge in the field is changing fast, Valencia said. For instance, the first paper on the structure of super-Earths (which Valencia published in Feb. 2007 inThe Astrophysical Journalas a graduate student at Harvard) is outdated because physicists have obtained new information about the chemicals inside our own planet.
Answering these questions is important, Duffy said, because they can tell us whether distant alien worlds have characteristics like plate tectonics, flowing magma and magnetic fields — and therefore, whether they could support life.