Eggshell Planets Have a Thin Brittle Crust and No Mountains or Tectonics

Planets without plate tectonics are unlikely to be habitable. But currently, we’ve never seen the surface of an exoplanet to determine if plate tectonics are active. Scientists piece together their likely surface structures from other evidence. Is there a way to determine what exoplanets might be eggshells, and eliminate them as potentially habitable?

The authors of a newly-published paper say there is.

The astronomy community hasn’t settled on a single method of classifying exoplanets yet. NASA likes to group them into four classifications: gas giants, super-Earths, Neptunians, and terrestrial. But that’s just a start. The Unified Astronomy Thesaurus uses 15 different exoplanet classifications. Other terms are used in scientific literature, too.

The number of classifications for exoplanets can be as granular as we’d like. Ultimately, each one is different. We’re in the early stages of understanding the variety of exoplanet types, and eventually, a comprehensive classification scheme will emerge.

One type of exoplanet that’s not often mentioned is the eggshell planet. They’ve caught researchers’ attention because they have thin, brittle crusts, no mountains, and no plate tectonics.

Eggshell planets are rare, as far as astronomers know. Only a few have been identified, but selection bias might play a role there. According to a new paper titled “The Effects of Planetary and Stellar Parameters on Brittle Lithospheric Thickness,” three have been found in exoplanet surveys. The lead author is Paul Byrne, Associate Professor of Earth and Planetary Sciences, at Trinity College, Dublin. The paper is published in the Journal of Geophysical Research: Planets.

Exoplanets are interesting in their own right, but a lot of what captures the interest of both scientists and the public is habitability. We want to know if there are planets out there that can support life. And while looking specifically for planets that could be habitable is one approach, another is discounting planets that, as far as we know, simply have no chance to support life.

“Understanding whether you’ve got the possibility of plate tectonics is a really important thing to know about a world…”

Paul Byrne, Associate Professor of Earth and Planetary Sciences, Trinity College, Dublin.

There’s strong evidence that plate tectonics is a necessary requirement for habitability. And since part of exoplanet hunters’ focus is finding Earth-like worlds, plate tectonics is a key. Without plate tectonics, we wouldn’t be here.

“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 lead author Byrne. “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.”

Plate tectonics occurs when a planet’s lithosphere is broken into chunks that float around on the mantle. Plate tectonics can help regulate a planet’s temperature by recycling the crust into the mantle over long geological timeframes. It regulates the atmosphere and helps remove carbon, avoiding a runaway greenhouse effect that could make the surface uninhabitable. The term “habitable zone,” which describes the region around a star where a planet can have liquid water, is usually calculated including active plate tectonics.

Earth's tectonic plates were mapped in the latter half of the 20th century. Image Credit: By Map: USGSDescription: Scott Nash - This file was derived from:  Tectonic plates.png, Public Domain, https://commons.wikimedia.org/w/index.php?curid=535201
Earth’s tectonic plates were mapped in the latter half of the 20th century. Image Credit: By Map: USGSDescription: Scott Nash – This file was derived from:  Tectonic plates.png, Public Domain, https://commons.wikimedia.org/w/index.php?curid=535201

A planet without plate tectonics is sometimes called a “stagnant lid planet.” They occur when the mantle isn’t energetic enough to fracture the crust into chunks. Instead, the crust is a single brittle chunk that covers the planet’s entire surface. In our own Solar System, Mercury has been a stagnant lid planet for billions of years. Some planets can exhibit episodic tectonic activity, where the crust is immobile for geological periods of time.

Since we have no way of observing the surfaces of exoplanets, astronomers are keen to find a way to detect them with other evidence. As the title of the new paper makes clear, the parameters of a planet and its star can provide evidence that a planet is an Eggshell planet.

“What we’ve laid out here is essentially a how-to guide or handy manual,” lead author Byrne said. “If you have a planet of a given size, at a given distance from its star and of a given mass, then with our results you can make some estimates for a variety of other features — including whether it may have plate tectonics.”

