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 …