A new study reveals that ‘outer shell planets’ with very thin outer layers and very shallow terrain may orbit other stars in the universe.
Researchers say that at least three of these extraordinary ‘exoplanets’ are already known outside our solar system, but more than that.
Such rare worlds may have an outer layer only a few miles thick, plate tectonics are unlikely to occur, and may or may not be habitable.
They are larger and smaller than other exoplanets with very thick outer layers that are likely to share the same solar system.
The surface of the egg-bearing planets resembles the lower regions of Venus, with vast lavas, extreme hot surface temperatures, and high terrain.
‘Exoskeleton Planets’ are rocky worlds with very thin crusts and no topography. The picture is of an artist rendering such an exoplanet
Strange planets with outer layers like egg shells are one of the richest varieties of possible exoplanets.
The new study was led by Paul Byrne, a planetary geologist at the University of Washington in St. Louis.
Byrne and his team of international collaborators wanted to see what cosmic elements play the most important role in determining the thickness of the outer layer of a planet known as the lithosphere.
This thickness, for example, helps determine if a planet can support a high terrain like mountains, or whether one part has the right balance of rigidity and flexibility to conquer another – the face of plate tectonics.
On Earth, this process helps our planet control its temperature on geological time scales, and this is why plate tectonics is considered to be an important factor in whether a planet is habitable or not.
“Understanding the potential of plate tectonics for you is very important to know about a world because plate tectonics may be needed to make a large rocky planet habitable,” Professor Byrne said.
‘This is especially important when we talk about exploring Earth-like worlds around other stars and depicting the habitation of the planets in general.’
So far, astronomers have discovered more than 4,000 exoplanets that have been confirmed to orbit other stars in our galaxy.
However, it is difficult to know exactly what exoplanets are made of, or whether they resemble Earth.
The thickness of the lithosphere – the outermost shell of a planet – plays an important role in the geological processes that determine their characteristics, the team says.
The surface of the planets with the egg is similar to the lower regions of Venus (here a false color radar image is depicted in the mosaic). The bright and fine peach lines are tectonic structures and the dark and purple areas are relatively smooth volcanic plains. Some small volcanoes appear near the center below. The image was made using radar data provided by NASA’s Magellan mission between 1990 and 1994 and covers an area of approximately 1,400 km (870 mi).
To understand how the planetary and stellar elements generally affect the thickness of a lithosphere, the team modeled multiple combinations of planetary mass, surface and mantle temperature, energy flow, and more.
For their modeling efforts, scientists have chosen a typical ‘Earth-sized’ rocky planet as a starting point.
‘Then we turned the dials,’ Byne said. ‘We literally drove thousands of models.’
According to the results, the surface temperature is the main factor controlling the thickness of the fracturing layer, although the mass of the planet, the distance to the star, and the age of a planet all play a role.
The smaller and older planets generally have thicker fracturing lithospheres similar to those of Mercury and Mars, while the larger and smaller planets have thinner fracturing lithospheres comparable to the lower regions of Venus.
However, some combinations of these parameters are likely to present worlds with ‘excessively thin brittle layers’, the team says.
Researchers have come a long way in depicting the planets’ surface with this egg, so they do not know what they look like other than speculating that they are similar to Venus.
‘We’ve photographed a few exoplanets, but they are light slots orbiting a star,’ Byrne said. ‘We do not yet have the technical ability to actually see the surface of exoplanets.
Scientists can use planned and future space telescopes to examine exoplanets in more detail and confirm their geographical features.
So far, astronomers have discovered more than 4,000 exoplanets that have been confirmed to orbit other stars in our galaxy. The picture is of an artist rendering an exoplanet and its moon
‘We know from published works that there are exoplanets in our solar system that experience conditions more intensely than we see,’ Byrne said.
‘They may be closer to their star than the planets we see in our own system, or they may be much larger, or have hotter surfaces.’
‘Ultimately, we want to help identify the assets that make the world habitable.
‘It’s livable, not just temporary, because we think life will take some time to move forward and become sustainable.’
The study has been published in Journal of Geophysical Research: Planets.
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