Rocky alien worlds need to be young to support life

As the search for life in the universe continues, scientists already know it’s not enough to find rocky planets in a star’s habitable zone, the region where a planet can contain the liquid water needed for life as we know it.

This is just a starting point. Indeed, other factors, such as nitrogen, may play a role in the habitability of the planet, as well as in the land-to-sea ratio. Now a team of scientists suggest that one of the key characteristics of a life-sustaining rocky exoplanet is that it must be young—a few billion years at the most. This is because to sustain life, the planet needs enough heat to power the carbon cycle, which usually comes from the radioactive decay of elements like uranium and thorium.

“Exoplanets without active outgassing are more likely to be cold snowball planets,” lead author Cayman Unterborn, a postdoctoral fellow at the Southwestern Research Institute in Texas, said in a statement. “Young temperate planets may be the easiest places to find other Earths.”

On the subject: What makes a planet habitable? Our assumptions may be wrong

This radioactive decay, in turn, causes volcanic degassing—the release of gases contained within the planet into the atmosphere through volcanoes—to the surface of the planet. Degassing brings carbon dioxide into the atmosphere and continues the carbon cycle. But older planets may have used up their radioactive resources and thus failed to retain their heat, the scientists explain in a new paper.

“We know that these radioactive elements are necessary for climate regulation, but we don’t know how long these elements can do this because they decay over time,” Unterborn said. “In addition, radioactive elements are not evenly distributed throughout the galaxy, and as planets age, they can run out of heat and stop outgassing.”

So how long can an exoplanet sustain radioactive decay? Since the amount of radioactive elements on each exoplanet can vary, these time frames can also differ.

“Under the most pessimistic conditions, we estimate that this critical age is only about 2 billion years for an Earth-mass planet and reaches 5 [billion] up to 6 billion years for higher-mass planets under more optimistic conditions,” Unterborn said.

On the subject: How habitable zones around planets work (infographic)

The problem is that current technology cannot determine what elements exist on an exoplanet. At present, the planetary composition can only be judged by the light of the planet’s star, which can indicate which 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 age of exoplanets and therefore the potential for these worlds to be heated by radioactive decay and surface degassing.

The study was published May 3 in The Astrophysical Journal Letters.

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