2023-11-18 18:26:50
On Earth, geological activity, and the resulting formation of continents, is suspected of having played a crucial role in the appearance and evolution of life. According to this principle, if an exoplanet has continents, its habitability potential increases. A recent study reveals that certain exoplanets in our Milky Way might have formed continents more than 2 billion years before our Earth… and life might have taken advantage of this to develop there.
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The continents on Earth were formed thanks to the movements of tectonic plates, enabled by the removal of internal heat from the planet. Scientists do not consider the existence of plate tectonics to be strictly necessary for the appearance of life: tectonic activity on Earth was still limited when the first living organisms emerged there. Plate tectonics, on the other hand, seems to have a regulatory role in the Earth’s climate and temperatures. It also offers exchanges of matter between the mantle and the atmosphere, and the evacuation 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 necessary for the appearance of life, plate tectonics appears crucial for its long-term development, and can provide an additional argument to characterize 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 mantle mainly come from the internal heat of our Planet, 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 during a collision between two neutron stars, or supernovae. The Earth thus stored these elements during its formation, which still continue today to disintegrate into other lighter elements, emitting heat.
If we know where terrestrial tectonic activity and the formation of continents on our Planet come from, observing these phenomena on rocky exoplanets remains unrealizable today; but the presence of radioactive elements in their cores can on the other hand 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 the planets and their host star form from the same pre-stellar cloud, the abundances of radioactive elements within the star reflect the chemical composition of the planets that orbit around it. By analyzing from previous studies the uranium and potassium abundances of neighboring stars, as well as the ages of these stars measured by the Gaia satellite, she is able to provide an estimate of the epoch from which hypothetical Rocky planets around the stars studied might have become hot enough for plate tectonics to emerge 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 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 which measures the mass of elements other than hydrogen or helium, the most abundant in the Universe) located in the “thin disk” of the Milky Way; on the other, the oldest and poorest stars in metals, located in the “thick disk” of our Galaxy. By analyzing the metallicity of the stars as well as their age, she is able to estimate how long a hypothetical rocky planet orbiting them might exhibit tectonic activity.
On Earth, plate tectonics as we know it began around 3 billion years ago; but according to the results of his study, the astronomer seems to have identified in his sample of young stars rich in metals hypothetical rocky planets where continents might have emerged more than 2 billion years earlier. The age of the onset of terrestrial plate tectonics appears to be median, compared to neighboring star systems.
However, two stars stand out from the rest, located respectively 70 to 110 light years from us, which might have formed continents up to 5 billion years earlier than on Earth. These stars have a low metallicity, well below that of our Sun. Thus, according to the astronomer, systems with stars having a lower metallicity than our Sun might be good candidates for searching for planets where life might have evolved, or even be more advanced than on Earth. Within its sample of only 29 stars, it estimates that two of these systems that might host planets with plate tectonics are close enough to be observable by future telescopes, such as the Habitable Worlds Observatory from NASA.
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