Generally, scientists have focused on planets that exist within a ‘Goldilocks zone’, an area where a planet can orbit its host star without it becoming too hot nor too cold for liquid water to form on its surface.
However, this is not the only important factor; sustaining the sort of climates necessary for life to develop requires a planet to have sufficient heat to power a carbon cycle.
A primary source of this is the radioactive decay of thorium and potassium, which can power convection in a planet’s mantle – the layer of rock between the crust and the outer core.
The resulting carbon dioxide gas escaping as a part of this process then reaches the atmosphere, which helps keep the planet warm.
“We know these radioactive elements are necessary to regulate climate, but we don’t know how long these elements can do this, because they decay over time,” said Dr. Cayman Unterborn, lead author of an Astrophysical Journal Letters paper about the research.
“Also, radioactive elements aren’t distributed evenly throughout the Galaxy, and as planets age, they can run out of heat and degassing will cease. Because planets can have more or less of these elements than the Earth, we wanted to understand how this variation might affect just how long rocky exoplanets can support temperate, Earth-like climates.”
Scientists believe that younger, rocky planets are more likely to support these kind of Earth-like, temperate climates. While current technology cannot measure the composition of an exoplanet’s surface - nor its interior – it can be used to measure how light interacts with the elements in a star’s upper layers. This data lets scientists infer its composition.
“Using host stars to estimate the amount of these elements that would go into planets throughout the history of the Milky Way, we calculated how long we can expect planets to have enough volcanism to support a temperate climate before running out of power,” Unterborn said.
“Under the most pessimistic conditions we estimate that this critical age is only around 2 billion years old for an Earth-mass planet and reaching 5–6 billion years for higher-mass planets under more optimistic conditions. For the few planets we do have ages for, we found only a few were young enough for us to confidently say they can have surface degassing of carbon today, when we’d observe it with, say, the James Webb Space Telescope.”
More experiments will help scientists quantify the parameters for possible life-supporting planets, with the possibility of detecting alien life. The research, ‘Mantle Degassing Lifetimes through Galactic Time and the Maximum Age Stagnant-lid Rocky Exoplanets can Support Temperate Climates’, has been published in The Astrophysical Journal Letters.
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