The paper outlines how knowledge of a planet’s size, age, and distance from its star could not only identify eggshell planets but other exoplanet types, too. Since astronomers can’t see the surfaces of exoplanets and are only now beginning to study their atmospheres, a planet’s other parameters are of highlighted importance.

“We have imaged a few exoplanets, but they are splotches of light orbiting a star. We have no technical ability to actually see the surface of exoplanets yet,” Byrne said. “This paper is one of a small but growing number of studies taking a geological or geophysical perspective to try and understand the worlds that we cannot directly measure right now.”

Nobody's ever seen the surface of an exoplanet. All we have is the work of scientific illustrators to fire our imaginations. This is an artist's impression of the view from the most distant exoplanet discovered around the red dwarf star TRAPPIST-1. Credit: ESO/M. Kornmesser.
Nobody’s ever seen the surface of an exoplanet. All we have is the work of scientific illustrators to fire our imaginations. This is an artist’s impression of the view from the most distant exoplanet discovered around the red dwarf star TRAPPIST-1. Credit: ESO/M. Kornmesser.

According to Byrne and his colleagues, the thickness of a planet’s brittle lithosphere is key to understanding if it has plate tectonics. And the lithosphere’s thickness is dictated not only by the characteristics of the planet but also its host star. “Factors inherent to the planet, such as size, interior temperature, composition, and even climate affect the thickness of this outer layer, but so too do factors specific to the host star, including how luminous and far away it is,” they write in their paper.

In order for a planet to have active tectonics, there needs to be a balance between a number of factors. For example, if the crust is too thick, the energy in the mantle might not be enough to trigger tectonics.

The team turned to computer models to better understand what factors lead to thicker exoplanet crusts.

The team started their models with a generic rocky world and went from there. “It was kind of Earth-sized — although we did consider size in there, too,” Byrne said. “And then we spun the dials,” he added. “We literally ran thousands of models.”

Prominent in the paper is the concept of BDT—brittle-ductile transition. The BDT is the zone in the lithosphere where dominantly brittle behaviour changes to dominantly ductile deformation. In this term ductile basically means pliable. The strength of a planet’s lithosphere is heavily reliant on its thickness, so the deeper the BDT, the stronger the crust.

Multiple factors go into determining a planet’s lithosphere thickness. Distance from the star, age, and planetary mass all factor into it. But the team found that surface temperature played a larger role. “Our 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 kilometres thick.” It’s these planets that the team calls eggshell planets, and that might resemble the lowlands on Venus.

This false colour image of lowlands on Venus' surface shows fine, light lines that are likely tectonic in nature. The darker areas are smooth volcanic plains. The image is a mosaic made of radar data from NASA's Magellan mission. The area in the image is about 1,400 km (870 miles) across. Image Credit: NASA.
This false colour image of lowlands on Venus’ surface shows fine, light lines that are likely tectonic in nature. The darker areas are smooth volcanic plains. The image is a mosaic made of radar data from NASA’s Magellan mission. The area in the image is about 1,400 km (870 miles) across. Image Credit: NASA.

Venusian lowlands are vast plains of lava. And they’re largely flat, too, with only wrinkled ridges. According to Byrne, the lithosphere in those areas is thin due to the planet’s extremely high surface temperatures.

This figure from the study shows the relationship between BDT depth and surface temperature. (g/ms2 is a measure of surface gravitational acceleration.) Image Credit: Byrne et al 2021.
This figure from the study shows the relationship between BDT depth and surface temperature. Each of the dots is one simulation result. (g/ms2 is a measure of surface gravitational acceleration.) Image Credit: Byrne et al 2021.

When it comes to exoplanets, mainstream media likes to announce the discovery of two categories of planets. Earth-like planets are always covered, and so are extremely weird planets, like the one that might rain molten iron.

