Record-Breaking Laser Experiment Could Teach Us About The Cores Of Super-Earths

An enormous laser targeted for shutdown by proposed US budget cuts has just done some record-breaking science in order to understand the centres of faraway exoplanets that scientists call "super-Earths".

The Omega Laser's target chamber.Photo: Laboratory for Laser Energetics

There's a whole lot we don't know about our own planet, like the exact nature of its interior that leads to its magnetic field and plate tectonics. But what about the interiors of rocky planets even larger than Earth? Scientists studied the nature of metals at high pressures using an incredibly intense laser experiment. In doing so, they performed the highest-pressure X-ray diffraction measurement yet.

"All the properties of the material depends on their composition and structure," study author Thomas Duffy from Princeton University told Gizmodo. By taking these measurements, the scientists could get a sense of what a super Earth's core might be like.

The researchers subjected iron-silicon alloys to pressures as high as 1300 gigapascals, over three times the pressure of Earth's core, with the Omega Laser at the Laboratory for Laser Energetics at the University of Rochester. They used X-rays to determine the density and structures these alloys take on at these super-Earth-like pressures.

This data could allow the researchers to determine how cores filled with different elements could affect the radius and density of such planets, according to the paper published in Science Advances.

Creating high pressures like these is difficult to do, and their results will definitely be useful, Cayman Unterborn, exogeologist at Arizona State University, told Gizmodo in an email. He felt it could help scientists more realistically model what these planets look like on the inside. "I think the really important thing is that this is an experiment that isn't just about the Earth, but representative of ALL rocky planets that have cores (and are large, of which there seems to be a lot)."

This experiment is far from a perfect model, of course. For one thing, we don't know that super-Earths would have iron cores that look like the iron we measure in a lab on regular earth. Also, exoplanets are way out in space, while the lab is in Rochester, New York.

But the laser is a truly amazing tool. "It stands 10 meters tall and is approximately 100 meters in length. OMEGA's 60 laser beams focus up to 40,000 joules of energy onto a target that measures less than 1 millimetre in diameter in approximately one billionth of a second," according to its website. It was originally designed to conduct nuclear fusion experiments.

Despite its awesomeness, just a few months ago the Department of Energy suggested de-funding the laser beginning with its 2019 budget. Scientists didn't like that.

"It's so shortsighted to cut out everything that allows us to ask questions for our interests," June Wicks, assistant professor from Johns Hopkins University and first author on the new paper, told Gizmodo. "Also, this is the science that allows us to train future students that will have the skills to be able to work on the hardest problems." Cut the laser, and you'll cut an opportunity to train scientists whose expertise could carry over to things like geoscience or even weapons development.

But for now, the laser is making record-breaking measurements (though others will soon publish with an even higher-pressure measurement, said Wicks). It's also teaching us about the universe.

Said Wicks: "This is just one step in that direction of taking what we think are the building blocks of planets and coming up with ways to describe them."

[Science Advances]

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