Toffee Planets” Hint at Earth’s Cosmic Rarity

It might not occur to us surface dwellers very often, but rocks can flow—more like the way exceedingly lethargic toothpaste would rather than water. Exposed to the extreme temperatures and pressures that reign in the hellish realms far below our feet, rocks can practically swim—slowly diving down and bobbing up through much of Earth’s subsurface.

For some rocky worlds around other stars, what is true for Earth’s innards may extend right up to the surface. Super Earths—sometimes rocky exoplanets that are bigger than our pale blue dot but smaller than massive ice giants such as Neptune—have comparatively strong gravitational fields. Thanks to this extreme gravity, some scientists suspect, rocks on such worlds would flow far closer to the surface.

This arrangement would mean rocks that snap, fracture and break might only be found in thin veneers on these exoplanets’ crust. If these rocky super Earths have thick, Venus-like atmospheres or are especially close to their parent star, they might exhibit no familiarly brittle geology at their surface at all. Instead, says Paul Byrne, a planetary scientist at North Carolina State University and lead author of a study on the Super Earths, their surface rocks would be strangely malleable over long timescales, flowing a bit like the stretchy, sugary confections on offer in any earthly candy shop.

Byrne and his colleagues’ work hinges on defining the point at which rocks deep below a planet’s surface no longer break in a mechanical way and instead begin to move like hot plastic. This point, known as the brittle-ductile transition (BDT), depends on how the pressure and temperature change with depth. For our own world’s crust, the BDT lies about 15.5 miles below the surface, although it varies quite a bit. But what about on super Earths, where greater gravitational forces would correspondingly increase pressures on rock? At what depths would BDTs emerge on such alien planets?

Taking inspiration from their own 2017 precursor paper, the researchers compiled data from 200 preexisting studies examining the lab-based deformation of basalt and other common rock types over a wide range of pressures and temperatures. They first used these data to calculate the BDT depth for Earth, calibrating their equations until sensible numbers emerged. Then they plugged in the estimated gravitational forces prevailing on five sizable, potentially rocky exoplanets found by NASA’s late, great Kepler space telescope, from the hefty Kepler-36b to the smaller Kepler-406c.