But that’s just a kind of cherry-picking. In the larger scientific picture, it’s imperative to grow our overall understanding of exoplanets. That’s where this study fits in, according to the authors.

“Our overall goal is more than just understanding the vagaries of exoplanets,” 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.”

Is the number of planets that sustain habitability small? Quite likely. And one of the factors that sustains habitability is long-term plate tectonics. Without that, life might be unlikely to develop complexity.

This figure from the study shows BDT depth and plate age, or planet age, with surface temperature keyed at the bottom. Each of the dots is one simulation result. Image Credit: Byrne et al 2021.
This figure from the study shows BDT depth and plate age, or planet age, with surface temperature keyed at the bottom. Plate age is used as a proxy for heat flow. Each of the dots is one simulation result. Image Credit: Byrne et al 2021.

Finding life somewhere else is a primal, driving force in science. And for these researchers, that centers around the planet Earth and how unique it might turn out to be.

“That is the big reach,” Byrne said. “Ultimately most of this work is tied into this final destination, which is ‘how unique, or not, is Earth?’ 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.”

An artist's illustration of exoplanet TOI 1235 b, a suspected eggshell planet. Image Credit: NASA
An artist’s illustration of exoplanet TOI 1235 b, a suspected eggshell planet. Image Credit: NASA

The authors acknowledge the simplicity of their model. Without detailed observations of exoplanet surface characteristics, this work is necessarily a starting point. “Of course, our study is necessarily simplistic, since we have essentially no geological observations of exoplanets with which to constrain our parameter space,” they write.

But it still serves a valuable purpose. It’s a kind of framework for understanding targets for further observation. “A key prediction we make here is that so-called eggshell planets will have little elevated topography. This prediction can be tested with future generations of telescopes capable of searching for constructional or orogenic topography on exoplanets,” they clarify.

As more powerful telescopes come online, astronomers will eventually be able to observe exoplanets much more closely. But we know of thousands of exoplanets, with more being discovered all the time. Observing time at the world’s most powerful observatories is always in high demand. Modelling studies like this one are a way of pre-sorting potential observation targets.

The authors say that we already know of three of these eggshell planets: TOI-1235 b, HD 136352 b, and L 168-9 b. They’re all very close to their stars and are likely far too hot to be habitable no matter if they have plate tectonics or not, but they’re good test cases for the overall method of detecting eggshell planets.

This figure from the study shows the three suspected eggshell planets as well as Mercury, Earth, and Mars. They're all shown in relation to their age, surface gravitational acceleration, and surface temperature. LHS 1140 b is also shown because surface gravity and surface temperature estimates are available for them, as they are for the other exoplanets. All four exoplanets are super-Earths. Image Credit: Byrne et al 2021.
This figure from the study shows the three suspected eggshell planets as well as Mercury, Venus, Earth, and Mars. They’re all shown in relation to their age, surface gravitational acceleration, and surface temperature. LHS 1140 b is also shown because surface gravity and surface temperature estimates are available for them, as they are for the other exoplanets. All four exoplanets are super-Earths. Image Credit: Byrne et al 2021.

Should those three be the focus of observation in the future? “We propose that these planets be examined with planned and future space telescopes to test if our models are correct,” the authors write.

And if the models are correct, the search for habitable planets will take another step forward.

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One Reply to “Eggshell Planets Have a Thin Brittle Crust and No Mountains or Tectonics”

  1. I don’t think there is any evidence that plate tectonics is a requirement for habitability. (But I have seen hypotheses along those lines, certainly.) Rather it is observed to extend the habitable zone and – as seen on Earth – makes for a productive and varied biosphere conducive for complex life.

    But when we take the first steps of characterizing atmospheres stagnant planets would be demoted in the priority list, since they have a lower likelihood to produce the O2+CH4 atmosphere biosignatures we can look for at first. Webb will only have so much time to study nearby planets, and the next generation of telescopes won’t see many candidates either.

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