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Rocky Exoplanets May Be 'Squishy' Worlds

Planets beyond the solar system that are bigger than Earth but smaller than gas giants like Neptune could have oceans of liquid metal and life-protecting magnetic shields.
/ Source: Discovery Channel

Planets beyond the solar system that are bigger than Earth but smaller than gas giants like Neptune could have oceans of liquid metal and life-protecting magnetic shields.

Under the heat and pressure that exist inside super-Earths, magnesium oxide and other minerals commonly found in the rocky mantles of the terrestrial planets, transform into liquid metals, laboratory tests show.

The research has implications for understanding conditions on super-Earths, including whether they might be favorable for supporting life.

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Scientists zapped a piece of magnesium oxide with high-powered lasers to simulate the heat and pressure that would exist on planets roughly three to 10 times as massive as Earth. They discovered that the clear ceramic mineral first morphed into a solid with a new crystal structure, then completely transformed into a liquid metal.

In that state, the liquid mineral may be able to sustain a physics phenomenon called a "dynamo" action, which is responsible for generating magnetic fields.

"It is often thought that a planetary magnetic field protects life on a planet's surface from harmful space radiation, like cosmic rays. What we find is that magnetic fields may exist on more super-Earth planets than expected, resulting from the transformation of the planet's rocks to metals in the deep interior. This could create new environments for life in the universe," geophysicist Stewart McWilliams, with the Carnegie Institution and Howard University in Washington DC, wrote in an email to Discovery News.

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"The field certainly affects the way life evolves. I think it is an open question as to whether its absence inhibits the development of life," added planetary scientist David Stevenson, with the California Institute of Technology in Pasadena.

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"It is not easy for a terrestrial planet to generate magnetic field because the high thermal conductivity of the core material also allows heat to leak out by conduction, thus reducing the likelihood of convection. It is actually best to have a poor electrical conductor," he continued.

The discovery not only complicates models for understanding how planets form and evolve, but also blurs the distinction between a planet's core and its mantle.

"Melting in planets is very important. In planets like the Earth, melting leads to many features of the world around us -- volcanoes, and the Earth's magnetic field, for example. In the early history of planets like Earth, it is possible the entire planet was liquefied, forming a deep ocean of magma on the surface. Even today, some super-Earth planets may have these magma oceans," McWilliams said.

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The experiment, however, shows a super-Earth's magma ocean may be made of liquid metal, he added.

Magnesium oxide is not the only mineral important for Earth-like planets, but other rocky materials including perovskite (magnesium silicate) and quartz show similar transformations at high pressure and temperature.

"They change from the transparent, insulating materials that we see on the Earth's surface, to conducting materials more similar to iron in the deep interiors of planets," McWilliams said.

Magnesium oxide has been studied for decades in computer simulations and theoretical models, but never before in an experiment that replicated conditions found inside super-Earths.

"To really understand a planet we need to model the whole thing," McWilliams said. "This can be done with advanced computer codes, for example, that describe the formation of a planet's magnetic field. I think the next step is to see if models can confirm our findings."

The research is published in this week's Science.