Welcome to the strange new worlds of the Exoplanet Era. A new category of exoplanets, known as eggshell planets, have ultra-thin, brittle crusts only one kilometer in depth, too thin to sustain tectonics and will be hostile to life. Meanwhile, Earth and Mars are crusty down to depths of 40 and 100 km according to a new international study that will help identify whether newly discovered planets could support Earth-like plate tectonics, adding a new geological dimension to exoplanet classification. Earth-like subduction zone plate tectonics are a critical component of planetary habitability, not just because they are the most Earth-like of all geological processes known to science.
“Earth is the only known planetary body that has robustly established plate tectonics of any kind,” said Alec Brenner, a member Harvard’s Paleomagnetics Lab who was not involved in the new University of St. Andrews study. “Tectonic plates began pushing and pulling in a process that helped Earth evolve and shaped its continents into the ones that exist today as early as four billion years ago.”
Plate Tectonics Common on Other Worlds?
“It really behooves us as we search for planets in other solar systems,” Brenner noted, “to understand the whole set of processes that led to plate tectonics on Earth and what driving forces transpired to initiate it. That hopefully would give us a sense of how easy it is for plate tectonics to happen on other worlds, especially given all the linkages between plate tectonics, the evolution of life and the stabilization of climate.”
“In effect, the new model provides a template with which to predict the nature of plate tectonics on exoworlds,” said Dr. Sami Mikhail, of the School of Earth and Environmental Sciences at the University of St Andrews. “You could say that we’re hunting for a new Goldilocks parameter, but instead of the right temperature for water we want to explore the right conditions for plate tectonics.”
The effects of plate tectonics are enhanced with volcanism and chemical weathering which have both endured for billions of years. These two factors, combined with the presence of some water, have regulated Earth’s climate, and keep Earth habitable. Earth’s tectonic activity of some 15 moving plates. The cycle of rigid tectonic plates in constant horizontal motion across the surface of the planet—makes Earth unique within the rocky planets of the solar system. Jupiter’s icy moon Europa, regarded as perhaps one of the solar system’s best bets to host alien life, has massive slabs of ice that are sliding over and under each other above a global ocean that reaches down 100 kilometers below the base of the ice –a depth 10 times greater than the Marianas Trench.
Another key ingredient for habitability is large scale magnetic fields, which provide a protective barrier in the upper atmosphere from stellar winds and flares. However, magnetic fields are generated by rotating, molten, metallic cores, and are thus difficult to associate with the activities on the rocky surface. In an email to The Daily Galaxy, Dr. Mikhail wrote: “Magnetic fields are super important, but tough to link with plate tectonics. For example, we know that a celestial body doesn’t need subduction zones to have a magnetic field. For example, Mars had one.”
Earth is Unique in the Solar System
“Earth is unique in the Solar System. However, there are only three other rocky planets—Mercury, Venus and Mars—and they are also distinct from one another,” added Mikhail. “Astronomers have discovered more than 4000 planets orbiting other stars, known as exoplanets. Are any of these Earth-like? What does Earth-like mean? And how representative are the planets of our Solar System to the wider cosmos?”
The research team 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. These predicted 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 that are too thin to sustain tectonics.
Resemble the Lowlands on Venus
These worlds, named ‘eggshell planets’ might resemble the planitia, lowlands, on Venus, and the term could potentially apply to at least three such extrasolar planets already known.
The outer layer of a rocky planetary body is generally rigid and behaves in a brittle manner. The thickness of this layer is important in governing numerous aspects of that body’s geological character, including whether it can support plate tectonics and even retain habitable conditions at the surface. 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.
The study considered several factors, including the size of the exoplanet, the distance from the host star, the surface temperature, and the internal temperature. The team found that the surface temperature was more important than all other factors—including the internal temperature of the planet.
The team believes the model should be used to predict which exoplanets are given telescope time for planned and future investigations that aim to determine the chemistry of the atmospheres around a given exoplanet, in the quest to find evidence of active geochemical processes and signatures left by putative biology beyond our Solar System.
The Last Word –Three Known Eggshell Worlds
“We predict that three known exoplanets should be Eggshell exoworlds, but we cannot speak to more at this stage,” Dr. Mihail told The Daily Galaxy. “However,” he added, “we’d also add that as more exoplanets are characterized in more detail with current and future telescopes (especially space-based) we anticipate more. We also anticipate more to be ‘Earth-like’.Exciting times ahead!”
The new computational model was developed by an international team of geologists based in the U.S., Switzerland, France and St Andrews, published in the Journal of Geophysical Research.
Source: Paul K. Byrne et al, The Effects of Planetary and Stellar Parameters on Brittle Lithospheric Thickness, Journal of Geophysical Research: Planets (2021). DOI: 10.1029/2021JE006952
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