discoveries of large Earth-like planets outside our Solar System, so-called “super-Earths,”

Super Earths' Will Have Plate Tectonics, Scientists Predict
The discoveries of large Earth-like planets outside our Solar System, so-called “super-Earths,” has prompted much speculation about just how Earth-like they may be. Recently, scientists from Harvard University suggested that these planets will, like Earth, have plate tectonics.

Plate tectonics, the movement of the giant plates that make up Earth's solid outer shell, are responsible for earthquakes, volcanoes, and other major geological events. In essence, they have dominated Earth's geological history. Earth is the only known planet that has plate tectonics, and this activity has been proposed as one necessary condition for the evolution of life.

However, in a paper published in The Astrophysical Journal, Harvard planetary scientist Diana Valencia and her colleagues predict that super-Earths – which are between one and ten times as massive as Earth – will fulfill one of the requirements for sustaining life by having plate tectonics.

“Some of these super-Earths may be in the 'habitable zone' of their solar systems, meaning they are at the right distance from their mother star to have liquid water, and thus life,” Valencia, the paper's corresponding author, told PhysOrg.com. “Ultimately, though, only these planets' thermal and chemical evolution will determine whether they are habitable. But these thermal and chemical properties are closely tied to plate tectonics.”

Using detailed models they developed of the interior of massive terrestrial planets, Valencia and her group determined how the mass of a super-Earth is related to the thickness of its plates and the magnitude of the stresses the plates experience. These stresses, part of the slow, slow convection of Earth's mantle, are the driving force behind the deformation and subduction (when one plate sinks below another) of the plates. For planets more massive than Earth, this driving force is larger than Earth's.

The group found that as planetary mass increases, there is an increase in the shear stress and a decrease in the plate thickness. Both of these factors weaken the plates and contribute to plate subduction, which is a key component of plate tectonics. Therefore, the scientists say, the conditions required for plate deformation and subduction are easily met by super-Earths. Their results show that this is particularly true for the larger super-Earths.

“Our work strongly suggests that super-Earths, even if they have no water, will exhibit plate tectonic behavior,” Valencia said.

In the future, it may be possible to verify these results using NASA's Terrestrial Planet Finder devices or the European Space Agency's Darwin project, which will consist of three telescopes to search out Earth-like planets.

Super Earths Emerge From Snowy Conditions
Many extrasolar planets have been discovered circling other stars, a few of which are 5-15 times the mass of the Earth, and thought to be solid like our planet. Astronomers were surprised to find these planets orbiting small, cooler red dwarf stars. Researchers believe these "super Earths" form in the chilly halo of snow, ice and frozen gasses that collect around red stars as they cool. There probably isn't enough solid material to form rocky planets much larger than Mercury in the star's habitable zone.

The 200 known planets that orbit other stars exhibit incredible variety. Among them are a handful of worlds that weigh between 5 and 15 times Earth. Astronomers believe these "super-Earths" are rocky iceballs rather than gas giants like Jupiter. While theorists can explain how such worlds form around Sun-like stars, the discovery of super-Earths around tiny red dwarf stars was surprising. New research suggests that some super-Earths build up rapidly when local temperatures drop and ices condense out of the surrounding gas.

"We believe that some super-Earths form during a cosmic 'snowstorm.' Only this snowstorm envelops the whole planet and lasts millions of years," said astronomer Scott Kenyon of the Smithsonian Astrophysical Observatory.

All planets form within a disk of gas and dust surrounding a newborn star. Rocky planets form close to the star, where it is warm, while icy and gaseous planets form farther out, where it is cold. When it was young, the Sun was relatively stable, leading to a natural progression of small, rocky worlds in the hot inner solar system and large, gaseous worlds in the cold outer solar system.

In contrast, planetary systems around small red dwarf stars undergo dramatic changes in their early history. As the young star evolves, it dims. The warm inner disk starts to freeze, creating conditions where water and other volatile gases condense into snowflakes and ice pellets.

"It's like a massive cold front that sweeps inward toward the star," explained first author Grant Kennedy of Mount Stromlo Observatory in Australia. "The ices add mass to a growing planet, and also make it easier for particles to stick together. The two effects combine to produce a planet several times the size of Earth."

The disks that surround small red dwarf stars tend to contain less material than the disk that formed the solar system. Without the "snowstorms" in these smaller disks, there is not enough material to make super-Earths.

Although astronomers have discovered a few super-Earths orbiting red dwarf stars, it may be tough to find worlds hospitable to humans. All of the known super-Earths are icy worlds with no liquid water. Red dwarf stars are so dim and cool that their warm "habitable zones" are very close to the star, where there is very little planet-forming material.

"It's difficult to make anything larger than Mercury or Mars in the habitable zone of a red dwarf.

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Super-Earths will have plate tectonics
Super-Earths" - rocky planets up to 10 times the mass of Earth that orbit other stars - probably have similar structures to our world, with a solid inner core surrounded by a liquid mantle and then a crust. They may even have plate tectonics, which some argue is necessary for life to evolve. Dimitar Sasselov of the Harvard-Smithsonian Center for Astrophysics and colleagues came to this conclusion after modelling geological processes on planets of various sizes. They found that as planetary mass increases, more heat is trapped and convection increases. As a result the shear stress within the crust increases too and plate thickness decreases. That means the plates are weaker and plate tectonics becomes "inevitable". Our own planet seems to lie at the threshold. If it were any less massive, it would probably not have plate tectonics. Plate tectonics may boost biodiversity by recycling chemicals and minerals through the crust.

"When it comes to habitability, super-Earths are our best destination," says Sasselov. "The idea is right," says Jack Lissauer of NASA's Ames Research Center in Moffett Field, California. "Plate tectonics is more likely on more massive planets."