On Earth, geological activity and the resulting formation of continents is suspected to have played an important role in the emergence and evolution of life. According to this theory, if an exoplanet has continents, its habitability increases. A recent study suggests that some exoplanets in our galaxy may have formed continents 2 billion years before our Earth… and life may have developed there taking advantage of this.

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The continents on Earth were formed due to the movements of tectonic plates, which made possible the removal of internal heat from the planet. Scientists do not consider the existence of plate tectonics to be strictly necessary for the presence of life: tectonic activity on Earth was limited even when the first living organisms emerged there. On the other hand, plate tectonics appears to have a regulatory role in Earth’s climate and temperature. It also provides the exchange of matter between the mantle and the atmosphere, and the extraction of internal heat from our planet allows the establishment of a magnetosphere, which protects us from cosmic radiation harmful to life. If it is not essential for the presence of life, plate tectonics appears to be important for its long-term evolution, and may provide an additional argument for characterizing an exoplanet’s habitability and the potential development of a complex biosphere. .

Simulation of the movement of tectonic plates on the Earth’s surface over the past billion years. © Meredith 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 flexible mantle. Movement in the mantle comes primarily from our planet’s internal heat, caused by the decay of radioactive elements in the core, such as uranium-238 or thorium-232. These very heavy elements can only form during very energetic cosmic events, such as a collision between two neutron stars or a supernova. Thus the Earth stored these elements during its formation, which are still disintegrating into other lighter elements emitting heat today.

Even if we know where terrestrial tectonic activity and the formation of continents originate on our planet, observing these phenomena on rocky exoplanets is still unrealistic today; But on the other hand, the presence of radioactive elements in their cores may make it possible to prove that tectonic activity is possible there. This is the challenge that Jane Greaves, an American astronomer, set out to take on: according to her, knowing that planets and their host stars formed from the same pre-stellar cloud, the abundance of radioactive elements within the star makes the chemistry Shows the structure of the planets revolving around it. By analyzing the uranium and potassium abundances of neighboring stars from previous studies, as well as the ages of these stars as measured by the Gaia satellite, he is able to provide an estimate of the age by which hypothetical rocky planets could have formed around the studied stars. . It’s hot enough for plate tectonics to emerge there. She presents her results in the journal Research Notes of the American Astronomical Society,

Discovery of radioactive elements in the galaxy

In her work, astronomer Jane Greaves studies 29 stars located 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 speak of metallicity , a quantity that measures the mass of) elements other than hydrogen or helium, which are most abundant in the universe) located in the “thin disk” of the galaxy; The oldest and poorest stars in metals, on the other hand, are located in the “thick disk” of our galaxy. By analyzing the metallicity of the stars as well as their age, he is able to estimate how long a hypothetical rocky planet orbiting them might exhibit tectonic activity.

On Earth, as we know it, plate tectonics began about 3 billion years ago; But according to the results of their study, astronomers have identified hypothetical rocky planets of young stars rich in metals in their sample, where continents may have emerged more than 2 billion years ago. The age of onset of terrestrial plate tectonics appears to be average compared to neighboring star systems.

However, two stars stand out from the rest, located at a distance of 70 to 110 light years from us respectively, which could have formed continents 5 billion years earlier than Earth. The metallicity of these stars is low, much lower than that of our Sun. Thus, according to astronomers, systems with stars with lower metallicity than our Sun may be good candidates for searching for planets where life could develop, or even be more advanced than Earth. Is. Within their sample of only 29 stars, it is estimated that two of these systems that may host planets with plate tectonics are close enough to be observed by future telescopes, such as Habitable Worlds Observatory From NASA.

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