According to one hypothesis, the continents of the Earth are necessary for the emergence and maintenance of life on it. Continents “float” on top of the Earth’s viscous mantle, and the heat of the planet’s core prevents the mantle from hardening and locking the continents in place.
The core is hot due to the presence of radioactive elements in it, resulting from collisions of neutron stars. Thus, it is possible to theoretically calculate when the first continents could have formed in the Universe on any of the planets. This is exactly what one of the researchers did.
Jane Greaves is Professor of Astronomy at the School of Physics and Astronomy at Cardiff University in Wales. Her work focuses on planet formation and habitability. Her new research is published in the journal Research Notes of the American Astronomical Society. Its title contains a simple question: “When did the first exocontinents appear?”
Greaves’ work aims to make the search for habitable worlds more efficient. If continents and the plate tectonics that support them are critical to life, then narrowing down the likely locations of rocky planets could make the search for habitable worlds more efficient.
First of all, why are continents and plate tectonics important?
Plate tectonics may not be absolutely necessary for life. However, they play an important role in regulating the Earth’s temperature. They allow heat to escape from the core, and excess heat in the core interferes with the Earth’s protective magnetosphere. They also help keep the Earth in what is called “habitable zone“However, some studies show that billions of years ago, when life began, plate tectonics was not very active. Thus, they may not be needed for the origin of life, but for the preservation of life and its evolution into more complex creatures such as as a person, they are most likely necessary.
Therefore, when searching for life and habitable planets, one should focus on rocky planets with plate tectonics. What we really want to do is find planets with continents. Planets with continents can support more biomass for longer periods of time than planets without continents, and plate tectonics creates continents.
Plate tectonics may not be a necessary condition for the origin of life on the planet. But tectonics and continents are probably necessary for the preservation and complexity of life.
Reeves has found a way to trace which planets might have continents—you just need to figure out which planets might have plate tectonics. This has a lot to do with heat. If the core of a rocky planet produces enough heat, then it likely has active plate tectonics, and we know why the Earth’s core produces heat.
The core contains the radioactive isotopes uranium-238, thorium-232 and potassium-40. Over geological time intervals, these elements break down into other elements and release heat. These elements do not appear randomly – they are formed in neutron stars and during supernova explosions.
There is a huge amount of detail in all this, and it is impossible to reflect them in one study. Greaves’ work is an attempt to understand all this from a broader perspective. “Here I present a research method for hypothetical Earth-like planets around stars whose photospheric abundance allows some inferences to be made about radiogenic heating of the planets,” she writes.
Not the least role in this is played by the connection between stars and the planets forming around them. Planets form from stellar nebula, the same material from which a star is formed. Therefore, the abundance of various chemical elements in a star is reflected in the planets that form around it.
Greaves took data from previous studies about the abundance of various elements in stars, and then combined them with data on the ages of stars obtained using Gaia. To be precise, she looked at two separate populations of stars: thin-disk stars and thick-disk stars. Stars with thin disks are typically younger and have higher metallicities, while stars with thick disks are older and poorer in metals.
This image of the Milky Way shows thick and thin disks. Thin disk stars are younger and have higher metallicity than older, metal-poor thick disk stars.
Her results show that the appearance of continents on Earth falls within the median of the graph.
Plate tectonics began about 3 billion years ago, or approximately 9.5 billion years from the beginning of the Universe. In Greaves’ sample, the first continents appeared 2 billion years before Earth on thin-disk stars. In stars with thick disks, rocky planets with continents appeared even earlier: approximately 4-5 billion years before Earth.
She also found that on most planets, continents would form more slowly than on Earth. To form continents, planets need the necessary amount of heat, and excess heat is unfavorable.
This figure from the study shows some of Greaves’ findings. The gray dots indicate stars of types F, G and K. Our Sun is a type G star, and F and K stars are similar enough that in this work they can be combined into one group. The pink dots represent the two thick-disk stars involved in the study, and the orange dot represents our Sun. The yellow dotted line separates planets where continents form either more slowly or faster than on Earth. The axes intersect at 12.5 billion years – the current age of the Universe. Image Credit: Jane S. Greaves 2023 Res. Notes AAS 7 195
Greaves also discovered a correlation between continents and Fe/H ratios in stars. “There is a general trend in the dependence of the iron content in stars: continents appear earlier at lower [Fe/H]”, she writes.
In this figure from the study, the x-axis shows the age of the universe, and the y-axis shows Fe/H, a broad measure of the metallicity of stars. The gray dots are F, G and K-type stars, and the pink dots are thick-disk stars that clearly stand out from the rest. Image Credit: Jane S. Greaves 2023 Res. Notes AAS 7 195
Greaves writes that stars with lower metallicity than our Sun may be a good place to look for habitable exoplanets with continents. “Systems with subsolar metallicity appear to be particularly interesting,” she writes. In her sample, all of these planets formed continents faster than Earth, making advanced life more likely there. Perhaps even more developed than ours.
Stars with thick disks are also intriguing because they apparently formed continents quickly. “The example of thick-disk systems is particularly advanced and deserves further study,” she writes, adding that of all the stars we know of that have exoplanets, only 7% are thick-disk stars.
With just a few years until the Habitable Worlds observatory launches, the scientific community has time to figure out the search criteria and what the best targets are. “Habitable Worlds Observatory has only 46 FGK stars on its top-tier target list,” she writes. But 15 of them are present in her results. If her work is correct, then “…only in this sample can there exist two systems with biospheres more developed than ours on Earth.”
Greaves concludes that the prospects for finding habitable planets with long-lived continents are good. “The prospects for searching for rocky exoplanets with continents appear very promising, given that nearby Sun-like stars have already given rise to several planet candidates,” she writes.
The next step is to study the stellar abundance of thorium and potassium isotopes that cause radiogenic heating. This “…could help discover older systems in which life on land may have arisen earlier than on Earth.”
Some elements are geophysically critical, especially the warming radiogenic ones such as U, Th and K. When you add Fe to them, this group of elements becomes critical to the size of the planet’s core, gravity and internal temperature. The planet’s internal temperature is very important not only because it regulates the life-supporting magnetosphere, but also because it helps create the conditions for plate and continental tectonics.
Previous studies show that the likelihood of Earth-like planets with continents was higher early in galactic history, and decreased as the galaxy evolved. But we still have a lot to learn about exoplanets, habitability, radiogenic heating, continents and plate tectonics, and a hundred other things.
We cannot say with certainty where we will find life and under what geophysical conditions. All we can do is study the Earth better, continue to build more powerful telescopes, and be patient.