Super-earths could be best places to find life

William Atkins
Monday, 22 October 2007 18:58

Science - Space

According to Harvard scientists, habitable worlds are best found on large, rocky planets that are up to ten times the size of Earth and contain plate tectonics.      

Diana Valencia and Richard J. O’Connell, of the Earth and Planetary Sciences department at Harvard University, and Dimitar D. Sasselov, of the Harvard-Smithsonian Center for Astrophysics, have written the paper “Inevitability of Plate Tectonics on Super-Earths.” It has been submitted to The Astrophysical Journal Letters.
The researchers found that super-Earths, or planets up to ten times the size of the Earth are the best places to find extraterrestrial life. These planets contain a solid inner core that is surrounded by a liquid mantle, and on top a crust. What is seen as critical to life on one of these large extrasolar planets (exoplanets)—or planets circling a star other then the Sun—is the presence of plate tectonics.

Thus, in the abstract of their paper, they state: “The recent discovery of super-Earths (masses less or equal to 10 earth-masses) has initiated a discussion about conditions for habitable worlds. Among these is the mode of convection, which influences a planet's thermal evolution and surface conditions. On Earth, plate tectonics has been proposed as a necessary condition for life. Here we show, that super-Earths will also have plate tectonics. We demonstrate that as planetary mass increases, the shear stress available to overcome resistance to plate motion increases while the plate thickness decreases, thereby enhancing plate weakness. These effects contribute favorably to the subduction of the lithosphere, an essential component of plate tectonics. Moreover, uncertainties in achieving plate tectonics in the one earth-mass regime disappear as mass increases: super-Earths, even if dry, will exhibit plate tectonic behaviour.” 

Plate tectonics is a geological theory that helps to explain large motions of the Earth’s lithosphere. The solid, outermost layer of the Earth is the lithosphere, which is made up of the rocky crust—the surface of the Earth and the uppermost part of the mantle. The next layer is the asthenosphere, the softer layer of the upper mantle, which steadily but very slowly flows like a liquid over millions of years. The lower layer of the mantle, which is below the asthenosphere, is a fairly rigid section because of extreme pressures around it. (The outer and inner core is the innermost layers of the Earth.)

Within the tithosphere, tectonic plates are present. On Earth, there are seven major tectonic plates and numerous smaller (minor) ones. These plates move together at about 0.75 centimeters each year, some moving apart and some moving together. This movement is what causes earthquakes, volcanoes, and other physical events within the Earth.

The researchers contend that such geological activity is essential for finding life on super-Earths. They have found that activities associated with plate tectonics help to enhance the chance for life, add to the diversity of life forms, and constantly move minerals, chemicals, and other life-giving substances throughout the soil.

According to Sasselov, “When it comes to habitability, super-Earths are our best destination.” [from New Scientist magazine, “Big rocky planets are best bet for life,” October 13-19, 2007, page 20]