The thickness of the brittle lithosphere — the outer portion of a planetary body that fails via fracturing — plays a key role in the geological processes of that body. The properties of both a planet and its host star can influence that thickness, and the potential range of those properties exceeds what researchers see in the Solar System. In new research, an international team of planetary scientists ran a large set of computer models to see how various combinations of planetary and stellar properties influence the thickness of a planetary body’s outer layer. Their models predict that worlds that are small, old, or far from their star likely have thick, rigid layers but, in some circumstances, planets might have an outer brittle layer only a few kilometers thick. These worlds, which the authors call ‘eggshell planets,’ might resemble the lowlands on Venus, and they suggest that at least three such extrasolar planets — TOI-1235b, HD 136352b, and L 168-9b — are already known.
An artist’s rendition of an eggshell exoplanet. Image credit: NASA / JPL-Caltech.
“Understanding whether you’ve got the possibility of plate tectonics is a really important thing to know about a world, because plate tectonics may be required for a large rocky planet to be habitable,” said Dr. Paul Byrne, a planetary geologist in the Department of Marine, Earth, and Atmospheric Sciences at North Carolina State University and the Department of Earth and Planetary Sciences at Washington University in St. Louis.
“It’s therefore especially important when we’re talking about looking for Earth-like worlds around other stars and when we’re characterizing planetary habitability generally.”
“We know from published work that there are exoplanets that experience conditions in a more extreme way than what we see in our Solar System,” he added.
“They might be closer to their star, or they might be much larger, or have hotter surfaces, than the planets we see in our own system.”
Dr. Byrne and colleagues wanted to see which planetary and stellar parameters play the most important role in determining the thickness of a planet’s outer brittle layer, which is known as the lithosphere.
This thickness helps determine whether, for example, a planet can support high topography such as mountains, or has the right balance between rigidity and flexibility for one part of the surface to dive down, or subduct, beneath another — the hallmark of plate tectonics.
It is this process that helps Earth regulate its temperature over geological timescales, and the reason why plate tectonics is thought to be an important component of planetary habitability.
For their modeling effort, the researchers chose a generic rocky world as a starting point.
“And then we spun the dials. We literally ran thousands of models,” Dr. Byrne said.
The team discovered that surface temperature is the primary control on the thickness of brittle exoplanet lithospheres, although planetary mass, distance to its star and even age all play a role.
Their new models predict that worlds that are small, old or far from their star likely have thick, rigid layers, but, in some circumstances, planets might have an outer brittle layer only a few kilometers thick.
“Although we are a long way from directly imaging the surfaces of these eggshell planets, they might resemble the lowlands on Venus,” Dr. Byrne said.
Those lowlands contain vast expanses of lavas but have little high-standing terrain, because the lithosphere there is thin as a result of searing surface temperatures.
A plot of brittle lithospheric thickness as a function of surface gravitational acceleration, g, surface temperature, T, and plate age (as a proxy for heat flow). The four planets of the inner Solar System are shown, as are four super-Earths for which radius, mass, and equilibrium temperatures are available: HD 136352b, L 168-9b, LHS 1140b, and TOI-1235b. The ranges of uncertainty in g and T for the exoplanets are shown with white lines. All planets are shown to scale; the illustrations of the four exoplanets are artistic impressions. The solid, dashed, and dotted lines denote where on this plot brittle lithosphere thickness is 5, 2, and 1 km for a range of plate ages from 0 to 300 million years. Image credit: Byrne et al., doi: 10.1029/2021JE006952.
“Our overall goal is more than just understanding the vagaries of exoplanets,” Dr. Byrne said.
“Ultimately we want to help contribute to identifying the properties that make a world habitable.”
“And not just temporarily, but habitable for a long time, because we think life probably needs a while to get going and become sustainable.”
The fundamental question behind this research is, of course, are we alone?
“That is the big reach. Ultimately most of this work is tied into this final destination, which is ‘how unique, or not, is Earth?’” Dr. Byrne said.
“One of the many things we are going to need to know is what kinds of properties influence a world like Earth.”
“And this study helps address some of that question by showing the kinds of ways these parameters interact, what other outcomes might be possible and which worlds we should prioritize for study with new-generation telescopes.”
The research is described in a paper in the Journal of Geophysical Research: Planets.
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Paul K. Byrne et al. The Effects of Planetary and Stellar Parameters on Brittle Lithospheric Thickness. Journal of Geophysical Research: Planets, published online October 20, 2021; doi: 10.1029/2021JE006952
