On Earth, geological activity and the resulting formation of continents are thought to have played a crucial role in the origin and development of life. According to this principle, if an exoplanet has continents, its habitability potential increases. A recent study shows that certain exoplanets in our Milky Way may have formed continents more than two billion years before our Earth… and that life may have taken advantage of this to evolve there.
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The continents on Earth were formed by the movements of tectonic plates, made possible by the dissipation of the planet’s internal heat. Scientists do not consider the existence of plate tectonics to be absolutely necessary for the emergence of life: tectonic activity on Earth was still limited when the first living organisms arose there. Plate tectonics, on the other hand, appears to play a regulating role in the Earth’s climate and temperatures. It also allows the exchange of matter between the mantle and the atmosphere, and the dissipation of our planet’s internal heat allows the formation of a magnetosphere that protects us from life-damaging cosmic radiation. Although not necessary for the emergence of life, plate tectonics appears to be crucial for its long-term evolution and may provide an additional argument for characterizing the habitability of an exoplanet and the possible development of a complex biosphere.
Simulation of the movement of tectonic plates on the Earth’s surface over the last billion years. © Merdith et al., 2020, EarthByte
Radioactivity as a driver of tectonic activity
On Earth, tectonic plates move relative to each other by “sliding” on the more ductile mantle. Movements in the Earth’s mantle are caused primarily by our planet’s internal heat, which is caused by the decay of radioactive elements in the core, such as uranium-238 or thorium-232. These very heavy elements can only be formed in very energetic cosmic events, such as a collision between two neutron stars or supernovae. During its formation, the earth stored these elements, which even today break down into other, lighter elements while giving off heat.
If we know where the tectonic activity on Earth and the formation of continents on our planet come from, observing these phenomena on rocky exoplanets is not yet feasible today; However, the presence of radioactive elements in their cores may, on the other hand, allow evidence that tectonic activity is possible there. This is the challenge that Jane Greaves, an American astronomer, has taken on: in her opinion, knowing that the planets and their host star arise from the same prestellar cloud, the amounts of radioactive elements within the star reflect the chemical’s composition Planets orbiting it. By analyzing the uranium and potassium abundances of neighboring stars, as well as the ages of these stars measured by the Gaia satellite from previous studies, it is able to provide an estimate of the epoch from which hypothetical rocky planets around the studied stars could have formed hot enough, that plate tectonics could arise there. She presents her results in the journal Research Notes of the American Astronomical Society.
Searching for radioactive elements in the Milky Way
In her work, astronomer Jane Greaves examines 29 stars that are relatively close to our solar system and divides them into two groups: on the one hand, the youngest stars and the richest in metals (we talk about metallicity, a quantity that measures mass ). made of elements other than hydrogen or helium, which are most common in the universe) is found in the “thin disk” of the Milky Way; on the other hand, the oldest and metal-poor stars that are located in the “thick disk” of our galaxy. By analyzing the metallicity of stars as well as their ages, she can estimate how long a hypothetical rocky planet orbiting them might exhibit tectonic activity.
On Earth, plate tectonics as we know it began about 3 billion years ago; However, according to the results of his study, the astronomer appears to have identified hypothetical rocky planets in his sample of young, metal-rich stars on which continents could have formed more than two billion years ago. The age at which plate tectonics began appears to be on average compared to neighboring star systems.
However, two stars stand out, each located 70 to 110 light-years away from us and which could have formed continents up to 5 billion years earlier than on Earth. These stars have a low metallicity, well below that of our Sun. Therefore, according to the astronomer, systems with stars that have lower metallicity than our Sun could be good candidates for searching for planets on which life could have evolved or that could even be more developed than on Earth. Within its sample of just 29 stars, it estimates that two of these systems, which could host planets with plate tectonics, are close enough to be observed by future telescopes such as NASA’s Habitable Worlds Observatory.