As telescopes get stronger and detection techniques get more sophisticated, especially with the assistance of artificial intelligence, astronomers are finding more exoplanets every day – including the first to be seen orbiting a star in another galaxy. However, contrary to other fields, it seems the more exoplanets we find, the less we know about them. Astronomers assumed other stars would have planets like those in our own solar system – especially the rocky ones with thick outer layers. Instead, a new study shows that many should be called “eggshell” exoplanets because their outer layers are so thin and brittle. Will future astronauts need to practice walking on eggshells?
“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. 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.”
Planetary geologist Paul Byrne, an associate professor in the Department of Earth and Planetary Sciences at Washington University in St. Louis, is the first author of the study, published in the Journal of Geophysical Research: Planets, led the development of the model used to prove the existence of these so-called “eggshell” planets. As he explains in the press release, Byrne and his collaborators focused on the outer layer – called the lithosphere – of exoplanets to determine their topography (do they have mountains?), thickness (can they be excavated?), temperature (are they habitable) and other features not visible to telescopes.
"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. By adjusting an exoplanet’s size, distance from its star, age and other factors, the researchers found that extremes created unusual planets … including three predicted to have eggshell-like lithospheres."
They discovered the biggest factor in creating eggshells is surface temperature. (Isn't that the same with chicken eggs?) Manipulating size, age and distance from their star had less effect in creating these brittle eggshell layers which Byrne says may be only a few kilometers thick. The only planet in our solar system with a similar topography is Venus, although it may not be eggshell thin across its entire surface.
“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.”
Are eggshell planets habitable? Byrne isn’t sure – his goal is to figure out how Earth formed and then search for stars and exoplanets with similar circumstances or histories.
Is he looking for a new place to live? Does he knows something we don’t? Now WE’RE walking on eggshells.