"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. —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 , 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.
Understandably, Byrne has dubbed such worlds "toffee planets."
The , presented at the in 2019 in the Woodlands, Tex., has yet to be peer-reviewed. That has not stopped Byrne's team speculating on what its findings might mean for the myriad super Earths already discovered beyond our solar system. The most striking possibility is that super Earths might not be able to sustain plate tectonics—the drifting of continents and cycling of crustal rock that intimately shapes Earth. Without that process, you can , the creation of oceans and plenty of a planet's volcanoes, and, just maybe, the evolution of complex life itself.
The science is already starting to stick in experts' mind. "It's a fascinating concept," says , a professor of astrophysics and planetary science at the Massachusetts Institute of Technology. In exoplanetary science, "you rarely see anything new like this. The fact that they came up with something new, that, in itself, is impressive."
Extrasolar Extrapolations
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 , 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 , from the hefty Kepler-36b to the smaller Kepler-406c.
The calculations revealed the BDT depths for those super Earths to be shockingly shallow, with some scarcely more than a mile beneath the surface. A nearby star, a suffocating atmosphere or an abundance of internal, radioactivity-generated heat could further bake the top of such a world, perhaps raising the BDT all the way to the surface, creating a full-blown toffee world.
It is always risky to make planet-scale extrapolations from a figurative handful of data points, and the researchers acknowledge their calculations make assumptions aplenty. One of them, notes , a volcanologist and experimental petrologist at NASA's Johnson Space Center, who was not involved in the work, is that real exoplanets most likely have complicated internal structures—a reality not taken into account in the study's simplified approach.
