Life without a moon

Posted on November 30, 2011

Formation of the MoonJaques Laskar and collegues of the Paris Observatory in France showed in 1993 that Earth’s moon helps to stabilise the tilt of the planet’s axis of rotation agauinst perturbations that would otherwise be caused by the planet Jupiter.Researchers calculated that without the moon Jupiter’s gravity would influence the planet so that its current tilt of some 23 degrees would wander chaotically between zero and 85 degrees causing huge swings in climate. Such swings would make it hard for life on Earth to survive, especially large land-based organisms such as ourselves.

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Since our moon is thought to have coalesced from the remains of a freak collision between the Earth and a Mars-sized planet, this result was thought to mean that life in the wider universe would be a rare event. Less than one on ten Earth-sized planets are expected to experience such events which would make large moons like our own – and consequently large-scale life such as our own – a rarity.

However, a new study from NASA’s Ames Research Center in California, led by Jack Lissauer, now suggests that moonless planets have been disregarded unfairly. “There could be a lot more habitable worlds out there,” he has said.

JupiterThe 1993 study showed that in the solar system, two fo the motions of the Earth would end up in synch which would allow Jupiter to have an influence larger than otherwise. The orbit of Earth around the Sun is elliptical and the long axis of the orbit shifts position over time. In addition, the axis of rotation also wobbles rather like a spinning top as the Earth turns. Without the moon, this wobble would be slower and would match up in just the right way with the elliptical orbit so that the effects of Jupiter’s gravity on the axis of spin would be magnified. It is this effects that would lead to big changes in the tilt.

What it seems Laskar’s study failed to take into account was just how fast these radical changes in tilt would take to happen. According to Lissauer, “The astrobiology community has taken it to mean there will be these really wild variations.”

That is just what Lissauer’s work with NASA has tested.He and his colleagues simulated a moonless Earth over 4 billion years – roughly the age of the planet – and found that the tilt varied only between 10 and 50 degrees. This is a much smaller range than suggested by the earlier work and included very long stretches of up to half a billion years when the tilt was particularly stable, between 17 and 32 degrees, which is close to our present tilt.

Much larger changes may, of course, happen over much larger periods than 4 billion years, Lissauer agrees, but in that case they may not be relevant anyway since stars like our own Sun have lives of only some 10 billion years before they burn out. Darren Williams, of Pennsylvania State University, agrees that large moons may not be required for life and suggests they could even be detrimental in some arrangement of the planets in a sysytem. All of this suggests that there could be a great deal more life out there to be found than has been assumed so far.