Exoplanets need right stuff to be habitable

SAN FRANCISCO — For some exoplanets, just being in the Goldilocks zone isn’t enough. Planets need to be made up of the right stuff to become a cradle of life, new research suggests.

Planets composed of certain element cocktails can’t host a continual recycling of Earth-like tectonic plates, new simulations of exoplanet interiors indicate. Measuring the compositions of stars could help astronomers narrow the list of potentially habitable planets, said Cayman Unterborn, who presented the work December 18 at the American Geophysical Union’s fall meeting.

“This is a new way of thinking — astronomers don’t think in geology terms,” said Unterborn, an extrasolar planetary scientist at Ohio State University. Exoplanet hunters currently treat an exoplanet as potentially habitable if it falls the right distance from its sun for water to exist in liquid form.

“When we talk about habitable planets, it may not be wise to just say Goldilocks zone — there may be a Goldilocks composition as well,” Unterborn said.

Earth’s surface is divided into a jigsaw puzzle of shifting rigid plates. When two plates collide, one can dive under the other and down into the mantle. This subduction helps regulate carbon dioxide levels in the atmosphere. Without plate tectonics, CO₂ from volcanic eruptions would accumulate unchecked and cook the planet, as on Venus.

A descending plate is initially too light to sink all the way into the mantle. As pressures mount during descent, however, the atoms in the plate undergo a reorganization that makes the plate denser. This phase change, at roughly 40 kilometers below ground, allows the plate to sink deeper into the mantle. Without this phase change, the sinking plate would stall and shut down plate tectonics.

Phase change properties depend on the composition of the planet. Astronomers currently can’t measure the compositions of exoplanets, but they can for stars. Since stars share very similar compositions with their planets, Unterborn and colleagues can use data from a star to estimate a planet’s elemental mix.

Key elements for rocky planets include magnesium and silicon. Together, these elements provide about 29 percent of Earth’s mass. The mix of the two is critical for which minerals form. Simulating the dynamics of exoplanets with various compositions, Unterborn and colleagues found that silicon-rich planets with a magnesium-to-silicon ratio around 40 percent smaller than Earth’s can’t support plate tectonics. Sinking plates on these planets still undergo phase changes but would never become denser than the surrounding material. The team is now calculating what exact range of stars cannot have tectonically active exoplanets.

Even the proper element mix doesn’t guarantee plate tectonics. Among our solar system’s four rocky planets, only Earth is known to currently have plate tectonics. The findings provide a way to cross exoplanets off the potentially habitable list, not put them on the list in the first place, Unterborn said.

The new work is a step in “just the right direction,” said planetary scientist Lindy Elkins-Tanton of Arizona State University in Tempe. But confirming the results will be difficult, she warned. “It’s almost impossible for us to detect whether exoplanets have plate tectonics,” she said. “It’s not going to happen in the coming couple of decades. We cannot even tell if Venus has plate tectonics, and it’s right next door.”