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
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