As the search for life in the universe continues, scientists already know that it is not enough to find rocky planets in a star’s habitable zone, the region where a planet can harbor liquid water, a requirement for life as we know it.
This is just the starting point. In fact, other factors, such as nitrogen, may play a role in the habitability of a planetas well as the land to sea ratio. Now, a team of scientists suggests that one of the key features of a rock structure exoplanet is that it must be young – only a few billion years at most. That’s because, to sustain life, a planet needs enough heat to feed a planet. carbon cyclewhich is normally created due to the radioactive decay of elements such as uranium and thorium.
“Exoplanets without active outgassing are more likely to be cold and snowball planets,” said lead author Cayman Unterborn, a research scientist at the Southwest Research Institute in Texas, in a statement. “Younger planets with temperate climates may be the simplest places to look for other Earths.”
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This radioactive decay, in turn, causes volcanic outgassing – the release of gases held within a planet into the atmosphere through volcanoes – onto a planet’s surface. Degassing contributes carbon dioxide to the atmosphere and continues the carbon cycle. But older planets may have used up their radioactive resources and therefore may not be able to retain their heat, the scientists explain in a new paper.
“We know that these radioactive elements are needed to regulate the climate, but we don’t know how long these elements can do that, because they decay over time,” said Unterborn. “Furthermore, radioactive elements are not evenly distributed throughout the galaxy, and as planets age, they can run out of heat and outgassing will cease.”
So how long can an exoplanet sustain radioactive decay? As the amounts of radioactive elements on each exoplanet can vary, so can these timeframes.
“Under the most pessimistic conditions, we estimate that this critical age is only about 2 billion years old for a planet with the mass of Earth and reaching 5 [billion] to 6 billion years for more massive planets under more optimistic conditions,” said Unterborn.
Related: How the habitable zones around planets work (infographic)
The problem is that current technology cannot determine which elements exist on an exoplanet. At the moment, planetary composition is only inferred by looking at a planet’s starlight, which can indicate what elements may be present in the system as a whole.
But with the James Webb Space Telescope, scientists will be able to determine the composition of exoplanet atmospheres, thus revealing more clues about the ages of exoplanets and therefore the potential ability of these worlds to be heated by radioactive decay and surface degassing.
The research was published May 3 in The Astrophysical Journal Letters.