Earth-sized planets are just right for life

By David Shiga

Auroras, like this one seen from Alaska, may be a sign that a planet can support life (Image: Joshua Strang, United States Air Force / Wikimedia Commons)
Auroras, like this one seen from Alaska, may be a sign that a planet can support life (Image: Joshua Strang, United States Air Force / Wikimedia Commons)

The discovery of extrasolar super-Earths — rocky planets about five to ten times the mass of Earth — has raised hopes that some may harbour life. Perhaps it's a vain hope though, since it now seems that Earth is just the right size to sustain life.

Life is comfortable on Earth in part because of its relatively stable climate and its magnetic field, which deflects cosmic radiation capable of damaging organic molecules as well as producing amazing auroras (see right).

The long-term stability of Earth's climate depends on the way the planet's crust is broken up into plates, which continually slide over and under one another in a process called plate tectonics. Carbon scrubbed from the atmosphere by natural chemical reactions gets buried and recycled within the Earth because of plate tectonics, part of a cycle that stabilises atmospheric carbon dioxide concentrations.

Now it seems rocky worlds have to be about the size of Earth to have both plate tectonics and magnetic fields, says Vlada Stamenkovic of the German Aerospace Center in Berlin. His team will present the work at the European Planetary Science Congress in Potsdam, Germany, on 15 September.

Heat from Earth's core creates the convection currents needed for plate tectonics. Such currents generate the force to tear the crust, produce multiple plates and move those plates around.

Stamenkovic's team found that the pressure and viscosity inside a super-Earth would be so high that a stagnant, insulating layer would form outside the core, weakening the convective currents needed to drive plate tectonics thus making the process unlikely. A 2007 study that concluded super-Earths were prone to plate tectonics did not account for the increase in viscosity that produces the stagnant layer (New Scientist, 13 October 2007, p 20).

The researchers also found that the slow transfer of heat out of the core in super-Earths would prevent a sufficiently rapid circulation of their molten cores, robbing them of a magnetic field.

Planets about 0.5 to 2.5 times the mass of Earth are most likely to support plate tectonics. The limits are fuzzier for magnetic field generation, but also favour Earth-sized planets. "Earth is special," says Stamenkovic.

But astrobiologist David Grinspoon of the Denver Museum of Nature and Science in Colorado points out that Venus seems to have recycled its crust in volcanic outbursts despite a lack of plate tectonics. While this has not stabilised Venus's climate, he says, the possibility that other forms of crustal recycling on super-Earths might do so should not be ruled out. "There may be super-Earths that have intelligent life that has concluded that no life is possible on puny planets such as ours," says Grinspoon.

September 7, 2009 — Return to cover.