Super Earths Hold Their Water

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When they apply observational data to statistics, astronomers reach the conclusion that there should be tens of billions of Earth-like planets in the Milky Way alone. But when we say Earth-like, it should not be taken too literally. For nearly all of the rocky worlds among the mostly gas giant 1000-plus extrasolar planets discovered so far, were found to be super Earths with masses up to 10 times that of the Earth and diameters a few times wider. But are they suited for life?

We know that liquid water is essential for the life as we know it. And again billions of Earth-like extrasolar planets orbit their stars within what is called the habitable zone where temperatures allow the presence of liquid water, astronomers tell us. Yet, being in the habitable zone does not guarantee being awash in water. Take, for instance the the sizzling Venus which lies within the Solar system’s habitable zone along with the Earth, but has been turned into a hellish world by a runaway greenhouse effect. Or take Mars, which is now a freezing desert where only traces of water, which filled large seas and lakes billions of years ago, is now buried under the surface or mixed with soil as ice.

Besides the presence of water as a prerequisite of life, its existence for long periods of time is necessary for complex organisms like us to evolve. We owe our existence to Earth’s ability to maintain its oceans for nearly 4.5 billion years. Now, a study shows that super Earths are even more proficient than our planet in preserving their water.

“When people consider whether a planet is in the habitable zone, they think about its distance from the star and its temperature. However, they should also think about oceans, and look at super-Earths to find a good sailing or surfing destination" says Laura Schaefer of Harvard-Smithsonian Center for Astrophysics (CfA) , who led the team that conducted the study.

 The researchers note that water, covering 70 percent of our planet’s surface, constitutes a very small proportion, just 10 percent of its mass, the remaining 90 percent being rock and ice. "Earth's oceans are a very thin film, like fog on a bathroom mirror," says co-author Dimitar Sasselov, also from the CfA.

But the Earth keeps enough water to fill up several oceans in the mantle under its crust, researchers point out. The water collects there as a result of the movement of the pieces of the Earth’s broken crust in a process called plate tectonics. As the heavier oceanic plates dive or “subduct” under the lighter continental plates to sink into the mantle , they drag along some of the ocean waters . Although normally the Earth should have lost all of its water eons ago, it has held on to its oceans by recycling the water in the mantle through volcanic activity, mainly at the mid ocean ridges where mantle rises to form new crust to replace the portion lost to subduction.

To see whether this process operates on super Earths, too, Schaefer and her team used computer simulations,working with planets of up to five Earth mass and 1.5 Earth diameter. The results showed that planets of up to four Earth masses were more successful at forming and preserving oceans compared to our planet.

Unless they are evaporated by a star evolving to red giant stage, these oceans maintain their existence for 10 billion years. By comparison, the Earth’s oceans will be gone a billion years from now according to calculations of planetary scientists. The simulations showed, however, that a world weighing five Earths would lag a billion years in forming its oceans due a thicker crust which delays the volcanic outgassing (that would carry the water in the mantle to the surface).

Results also provide clues concerning the hunt for habitable planets: Once physical properties and locations mark a planet as habitable, astronomers seek signatures of putative life in its atmosphere. But the emergence of chemical processes to kickstart life takes time and extra time is required for the emergent life to alter the atmosphere of the host planet. Thus, supposing the evolution proceeds with the same speed as on Earth, signatures of advanced life forms should be sought on planets at least a billion years older than the 4.5 billion-year-old Earth.