Some things lethal to humans can also serve as the basis for habitable conditions.
The search goes on for alien worlds orbiting nearby stars capable of supporting life as we know it, but scientists at the Southwest Research Institute (and others) have found that younger rocky exoplanets are the most likely candidates for habitable worlds beyond our solar system, according to a study published in Astrophysical Journal Letters.
And the reason, perhaps ironically, has to do with the presence of dangerous materials. Ones that are more abundant on young rocky worlds, than old ones.
Alien worlds are warmed by CO2 released via radioactive elements
In the past, scientists have emphasized planets located inside their star's habitable zone in their search for Earth-like planets. This is at first glance wise, since too far is too cold and too close is too hot for liquid water to persist. But even inside the coveted "Goldilocks zone," alien worlds may still end up with atmospheres that aren't very friendly to life.
To keep viable temperatures, planetary climates need to possess enough heat for a global carbon cycle. A crucial resource for an active carbon cycle is the decay of radioactive isotopes of thorium, potassium, and uranium.
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These serve as a critical means of heating — and thus powering — rocky planets' motion mantle convection, which is the slow, eon-spanning process by which a planet's core and crust interact. Volcanic "degassing" (or unleashing) of internal gas is the primary means for carbon dioxide (CO2) to make it into the atmosphere. This, in turn, warms the planet.
But without mantle degassing, a rocky planet is likely to host an uninhabitable atmosphere. In essence, this means that radioactive materials are crucial in the formation of an atmosphere capable of supporting life.
Judging alien worlds by the composition of their stellar hosts
And, since radioactive material decays over time, that means older planets are less likely to have them, which means it's less likely to be warm enough to support a habitable atmosphere.
"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 Cayman Unterborn, the lead author of the study, in a press release.
"Also, radioactive elements aren't distributed evenly throughout the galaxy, and as planets age, they can run out of heat and degassing will cease," added Unterborn. "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."
And it's not easy to confirm this in the physical universe. Modern-day technology can't measure the composition of exoplanet surfaces — let alone what's going on under the crust. But we can check out the abundance of elements in a star on a spectroscopic basis, by observing the way light interacts with elements in the upper layers of the star.
The James Webb Space Telescope could reveal signs of habitable conditions on alien worlds
With this data, scientists are capable of inferring the composition of the star's orbital planets. "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," said Unterborn, in the release.
"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," added Unterborn. And, looking at the range of planet ages we know so far, Unterborn and his collaborators confirmed only a handful of systems whose planets should "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."
While there could be a natural limit to the number of habitable planets — maybe they're mostly old — when the James Webb Space Telescope begins its science missions this summer, there's a good chance we might spot a few rocky planets that are just the right temperature, at just the right age, with traces of exactly the right kinds of elements that signify a high probability of habitable conditions on an alien world.