Earth's Early Temperatures Might Make It "One-of-a-Kind," Study Says

Tuesday, 23 August 2016 - 5:01PM
Astrobiology
Earth
Alien Life
Tuesday, 23 August 2016 - 5:01PM
Earth's Early Temperatures Might Make It "One-of-a-Kind," Study Says
For many years, scientists have all but taken for granted that, statistically speaking, there must be habitable planets within stars' "Goldilocks zone," or the distance from a star at which liquid water can exist. But according to a new study, we may be overlooking a huge factor—a planet's ability to regulate its temperature—which would make habitable planets much rarer than previously thought.

When searching for potentially habitable planets, astronomers tend to look for roughly Earth-sized rocky planets that are within the star's Goldilocks zone—distant enough that water won't evaporate, but close enough that water won't freeze. But this theory assumes that other rocky planets are similar to Earth in the sense that they have the ability to regulate their surface temperatures. On Earth, about 200 million years after the planet's formation, the mantle began to regulate its temperature through convention—the constant shifting of hot rock toward the surface and cold rock toward the center. 

But planets with a "stagnant lid," or a static crust that isn't capable of convection, will eventually lead to uninhabitable temperatures. Venus, for example, has a stagnant lid that is unable to pull carbon down towards the core, which led to the extreme greenhouse effect and intolerable temperatures we observe today. And according to modeling performed by Yale University researcher Jun Korenaga, this ability to self-regulate is relatively rare among rocky planets. In addition, his study claims that Super-Earths, like the ones found by exoplanet hunters, would likely not have the ability to self-regulate, and therefore wouldn't be able to maintain habitable temperatures.

Opening quote
"Most ... previous studies have assumed that Earth-like planets self-regulate, and we need to lift the assumption and become much more open-minded," study author Jun Korenaga of Yale University told Business Insider.
Closing quote

And even if some rocky planets mantles that are capable of convection, that still might not mean they have habitable temperatures. While it was previously assumed that self-regulation could correct a planet's internal temperature if it were too cold or too hot for life, Korenaga says mantle convection is "rather indifferent" to internal temperature.

Opening quote
"What we take for granted on this planet, such as oceans and continents, would not exist if the internal temperature of Earth had not been in a certain range," Korenaga said in a statement, "and this means that the beginning of Earth's history cannot be too hot or too cold."
Closing quote

As a result, that "self-regulation" may only apply to maintaining already-habitable temperatures. And since the initial temperature is dependent on the formation mechanism of a planet, and Earth was formed by a relatively arbitrary process involving object collisions, the chances of another planet with livable temperatures may be slim.

Opening quote
"The lack of the self-regulating mechanism has enormous implications for planetary habitability," Korenaga said. "Studies on planetary formation suggest that planets like Earth form by multiple giant impacts, and the outcome of this highly random process is known to be very diverse."
Closing quote

This means that there may be a "Goldilocks zone" for initial temperature as well as distance from a star, and that a planet like Earth is a much rarer occurrence than we once believed.

Opening quote
"[A] planet like Earth could well be the one of a kind in the universe," said Korenaga
Closing quote
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