Eggshell exoplanets: rocky worlds with ultra-thin brittle lithospheres could theoretically exist

The thickness of the fragile lithosphere – the outer part of a planetary body that fails by fracturing – plays a key role in the geological processes of that body. The properties of a planet and its host star can influence this thickness, and the potential range of these properties exceeds what researchers see in the solar system. In new research, an international team of planetary scientists have run a large number of computer models to see how various combinations of planetary and stellar properties influence the thickness of the outer layer of a planetary body. Their models predict that worlds that are small, old, or far from their star likely have thick and rigid layers, but under certain circumstances planets can have a fragile outer layer only a few miles thick. These worlds, which the authors call “eggshell planets,” might resemble the lowlands of Venus, and they suggest that at least three of these extrasolar planets – TOI-1235b, HD 136352b and L 168-9b – are already known.

Artist’s impression of an eggshell-shaped exoplanet. Image credit: NASA / JPL-Caltech.

“Understanding if you have the possibility of plate tectonics is a really important thing to know about a world, because plate tectonics can be necessary for a large rocky planet to be habitable,” said Dr Paul Byrne, geologist. planetary in the Department of Marine, Earth and Atmospheric Sciences at North Carolina State University and Department of Earth and Planetary Sciences at Washington University in St. Louis.

“So this is especially important when we talk about looking for Earth-like worlds around other stars and when we characterize planetary habitability in general.”

“We know from published work that there are exoplanets that experience more extreme conditions than what we see in our solar system,” he added.

“They could be closer to their star, or they could be much bigger, or have hotter surfaces, than the planets we see in our own system.”

Dr Byrne and his colleagues wanted to see which planetary and stellar parameters play the most important role in determining the thickness of a planet’s outer fragile layer, known as the lithosphere.

This thickness helps determine whether, for example, a planet can withstand high topography such as mountains, or has the right balance of stiffness and flexibility for one part of the surface to dip, or subduct, under another – the characteristic of plate tectonics.

It is this process that helps the Earth to regulate its temperature on geological time scales, and it is the reason why plate tectonics are considered 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 have literally run thousands of models, ”said Dr. Byrne.

The team found that surface temperature is the primary control over the thickness of the fragile lithospheres of exoplanets, 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 and rigid layers, but under certain circumstances planets can have a fragile outer layer only a few miles thick.

“Although we are far from directly imagining the surfaces of these eggshell planets, they could resemble the lowlands of Venus,” said Dr Byrne.

These lowlands contain vast expanses of lava but have little high relief, as the lithosphere is thin there due to scorching surface temperatures.

A plot of brittle lithospheric thickness as a function of surface gravitational acceleration, g, surface temperature, T, and plate age (as an indicator of heat flow).  The four planets of the inner solar system are shown, as well as four super-Earths for which radius, mass and equilibrium temperatures are available: HD 136352b, L 168-9b, LHS 1140b and TOI-1235b.  The uncertainty ranges in g and T for exoplanets are represented by white lines.  All the planets are shown to scale;  the illustrations of the four exoplanets are artistic impressions.  Solid, dotted, and dotted lines indicate where on this graph the thickness of the fragile lithosphere 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.

A plot of brittle lithospheric thickness as a function of surface gravitational acceleration, g, surface temperature, T, and plate age (as an indicator of heat flow). The four planets of the inner solar system are shown, as well as four super-Earths for which radius, mass and equilibrium temperatures are available: HD 136352b, L 168-9b, LHS 1140b and TOI-1235b. The uncertainty ranges in g and T for exoplanets are represented by white lines. All the planets are shown to scale; the illustrations of the four exoplanets are artistic impressions. Solid, dotted, and dotted lines indicate where on this graph the thickness of the fragile lithosphere 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,” said Dr Byrne.

“Ultimately, we want to help help identify the properties that make a world livable.”

“And not just temporarily, but habitable for a long time, because we believe that life probably needs time to start and become sustainable.”

The fundamental question behind this research is, of course, are we alone?

“That’s the big reach. Ultimately, most of this work relates to this final destination, which is “how unique is the Earth or not?” “Said Dr Byrne.

“One of the many things we’ll need to know is what kinds of properties influence a world like Earth.”

“And this study helps answer some of those questions by showing the types of ways these parameters interact, what other outcomes might be possible, and which worlds we should prioritize for study with next-generation telescopes.”

The research is described in an article in Journal of Geophysical Research: Planets.

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Paul K. Byrne et al. The effects of planetary and stellar parameters on the brittle lithospheric thickness. Journal of Geophysical Research: Planets, published online 20 October 2021; doi: 10.1029 / 2021JE006952

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