Super-earths not a place for plate tectonics
The surfaces of Earth’s supersized cousins outside the solar system probably don’t quake and move like those on our home planet. Or that’s what new research suggests, anyway.
Our solar system hosts only eight planets. But scientists have found nearly 2,000 others orbiting stars beyond our sun. These are exoplanets. Some are rocky like Earth, but much bigger. These giants are called super-Earths.
The surface of our planet is broken into about a dozen large tectonic plates. These plates migrate slowly over time. Their movements can cause big earthquakes where the plates collide. Plate tectonics also drives the carbon cycle. Carbon flows between Earth’s land, oceans and atmosphere. That cycle helps regulate the planet’s temperature and allows life to flourish, notes Takehiro Miyagoshi. He is an earth and planetary scientist at the Japan Agency for Marine-Earth Science and Technology in Yokohama.
The interiors of super-Earths experience supersized pressures. In the new study, Miyagoshi’s team simulated these extreme pressures. That work suggested that these exoplanets probably have thick, stagnant outer shells. Deeper within these planets, gooey rock circulates sluggishly. Those properties make the existence of tectonic plates unlikely, the scientists conclude. They report their findings in a paper to be published soon in the Journal of Geophysical Research: Planets.
Scientists have yet to find a planet other than Earth that is capable of supporting life. The new study suggests that the lack of plate tectonics on super-Earths makes these planets less likely to support a climate friendly to life. The reason: These planets are unlikely to have a vigorous carbon cycle, one that removes and adds carbon dioxide.
“We think super-Earths are boring,” Miyagoshi says. “This point should be kept in mind in our search of habitable planets.”
Earth’s tectonic plates are driven by a conveyor belt of sinking and rising rock. Previous studies had predicted that the extra heat inside super-Earths would easily power similar movement.
Those studies, however, adapted previously created simulations of Earth’s internal movements. The studies did not consider the changes that come with a bigger planet, Miyagoshi says. Larger planets put more pressure on their interiors. That boosts temperatures at lower depths. And it changes how rocks and magma — liquid rock — move through the planet.
Miyagoshi and his colleagues simulated a planet with 10 times Earth’s mass. Blobs of cold rock descended into the simulated interior. Then, rising pressures heated the rock and stalled its fall. Similarly, rising plumes of magma cooled and slowed as they climbed toward the surface. This lethargic movement created a stagnant shell around the planet that was roughly 1,800 kilometers (1,100 miles) thick. That’s about equal to the radius of the moon.
The new work isn’t the final word on plate tectonics on super-Earths, says Brad Foley. A geoscientist, he works at the Carnegie Institution for Science in Washington, D.C. Scientists don’t fully understand why Earth has plate tectonics while other planets such as Venus don’t, he says. “Until we know that well, we’ll always be all over the place when we try to predict what will happen on super-Earths.”
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carbon The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules.
exoplanet A planet that orbits a star outside the solar system. Also called an extrasolar planet.
geophysics The study of matter and energy on Earth and how they interact.
geoscience Any of a number of sciences, like geology or atmospheric science, concerned with better understanding the planet.
magma The molten rock that resides under Earth’s crust. When it erupts from a volcano, this material is referred to as lava.
moon The natural satellite of any planet.
planet A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and it must have cleared other objects out of the way in its orbital neighborhood. To accomplish the third feat, it must be big enough to pull neighboring objects into the planet itself or to sling-shot them around the planet and off into outer space. Astronomers of the International Astronomical Union (IAU) created this three-part scientific definition of a planet in August 2006 to determine Pluto’s status. Based on that definition, IAU ruled that Pluto did not qualify. The solar system now consists of eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
plate tectonics The study of massive moving pieces that make up Earth’s outer layer, which is called the lithosphere, and the processes that cause those rock masses to rise from inside Earth, travel along its surface, and sink back down.
plume (in biology) A single large feather. (in environmental sciences) The movement of some gas or liquid, under the direction of gravity, winds or currents. It may be in air, soil or water. It gets its name from the fact that it tends to be long and relatively thin, shaped like a large feather. (in geology) Magma that rises into Earth’s crust.
radius A straight line from the center to the circumference of a circle or sphere.
simulate (in computing) To try and imitate the conditions, functions or appearance of something. Computer programs that do this are referred to as simulations.
solar system The eight major planets and their moons in orbit around the sun, together with smaller bodies in the form of dwarf planets, asteroids, meteoroids and comets.
star The basic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become dense enough to sustain nuclear-fusion reactions, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.
sun The star at the center of Earth’s solar system. It’s an average size star about 26,000 light-years from the center of the Milky Way galaxy.
super-Earth A planet (in a distant solar system) with between one and 10 times the mass of Earth. Our solar system contains no super-Earths: All of the other rocky planets (Mercury, Venus, Mars) are smaller and less massive than Earth, and the gas giants (Jupiter, Saturn, Neptune and Uranus) are all larger, containing at least 14 times the mass of Earth.
tectonic plates The gigantic slabs — some spanning thousands of kilometers (or miles) across — that make up Earth’s outer layer.
Venus The second planet out from the sun, it has a rocky core, just as Earth does. However, Venus lost most of its water long ago. The sun’s ultraviolet radiation broke apart those water molecules, allowing their hydrogen atoms to escape into space. Volcanoes on the planet’s surface spewed high levels of carbon dioxide, which built up in the planet’s atmosphere. Today the air pressure at the planet’s surface is 100 times greater than on Earth, and the atmosphere now keeps the surface of Venus a brutal 460° Celsius (860° Fahrenheit).
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Original Journal Source: T. Miyagoshi et al. Thermal convection and the convective regime diagram in super-Earths. Journal of Geophysical Research: Planets, in press, 2015. doi: 10.1002/2015JE004793.
Learn more about the carbon cycle from the National Aeronautics and Space Administration.