When looking for life beyond our planet, there are a lot of things to take into consideration, from an exoplanet’s size, to its distance from its host star. A further condition that contributes to Earth’s habitability is the existence of plate tectonics in our planet’s solid outer crust. Not only does it regulate Earth’s surface temperature but it also allows for the development of our protective magnetic field.
However, new research, presented at the Goldschmidt Virtual 2020 Conference, suggests that the formation of plate tectonics on a planet is dependent on the age of its home galaxy – the younger the galaxy, the more likely this life-supporting movement developed.
“Plate tectonics is important for habitability, and it looks like the optimum conditions plate tectonics existed for planets forming early in the galaxy's lifespan, and may be unlikely to easily recur,” Professor Craig O'Neill, Director of the Planetary Research Centre at Macquarie University, Australia, said in a statement. “For life, maybe that was as good as it gets.”
O’Neill and his group carried out huge simulations to determine the development of the interior of planets using known data on exoplanets such as position, temperature, and some aspects of geochemistry. Whilst they had expected to find that the abundance of iron in Earth’s core was the critical element for tectonic movement, their simulations showed that the galaxy’s age was a more prevalent factor.
“The Earth has a lot of iron in its core, and we had assumed that this would be necessary for tectonic development,” O’Neill said. “However we found that even planets with little iron may develop plate tectonics if the timing is right. This was completely unexpected.”
As a galaxy evolves, so too does its overall chemical balance; material clumps together to form stars and planetary bodies, and supernovas eject stored-up and newly-created elements throughout space. At different points in a galaxy’s timeline, the interstellar material used to form planets changes, and therefore so too does its chances of sustaining life.
“Planets which formed later may not have developed plate tectonics, which means that they don't have this built-in thermostat,” O’Neill explained. “This doesn't just affect the surface temperature, this means that the core stays hot, which inhibits the development of a magnetic field. If there's no magnetic field, the planet is not shielded from solar radiation, and will tend to lose its atmosphere. So life becomes difficult to sustain.”
Overall, O’Neill and colleagues concluded that planets that formed earlier “did so in conditions favorable to allow the development of life.” Conditions that in our galaxy are becoming “increasingly rarer.” Nonetheless, simulations such as O’Neill’s contain valuable knowledge that helps to continue our current search for life.
“It is so important to combine observing campaigns with large simulation projects like this, that really tell us something about the geological evolution of planets formed at different stages of galactic evolution,” Professor Sara Russell, leader of the Planetary Materials Group at the Natural History Museum, London, who was not involved in the work, said. “This enables us to build a picture of what these strange worlds might look like, and how habitable they may be.”
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