Astronomers have discovered hundreds of Jupiter-like planets in our galaxy. However, a handful of the planets found orbiting distant stars are more Earth-sized. This gives hope to astrobiologists, who think we are more likely to find life on rocky planets with liquid water.

The rocky planets found so far are actually more massive than our own. Dimitar Sasselov, professor of astronomy at Harvard University, coined the term "Super-Earths" to reflect their mass rather than any superior qualities.

But Sasselov says that these planets – which range from about 2 to 10 Earth masses – could be superior to the Earth when it comes to sustaining life.

On Shaky Ground

It is said that 99 percent of all species that ever lived have gone extinct. Earth, it seems, is a tough place to call home. Our planet has gone through Ice Ages and global warming trends, it has been hit by comets and asteroids (leading, in one case, to a mass extinction that felled the mighty dinosaurs), and the amount of oxygen in the atmosphere has risen and fallen over time. Our planet is always in a state of flux, and life must adapt to these changes or die.

The shifting of tectonic plates is another example of Earth's restless nature. Continents bang together, forming mountains, only to be later torn apart. Islands grow from underwater volcanoes, and elements are liberated from rocks when they are melted beneath the crust.

While all this geologic activity makes us literally stand on shaky ground, scientists have come to believe that tectonics is one of the key features of our planet which makes life possible. If not for tectonics, carbon needed by life would stay locked within rocks.