Newly theorized "eggshell planets" could develop ultra-thin, ultra-smooth outer layers, according to a study published in the Journal of Geophysical Research: Planets. Remarkably, three such worlds may already have been detected.
Over the past two decades, astronomers have confirmed more than 4,500 exoplanets. Many defy resemblance to planets in our solar system, including super-Earths and hot Jupiters—with even more exotic types potentially awaiting discovery.
In this study, researchers modeled how a rocky planet's characteristics—such as size, interior temperature, and composition—along with its star's properties like brightness and distance, influence lithosphere thickness.
A rocky planet's rigid outer lithosphere dictates key geological features, including plate tectonics. Planetary factors like size, heat, composition, and climate, plus stellar influences, determine its thickness.
Led by Paul Byrne at Washington University in St. Louis, the team ran extensive computer simulations exploring various planetary and stellar combinations.
Results suggest small, ancient planets or those distant from their stars form thick, rigid lithospheres. Conversely, under specific conditions, exoplanets could develop a fragile lithosphere just a few kilometers thick—dubbed "eggshell planets." These might mirror Venus's lowlands, with vast lava plains and minimal topography.
Such thin layers preclude plate tectonics, rendering these planets unlikely habitats for life as we know it. On Earth, tectonics recycle carbon-capturing minerals, stabilizing CO2 levels and climate. Without it, runaway greenhouse effects—like Venus's—could dominate.
"Plate tectonics regulate Earth's temperature, supporting habitable surface conditions," notes Byrne. "We don't know if it's essential for habitability, but it plays a crucial role."
The team highlights three candidate eggshell planets already observed, urging scrutiny with advanced telescopes like the James Webb Space Telescope.
