Scientists wielded giant lasers to simulate an exoplanet's super-hot core

1/15/2022 1:00:00 AM

Scientists wielded giant lasers to simulate an exoplanet's super-hot core

Scientists wielded giant lasers to simulate an exoplanet's super-hot core

Researchers explored how exoplanets form magnetospheres—magnetic fields that protect from solar radiation—as a window into habitability.

The molten cores of larger rocky exoplanets should stay hot longer than those within small worlds, according to a study published Thursday in the journal Science. That’s good news for interstellar explorers–because a molten core is probably required for life to develop on a planet.

, a physicist at Lawrence Livermore National Laboratory who led the study.Magnetic fields are a result of molten planetary cores. Earth has a core composed mostly of iron, split into a solid inner core and a liquid outer core. Earth’s magnetic field is caused by the convection of the liquid iron, meaning how it swirls: The cooler, denser liquid areas sink to the bottom, while the hotter ones rise like wax in a lava lamp.

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The National Ignition Facility's target chamber is superimposed over the mantle. John Jett, LLNL SHARE The molten cores of larger rocky exoplanets should stay hot longer than those within small worlds, according to a study published Thursday in the journal Science. If these super-Earths’ cores are like our Earth’s, they may have a protective magnetosphere that. That’s good news for interstellar explorers–because a molten core is probably required for life to develop on a planet. Fortunately, astronomers have now noticed something else unusual about this recently discovered middle-range giant, a finding that could help tell us why it's so special: there happen to be hints of water floating in its atmosphere. Determining this feature of exoplanets required an experiment with giant lasers and an incredibly thin sliver of iron placed under unprecedented pressure. “We’re finding so many planets, and [one of] the big questions people have are: are these planets potentially habitable?” says Rick Kraus , a physicist at Lawrence Livermore National Laboratory who led the study. Vose said chances are 50-50 that at least one year in the 2020s will hit 1.

To answer this question, researchers don’t normally start with thinking about a planet’s core.   Planets that form beyond a point where the star's radiation can easily sublimate ice into gas will have a much better chance of holding onto water, for example, raising the chances that a short-orbit Neptune with plenty of water is far from its place of birth. Instead, they ask whether the planet is the right distance from its star or whether it has water. But Kraus and his team wanted to find other ways to discern whether a planet is habitable. They explored a planet’s ability to form a magnetosphere—a magnetic field that protects it from solar radiation, like the one around Earth does for us—as a window into habitability, Kraus says. But it's already proved interesting enough for researchers to turn other instruments, such as the Hubble Space Telescope, towards the planet to probe its secrets. Life as we know it wouldn’t be possible without the Earth’s magnetic field. The global average temperature last year was 58.

Magnetic fields are a result of molten planetary cores. Earth has a core composed mostly of iron, split into a solid inner core and a liquid outer core. Better still, at a distance of just 150 light years away, the whole planetary system is more or less in our backyard. Earth’s magnetic field is caused by the convection of the liquid iron, meaning how it swirls: The cooler, denser liquid areas sink to the bottom, while the hotter ones rise like wax in a lava lamp. Studying an exoplanet’s core in a laboratory is difficult because there are few ways to recreate such intense pressures and temperatures. This is the first experiment to use iron under pressures that exceed those in Earth’s core, Kraus says. Knowing how TOI 674b fell into such a hot embrace with its star will help us fill in the bigger picture of how other solar systems evolve, and whether our own is boringly normal or a unique gem in an ocean of chaos. Last year, 1.

To achieve these extremes, the team needed some big lasers, specifically the National Ignition Facility in Lawrence Livermore National Lab–where big lasers are their specialty. In the experiment, those lasers blasted a multilayered sample of iron. Layers of beryllium, a metal element, and some filters formed the outside of a super-thin iron “sandwich” while a piece of transparent lithium fluoride made up the other half, Kraus says. The outer beryllium layer heated up thousands of degrees in “a fraction of a billionth of a second,” he says, and that side of the sandwich cooked into a plasma. The plasma then expanded, driving an intense shockwave into the sample..

[Related: Saturn has a slushy core and rings that wiggle ] This process emulates the conditions that a section of hot iron would experience as it descends through a planet’s molten core. “You’re shocking the hell out of it,” says Peter Driscoll , geophysicist at Carnegie Science who models the core of Earth and other planets and was not involved in the study, who adds this was a difficult process to study. It destroys the sample, so the experimenters have to collect their data in one go. The process only produced “a couple of data points,” he says, but “these kinds of experiments are very valuable.” As the sample reached peak pressures, another instrument tested whether that iron remained solid or liquid at key times, to help researchers home in on iron’s behavior at these high pressures and temperatures. The last time Earth had a cooler than normal year by NOAA or NASA calculations was 1976.

The team found that “as you increase the pressure, the temperature increases, quite rapidly,” Kraus says. For exoplanets, that means the larger they get, the longer it will take their cores to solidify. The super-Earths that are between four and six times Earth’s mass would take the longest, he says. The team estimates that it will take a total of 6 billion years for Earth’s core to solidify, whereas cores in large exoplanets of similar composition to Earth should take up to 30 percent longer. “While that may sound sort of intuitive,” it wasn’t a given with all the different factors, Kraus says.

Measuring how iron melts in extreme conditions is so important because it tells you if and how a planet’s core will solidify, Driscoll says. Even at the boundary between Earth’s solid inner core and liquid outer core, scientists don’t know precisely what the temperature is–though it’s estimated to be near that of the sun’s surface, at roughly 10,000°F. One issue with extrapolating these results to exoplanets is that those super-Earths can contain elements other than iron in their core, which would change their melting temperature by an unknown amount, Driscoll says. It will also be hard to predict how exoplanets cool because the mantle, the layer of hot rock surrounding the core, plays a huge role in how quickly the core can cool. And those exoplanet mantles could be made of “pretty much anything,” he says.

Venus, Driscoll says, is the go-to example of this disconnect. On paper its composition is very similar to Earth’s, but it lacks a magnetic field and plate tectonics. Other factors can decide whether a magnetosphere will form, too, such as how well the material in the core lets heat or electricity flow through it. But these characteristics are difficult to measure, even on Earth–scientists have only succeeded in measuring the flow of heat through Earth’s core in the last decade. Still, Driscoll says, that would be, “the next thing to go after.

” Leto Sapunar Leto is a freelance science journalist. He studied physics at Oregon State University, then got a master’s at NYU’s Science Health and Environmental Reporting Program (SHERP). You can also find his bylines at Scientific American, Inside Climate News, and Retraction Watch among other outlets. When he’s not working, he’s writing science fiction or climbing something, anything. .