Birth of the moon
A theory of how the moon was formed is at stake. Robin Canup and her colleagues at the University of Colorado, USA, say if a collision between the Earth and a planet-sized body would have led to formation of the moon, the Earth and moon should have more angular momentum than they actually do. Angular momentum is the quantity of rotation of a body. For the formation of the moon, an object should be much more massive than the planet Mars. The researchers say a stray planet three times as big as the Mars might have crashed into the youthful Earth that may have led to formation of the moon (New Scientist , Vol 155, No 2093).