Radioactive elements may be key to exoplanet habitability

The amount of radioactive elements in a planet’s core may be a key factor in determining its habitability. In this image, too much radiogenic heating stifles convection and the generation of a magnetic field while triggering mass extinction-level volcanism. All images: University of California-Santa Cruz/Melissa Weiss

As it turns out, organic material, liquid water, sunlight and, possibly, a large moon might not be enough to ensure an exoplanet’s habitability. It also may depend in part on whether enough long-lived radioactive elements are present in the planet’s deep interior.

That’s because the radioactive decay of uranium and thorium produce the heat needed to power plate tectonics and volcanism, driving convection in a planet’s molten metallic core. That convection, in turn, creates an internal dynamo in Earth’s interior that produces a protective magnetic field, shielding the surface from the harmful effects of solar radiation.

A planet like Earth, depicted here, has just the right amounts of thorium and uranium in its core to generate dynamo-creating convection and a magnetic field that shields the surface from the harmful effects of solar radiation.

In a paper published 10 November in Astrophysical Journal Letters, researchers at the University of California-Santa Cruz considered how different amounts of radioactive elements might affect a planet’s evolution.

“What we realised was that different planets accumulate different amounts of these radioactive elements that ultimately power geological activity and the magnetic field,” said Francis Nimmo, professor of Earth and planetary sciences and lead author of the study.

“So we took a model of the Earth and dialled the amount of internal radiogenic heat production up and down to see what happens.”

A planet with too little radiogenic heating, seen here, may have a magnetic field, but no plate tectonics or volcanism and is geologically dead.

They found radiogenic heating is too high, a dynamo cannot be sustained indefinitely because most of the thorium and uranium will end up in the mantle, producing enough heat to make the mantle act as an insulator. That, in turn, would prevent a molten core from losing heat fast enough to generate the convection required for a magnetic field. It also could trigger rampant volcanism and frequent mass extinction events.

Conversely, too little radiogenic heating in the core results in a geologically dead planet.

“Just by changing this one variable, you sweep through these different scenarios, from geologically dead to Earth-like to extremely volcanic without a dynamo,” Nimmo said.

“Now that we see the important implications of varying the amount of radiogenic heating, the simplified model that we used should be checked by more detailed calculations.”