Earth Barely Big Enough for Life, Study Says

Astronomers searching for habitable worlds might do best to look for rocky planets....

  Astronomers searching for habitable worlds might do best to look for rocky planets several times larger than Earth. That's because, according to a new study, our planet is at the lower end of the size range needed for plate tectonics—which scientists believe are vital for stabilizing temperatures enough for life. Tectonics—the continent-shifting forces that build mountains and fuel volcanoes—recycle Earth's crust by drawing it underground, where it melts and later re-emerges as magma, pointed out Diana Valencia of Harvard University. That helps keep carbon dioxide levels in the atmosphere more or less stable, as excess gas is removed from the atmosphere by reacting with fresh rocks in a process called "weathering." The carbon dioxide is later returned to the atmosphere via volcanic gases. "Plate tectonics is important for the carbon cycle to operate," Valencia said. "Carbon cycling is the Earth's climate thermostat. Over time, it's kept the Earth's temperature around [that of] liquid water, allowing life to emerge." Valencia, who presented her study yesterday at a meeting of the American Astronomical Society in Austin, Texas, and her team looked at factors that determine whether rocky planets experience tectonics. Rocky worlds significantly larger than Earth—commonly known as "super-Earths"—are more likely to have plate tectonics, the researchers concluded. "It gives us hope" for finding habitable worlds, Valencia said. "Finding an Earth analog is going to be hard. But finding super-Earths is easier." Plate tectonics occur because of convection currents in a planet's mantle, which cause the surface to crack into plates that slide past one another