Read more: Cosmic accidents: 10 lucky breaks for humanity
A colossal interplanetary collision doesn鈥檛 sound like a good thing 鈥� but without it, things might have turned out very differently
One hundred million years into the life of the sun, the dust left over from its formation has gradually coagulated into orbiting bodies in the nascent solar system. There are small rocky lumps close to the sun and larger, icier bodies in the cold outer recesses. As yet, though, there is little to distinguish the third rock from the sun from any other.
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The solar system in which the infant Earth found itself was an unsettled environment, filled with lumps of rock whizzing around on irregular orbits. Some 4.5 billion years ago one of these, a Mars-sized body, clobbered our planet. The result was a comprehensive rearrangement. Some of the impacting material stuck, while the rest was blasted into orbit along with bits of Earth excavated by the collision, where it formed the moon.
It does not sound a particularly propitious event. But luckily, it resulted in a satellite that is anomalously big in comparison to its parent planet. There is nothing else like it in the solar system, where satellites are relatively small bodies that either accreted slowly from orbiting debris or were captured in passing. Elsewhere it seems a similar story. Giant collisions in other solar systems would produce abundant dust visible to the infrared Spitzer Space Telescope, but although a few such dusty systems have been found, collisions big enough to produce something like the moon seem to happen in only 5 to 10 per cent of solar systems 鈥� with the number of instances where this has actually happened considerably smaller even than that (The Astrophysical Journal, vol 670, p 516).
Why does this matter? Because the moon鈥檚 size provides a steadying gravitational hand that helps to stabilise the tilt, or 鈥渙bliquity鈥�, of Earth鈥檚 axis. That prevents wild changes in the pattern of solar heating on the planet鈥檚 surface that could lead to extreme climate swings, including frequent periods where the whole planet freezes over. That鈥檚 a big deal for us. 鈥淐onditions might be bad for complex land-based life if there were no moon and obliquity varied significantly,鈥� says , a planetary scientist at Princeton University.
Earth might still have spawned life without its outsize moon 鈥� even with a frozen surface, the water beneath could offer a decent habitat for sea creatures, Spiegel says. It鈥檚 just unlikely that we would be around to appreciate it.
