Strange 'eggshell' exoplanets may have ultra-smooth surfaces - Space.com

Strange, newly theorized "eggshell planets" may possess super-thin outer layers with ultra-smooth surfaces unlike those seen on any world to date, a new study reports.

Astronomers may have already detected at least three eggshell planets, scientists noted.

Over the last 25 or so years, astronomers have confirmed the existence of more than 4,500 exoplanets, worlds that orbit other stars. Many of these worlds are quite unlike any planets in the solar system. For example, so-called "hot Jupiters" are gas giants that orbit their host stars more closely than Mercury does the sun.

In the new study, the researchers wanted to investigate what might be possible in terms of a planet's crackable outer layer, known as its brittle lithosphere. They modeled how different features of a rocky planet and its star might influence its brittle lithosphere's thickness.

The new models predicted that worlds that are small, old or far from their star likely have thick and rigid outer layers. However, under the right circumstances, exoplanets might possess a brittle lithosphere only a few miles thick — so-called eggshell planets.

The extremely thin brittle lithospheres of eggshell planets may not prove rigid enough to support high-standing mountains, leading to relatively smooth surfaces. In addition, their outer layers may not prove strong enough to produce the strong dense plates needed for plate tectonics, which drives continental drift and mountain-building on Earth.

"I love the idea that there are big, rocky worlds out there with surfaces that might look completely different to what we're used to because of these particular combinations of size, surface temperature and age," study lead author Paul Byrne, a planetary scientist at Washington University in St. Louis, told Space.com. "The types of possible planets out there are considerably beyond what we see in the solar system."

The absence of plate tectonics suggests that eggshell planets may not prove habitable to life as we know it. Among other things, plate tectonics helps ensure that minerals that draw carbon from their air get locked away in Earth's interior. As such, plate tectonics helps control atmospheric levels of the global warming gas carbon dioxide, which helps keep a planet from experiencing the kind of runaway greenhouse effect that has doomed Venus with a hellish climate.

"Plate tectonics helps regulate Earth's temperature, which in turn helps it retain relatively comfortable temperatures at the surface," Byrne said. "We don't know if plate tectonics is a necessity for a world to be considered habitable, but it sure helps."

The roiling of magma in the mantle layer under the surface of eggshell planets could lead to some kind of deformations. "Places where the mantle is upwelling could stretch the upper surface, and places where the mantle downwells could push parts of the crust together," Byrne said. "But the style, the appearance, of that surface deformation probably wouldn't look like tectonic plates the way we know them on Earth."

Previous astronomical surveys have detected at least three exoplanets that may be eggshell planets, Byrne said. Although scientists are a long way from directly imaging the surfaces of these eggshell planets, he noted they might resemble the lowlands on Venus, which contain vast expanses of lava but have little high-standing terrain, because the lithosphere there is thin as a result of scorching surface temperatures.

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Strange 'eggshell' exoplanets may have ultra-smooth surfaces - Space.com

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