Planets without plate tectonics are not likely to be hospitable. We have never seen the surface of an exoplanet to determine if plate tectonics are active. Scientists piece together their surface structures from other evidence. Is there a way to determine what exoplanets are eggshells and eliminate them as potentially habitable?
The authors say there is.
The astronomy community has not settled on a single method to classify exoplanets. Gas giants, super-Earths, Neptunians, and terrestrial are what NASA likes to group them into. That is just the beginning. 15 different exoplanet classifications are used in the Unified Astronomy Thesaurus. Other terms are used in scientific literature.
The number of classifications for exoplanets can be as small as we would like. Each one is different. A comprehensive classification scheme will emerge once we understand the variety of exoplanet types.
The eggshell planet is a type of exoplanet that is not often mentioned. They have thin, brittle crusts, no mountains, and no plate tectonics, which has caught the attention of researchers.
Astronomers don't know if shell planets are rare. There are only a few that have been identified. Three have been found in exoplanet surveys according to a new paper. The lead author is a professor at Trinity College, Dublin. The Journal of Geophysical Research: Planets has a paper in it.
Habitability is one of the things that captures the interest of both scientists and the public. We want to know if there are planets that can support life. One approach is to look for planets that could be hospitable, but another approach is to ignore planets that have no chance of supporting life.
It is important to know if you have the chance of plate tectonics.
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Trinity College, Dublin has an Associate Professor of Earth and Planetary Sciences.
There is strong evidence that plate tectonics is needed for habitability. Since part of the exoplanet hunters focus is finding Earth-like worlds, plate tectonics is a key. Without plate tectonics, we wouldn't be here.
It is important to know if plate tectonics is possible for a large rocky planet to be a good place to live. It is important when we are talking about looking for Earth-like worlds around other stars and when we are describing planetary habitability generally.
Plate tectonics occurs when chunks of a planet are broken into into smaller pieces on the mantle. Plate tectonics can help regulate a planet's temperature by recycling the crust into the mantle. It helps remove carbon from the atmosphere and avoids a greenhouse effect that could make the surface uninhabitable. Thehabitable zone is a region around a star where a planet can have liquid water.
The plates were mapped in the last half of the 20th century. The file was derived from the public domain.
A planet without plate tectonics is sometimes called a "stagnant lid planet." They occur when the mantle isn't strong enough to break the crust into pieces. The planet's entire surface is covered by a brittle chunk of the crust. Mercury has been a stagnant planet for billions of years. Some planets can have periodic tectonic activity, where the crust is immobile for geological periods.
Astronomers want to find a way to detect exoplanets with other evidence since we don't have a way of observing their surfaces. The parameters of a planet and its star can provide evidence that a planet is an Eggshell planet.
The how-to guide is a handy manual, according to lead author Byrne. If you have a planet of a given size, at a given distance from its star and of a given mass, you can make some estimates for a variety of other features.
The paper shows how knowledge of a planet's size, age, and distance from its star could be used to identify eggshell planets. Astronomers can't see the surfaces of exoplanets and are only now beginning to study their atmospheres, so the other parameters are of importance.
There are a few exoplanets that are not in the picture. We don't have a way to see the surface of exoplanets. This paper is one of a growing number of studies that are taking a geological or geophysical perspective to try and understand the worlds that we cannot directly measure right now.
Nobody has ever seen the surface of an exoplanet. The work of scientific illustrators is all we have. The view from the most distant exoplanet discovered around the red dwarf star TRAPPIST-1 is an artist's impression. Credit: M. Kornmesser.
The thickness of a planet's brittle lithoosphere is important to understand if it has plate tectonics. The host star and the characteristics of the planet are just some of the factors that affect the thickness of the lithoosphere. They write in their paper that factors inherent to the planet, such as size, interior temperature, composition, and even climate, affect the thickness of this outer layer.
There needs to be a balance between factors in order for a planet to have active tectonics. The energy in the mantle might not be enough to cause tectonics if the crust is too thick.
The team used computer models to better understand factors that lead to thicker exoplanet crusts.
The team started with a generic rocky world. It was kind of Earth-sized, although we did consider size in there. He said that they spun the dial. We ran thousands of models.
BDT is a concept in the paper. The BDT is the zone where brittle behavior changes to ductile behavior. The term ductile means that it is flexible. The deeper the BDT, the stronger the planet is.
There are multiple factors that go into determining a planet's thickness. The distance from the star, age, and planetary mass all factor into it. The team found that surface temperature was a bigger factor. Our models show that worlds that are small, old, or far away from their star may have thick, rigid layers, but that planets may have an outer brittle layer only a few kilometres thick. The team calls these planets eggshell planets, and they might look like the lowlands on Venus.
The false colour image of the lowlands on Venus shows lines that are likely tectonic. The dark areas are volcanic plains. The image is made of radar data. The area in the image is over 800 miles. The image is from NASA.
Venusian lowlands are covered in lava. They are mostly flat, with only wrinkled ridges. The planet's high surface temperatures cause the lithoosphere in those areas to be thin.
The figure shows the relationship between BDT depth and surface temperature. Each dot is a simulation result. The image is from the paper by Byrne et al.
Mainstream media likes to announce the discovery of two categories of planets. Earth-like planets are always covered, and so are extremely weird ones, like the one that might rain molten iron.
That is a kind of cherry-picking. It is important to grow our understanding of exoplanets in the larger scientific picture. The authors say that this study fits in.
Our overall goal is more than just understanding exoplanets. We want to help identify the properties that make a world a good place to live. We think life needs a while to get going and become sustainable, so we think it's temporary.
Is the number of planets small? Quite likely. Long-term plate tectonics is one of the factors that sustains habitability. Life might not develop complexity without that.
The figure shows plate age and BDT depth, with the surface temperature at the bottom. The age of the plate is used as a proxy. Each dot is a simulation result. The image is from the paper by Byrne et al.
Finding life somewhere else is a driving force in science. For these researchers, it's about the planet Earth and how unique it might be.
That is the largest reach, according to Byrne. Most of this work is tied into the question of how unique or not Earth is. We need to know what kinds of properties influence the world we live in. The study shows the ways in which these parameters interact, what other outcomes might be possible, and which worlds we should prioritize for study with new-generation telescopes.
An artist's illustration of a suspected eggshell planet. The image is from NASA.
The authors acknowledge the simplicity of their model. This work is a starting point without detailed observations of exoplanet characteristics. Our study is simplistic since we have no geological observations of exoplanets with which to constrain our space, they write.
It still serves a purpose. It is a framework for understanding targets. eggshell planets will have little elevated topography. Future generations of telescopes capable of searching for constructional or orogenic topography on exoplanets can test this prediction.
Astronomers will eventually be able to observe exoplanets much more closely as more powerful telescopes come online. We know of thousands of exoplanets, with more being discovered all the time. The world's most powerful observatory is always in high demand for observing time. Modelling studies are used to sort potential observation targets.
The authors say that we already know of three eggshell planets. They are all very close to their stars and are likely too hot to be hospitable, but they are good test cases for the overall method of detecting eggshell planets.
The three suspected eggshell planets are shown in this figure. They are shown in relation to their age, surface temperature, and surface gravitational acceleration. The other exoplanets have surface gravity and surface temperature estimates available for them. Four planets are super-Earths. The image is from the paper by Byrne et al.
Should those three be the focus of observation in the future? The authors propose that the planets be examined with future space telescopes to see if their models are correct.
If the models are correct, the search for planets with water will continue.
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