Lasers squeezed iron to mimic the conditions of exoplanet cores

Physicists have simulated the cores of some large rocky exoplanets by pummeling iron with lasers. The resulting measurements give the first clue to how iron might behave inside planets outside the solar system that are several times the mass of Earth, researchers report April 16 in Nature Astronomy.

“Until now, there’s been no data available on the state of these materials at the center of large exoplanets,” says Ray Smith, a physicist at Lawrence Livermore National Laboratory in California.

Working at the National Ignition Facility, Smith and his colleagues aimed 176 lasers at a pellet of iron a few micrometers thick wrapped in a gold cylinder. The lasers delivered enough energy over 30 billionths of a second to compress the iron to pressures up to 14 million times Earth’s atmospheric pressure at sea level. The researchers measured how the iron’s density changed at different pressures.

These high pressures are thought to be found in the iron cores of rocky exoplanets that are between three and four times Earth’s mass, Smith says. Although our solar system doesn’t have any planets of this size, they are the most common type of exoplanet in the galaxy. Previous simulations suggest that some of these rocky worlds may have interior compositions similar to Earth’s.

That similarity raises hopes that the exoplanets may have features that make the worlds hospitable to life, such as a magnetic field or plate tectonics (SN: 4/30/16, p. 36). NASA’s latest exoplanet-hunting telescope called TESS, scheduled to launch the evening of April 16, is expected to find hundreds of planets in this size range (SN Online: 4/12/18).

But the details of exoplanet interiors are hard to tease out. Until now, researchers had to extrapolate iron’s behavior at high pressures from measurements made at low pressures, which introduced uncertainties. With the new measurements, scientists can be more confident that their simulations reflect actual planets.

Previously, Smith and colleagues had made some headway by compressing diamond to even greater pressures (SN: 8/9/14, p. 20). Diamond is easy to compress but is not likely found deep inside rocky exoplanets.

“This is one of the first mineral physics experiments that is going to be much more applicable to every planet,” says exogeologist Cayman Unterborn of Arizona State University in Tempe. “That’s going to be really important once we start characterizing exoplanets with all of these new missions.”