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.
Understandably, Byrne has dubbed such worlds “toffee planets.”
The research, presented at the 50th Lunar and Planetary Science Conference in the Woodlands, Tex., has yet to be peer-reviewed. That has not stopped Byrne’steam 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 say goodbye to the building of mountains, 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 Sara Seager, 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.”
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.
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 Kayla Iacovino, 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.
Although approving of Byrne’s team’s first-order calculations and the principles behind them, Brad Foley, a geodynamicist at Pennsylvania State University, who was also not involved in the research, is skeptical of another key assumption: that the lithosphere, the uppermost mechanical layer of a planet, would be extremely thin on these rocky super Earths. A planet with ductile rocks right up to its surface could still have a thick lithosphere, he says, because lithospheric thickness is determined by how vigorously a world’s underlying mantle is churning. Because this churning is not something that the BDT significantly influences, one cannot use the BDT to gauge how thick the lithosphere is.
Although the model remains a work in progress, Iacovino says that it already demonstrates a “really cool way to take a very small data set and make inferences about what the lithospheres of extrasolar worlds might look like. It’s extremely simplified,” but because of a huge dearth of exoplanetary data, “it’s what we have to work with.”
Paint by Numbers
Byrne acknowledges that the only way to test the validity of the model is to obtain direct observational data of candidate rocky super Earths. Although there is some hope that colossal, next-generation exoplanet-spying telescopes will be able to get some vague sense of their topography, for now, such feats remain well beyond our capabilities.
With that in mind, Robert Stern, a geoscientist at the University of Texas at Dallas, who was not involved in the research, says that this ambitious work deserves credit, because these sorts of numerical models will help paint a picture of what exoplanetary geology may be like. “In my lifetime, I’ve seen the solar system turn from something that belonged to astronomers to something that belonged to geologists,” he says. “We’re not there yet with exoplanets, but you can see this is a step in that direction.”
Although incremental and provisional, the toffee worlds hypothesis could represent a sizable step indeed, as it directly addresses a question foremost in many an exoplanet-pondering geologist’s mind: Are worlds with plate tectonics common as dirt or vanishingly rare? Either way, the answer has game-changing implications.
In order for plate tectonics to exist, a planet needs a few ingredients. Water is probably vital, because it weakens the mantle and permits chunks of the planet to slip and slide that otherwise would remain immobile. A world’s plates also must be sufficiently thick and dense to sink into the mantle—a crucial step for initiating and stabilizing the tectonic cycle over eons. Water or no, toffee worlds’ wafer-thin, brittle layers would not be able to dive deep, short-circuiting the “engine” of plate tectonics before it could even start.
This concept reinforces the notion that plate tectonics is a rare feature in the cosmos, Stern says. After all, as far as we can presently see, Earth is the only planet where it operates.
Aside from making toffee planets geologically dull, the absence of plate tectonics could also significantly reduce continental erosion and runoff into any oceans. This, Stern explains, would rob toffee worlds of a nutrient pump than may have given life a huge boost on ancient Earth. Plate tectonics also acts as Earth’s thermostat, keeping the planet’s temperature stable on geological timescales by buffering the levels of atmospheric greenhouse gases. Both of these tectonic side effects may be essential for the development of complex life.
Plate tectonics certainly gives Earth’s long-term biogeochemical cycles a constant refresh, Seager says, but she speculates that having a mantle far closer to the surface could create an entirely different refreshing mechanism. Although currently unknown to science, there is nothing to say that this situation could not prop up toffee worlds’ very own biogeochemical cycles, too.
In any case, “we don’t know that plate tectonics is a requisite for habitable planets,” Byrne says, “so we certainly can’t say that toffee planets are uninhabitable.”
The most contentious aspect of this thought-provoking study is not actually about any of the science. It is about the name for these possible exoplanets.
The team is composed of researchers hailing from the U.K. and Ireland. “To us,” Byrne says, “‘toffee’ means something soft and chewy.” Scientists from North America tend not to see the word that way, considering toffee to be a hard, crunchy treat. Byrne suggests “taffy” or “fudge” might be better, and a Twitter poll he ran put “squidgy” on par with “toffee” as participants’ preferred nomenclature.
Things have become even more complicated with the discovery by NASA’s Transiting Exoplanet Survey Satellite (TESS) of a brand-new confirmed rocky super Earth, HD 213885b. Byrne’s calculations suggest that this newfound world might be a toffee planet, with a brittle layer just more than two miles thick. The problem is that the radiation from HD 213885b’s parent star is akin to that of 55 Cancri e, another known rocky super Earth whose dayside is entirely molten.
“If HD 213885b is similarly hot, then any lack of rigidity at the surface won’t be from relatively higher surface gravity so much as the floor being lava,” Byrne says. It’s not quite a toffee planet, then, but something very close.
Maybe, he suggests, “fondue planets” are a thing, too.