Planets are born to support life – or not

Location, location, location. It’s one of the most important factors when it comes to a planet’s habitability, and Earth is square in the “Goldilocks zone”. The sun provides the vast majority of our planet’s heat – not too much, and not too little. 

But how a planet evolves – and if it’s habitable – also hinges on the amount of heat it contained when it was born, Jun Korenaga, a geophysicist at Yale University, writes in Science Advances.

Just as you might put on a coat when you go outside in winter, Earth and other rocky planets are thought to add their own blanket in the form of a crusty layer as they cool over time. This “self-regulation” stems heat loss.

Today, the Earth radiates around 44 terawatts of heat into space. Much of this is a relic of energy from the planet’s first few hundred million years.

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Once things settled down, Earth started to cool. And in the four billion years since, it settled into layers: a solid iron inner core and liquid iron outer core – which circulates and creates our magnetic field – sheathed in a 2,900-kilometre-thick layer of hot rock called the mantle. The mantle also circulates, albeit sluggishly compared to the outer core.

This is all topped by the crust, which formed as the mantle cooled. There are two types – oceanic crust, which is only a few kilometres thick, and continental crust, which is 30 to 50 kilometres thick.

Heat is also pumped out by decaying radioactive elements such as uranium-238 and thorium-232. Exactly how much of Earth’s total heat loss comes from radioactive decay is unknown, but recent estimates hover around half or more.

The idea that rocky planets, such as Earth, self-regulate their temperature has been around since American chemist and Nobel prize winner Harold Urey proposed the idea in 1955. Now, geophysicists cite the “Urey ratio”: calculated as the amount of heat generated in the mantle divided by heat loss.

A Urey ratio of one means a planet’s in thermo-equilibrium – in other words, the amount of heat produced inside a planet is the same as the heat shrugged off into space. Estimates peg the Earth’s current Urey ratio between 0.16 and 0.76.

But, Korenaga argues, this self-regulation is not the full story. His numerical modelling suggests the internal temperature of the Earth has little effect on mantle convection. 

Oceans, he says, would never have existed if the planet had started outside a certain range of temperature. “This means that the beginning of Earth’s history cannot be too hot or too cold.”

The paper overlooks a crucial element, says Louis Moresi, a geophysicist at the University of Melbourne in Australia. “You’re in trouble if you ignore the continents,” he says.

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Continental crust, being thicker than oceanic crust, comprises the thick, fluffy parts of the planet’s natural blanket.

Korenaga’s equations, he says, are one-dimensional – they don’t take the planet’s varying crust thickness into account, nor do they address how mantle might move laterally beneath the surface or how the planet “fine-tunes” itself via heat hissing from volcanoes. 

In a 2011 paper, he and colleagues in the US calculated the Urey ratio but included the blanketing effect of continents. They ended up with much higher values than before – up to 0.76.

Still, there’s plenty about the inner workings of the Earth left to be discovered. The Borexino experiment at the Gran Sasso National Laboratory in Italy, for instance, is detecting the slippery anti-particles called geoneutrinos produced during radioactive decay. 

The more it snares, the better scientists will be able to estimate Earth’s composition. “We’ll just have to wait and see,” Moresi says.