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Talk about being over the moon. It seems planets don't need a big satellite like Earth's in order to support life, increasing the number on which life could exist.

In 1993, Jacques Laskar of the Paris Observatory in France and colleagues showed that the moon helps stabilise the tilt of Earth's rotation axis against perturbations by Jupiter's gravity. The researchers calculated that without the moon, Jupiter's influence would make the current tilt of some 23 degrees wander chaotically between 0 and 85 degrees. That could cause huge climate swings, making it hard for life to survive, especially large, land-based organisms like us.

The result was taken by many to imply that complex life is rare in the universe, since Earth's large moon is thought to have coalesced from the debris of a freak collision between a Mars-sized planet and Earth. Less than 10 per cent of Earth-sized planets are expected to experience such a trauma, making large moons a rarity.

But a study now suggests moonless planets have been dismissed unfairly. "There could be a lot more habitable worlds out there," says Jack Lissauer of NASA's Ames Research Center in Moffett Field, California, who led the research.

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